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This Volume is for
REFERENCE USE ONLY

2529(1 1

RADIO BROADCAST

VOLUME XIV

NOVEMBER, 1928, to APRIL, 1929

GARDEN CITY NEW YORK

DOUBLEDAY, DORAN & COMPANY, INC.

1929

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INDEX

(*Illustrated Articles. Editorials in Italics)

PAGE

* A AND B Power from the D. C. Lines

r\. (William B. Dall) 34

*A-Power Unit, The Improved Knapp. 43

Aircraft Radio 18, 234

Aircraft Radio, Progress in the Field of. 93
*A11 About the Dynamic Loud Speaker

(Joseph Morgan) 159

Allocation Plan, WGY's Attack on the. . . 163
Allocation Plan, WGY Protests New. ... 92
Allocation Repairs the Broadcast Struc-
ture, New 16

Allocations Attempted, Regulation of. . . . 373
Allocations, Fitting Receivers to the New. 234
"A. C. Band-Pass Screen-Grid Receiver,
An— The Master "Hi-Q 29" (William

E. Barton) 117

*A. C. Operated Tube Tester, A Simple

(The Laboratory Staff) 173

*A. C. Screen-Grid Browning-Drake Re-
ceiver, The New (Glenn H. Browning) 115
*A. C. Tubes. The "Chronophase" for

(Bert E. Smith) 126

Amateur and Commercial Radio 374

*Amplifier and Power Supply for the

"Vivetone 29," An (R. F. Goodwin). 45
Amusement Industries, Radio May Yet

Become the Cornerstone of 91

*Apparatus, New 52

* Apparatus and Its Applications, New

127, 194

*Are Filters Needed in A. F. Amplifiers?
(Keith Henney) 250

*Armchair Chats on Short- Wave Sub-
jects (Robert S. Kruse) 189

*As the Broadcaster Sees It (Carl
Dreher) 32, 102, 185

*Audio Oscillator, An Inexpensive
. (Edward Stanko) 186

*Audio System of Dynamic Speaker
Reproduction, A New (Frank C.
Jones) 29

*Automatic Volume Control System.
New (Charles Williamson) 299

Aviation, A New Radio Service to 297

*TD POWER from the D. C. Lines, A

JD and (William B. Dall)

'Band-Pass Filters, Some Experiments

with (Kendall Clough) 104

*Band-Pass Screen-Grid Receiver, An
A. C.— The Master "Hi-Q 29" (Wil-
liam E. Barton) 117

'Battery-Operated Receiver, An Eco-
nomical (Howard E. Rhodes) 257

Book Reviews:

Advertising by Radio, by Orrin
E. Dunlap, Jr. (Edgar H.

Felix) ... 388

B. B. C. Handbook, The 1929,
by The British Broadcasting
Corporation (Carl Dreher).. 252
Laboratory Treatise on B Bat-
tery Eliminator Design and
Construction, A (Howard E.

Rhodes) 125

Practical Radio, by James A.
Moyer and John F. Wostrel

(Carl Dreher) 162

Practical Television, by E. T.
Larner (Carl Dreln 354

PAGE

Radio, by Elmer E. Burns

(Carl Dreher) 252

Treatise on 25 Testing Units
for Service Men, A (Howard

E.Rhodes) 116

*Broadcast Engineering (Carl Dreher)

246, 311, 402

Broadcasting, In the World of 297, 373

Broadcasting Increases, Commercial .... 18

Broadcasting Stations, Mexican 358

Broadcasting Stations, With the. .93, 164, 235
"Browning-Drake Receiver, A Loop-
Operated (J. H. Gockel) 171

*Browning-Drake Receiver, The New
A. C. Screen-Grid (Glenn H. Brown-
ing) 115

*Building Receivers for Television (Zeh

Bouck and James Millen) 35

*Business Side of Radio Servicing, The
(John S. Dunham) 236

CHARACTERISTICS of Power

V^ Rectifiers (Roger Wise) 393

"Chart for Making DX Measurements,

A (James B. Friauf) 188

*"Chronophase" for A. C. Tubes, The

(Bert E. Smith) 126

*" Chronophase" Screen-Grid Receiver,

The (Bert E. Smith) 49

Commercial Broadcasting Increases 18

Commerical Radio Telegraphy and Tele-
phony 165

Commissioner Robinson Stands Firm . . 163
"Condenser Type Loud Speaker, The

(Joseph Morgan) 369

Congress Considers the Commission's

Record 372

"Crystals, Quartz (R. C. Hitchcock) ... 85
"Cuban Short- Wave Receiver, A (Frank

H. Jones) 313

"TAX Measurements, A Chart for

LJ Making (James B. Friauf) .... 188
Davis-Dill Publicity Barrage Begins, The 233

Decisions of the Courts 236

"Double-Detection Short-Wave Set, A

(Robert S. Kruse) 309

"Dynamic Loud Speaker, All About the

(Joseph Morgan) 159

"Dynamic Speaker Reproduction, A

New Audio System for (Frank C.

Jones) 29

"Dynamic Speakers, Measurements on

(Frank C. Jones) 316

* T? CONOMICAL Battery-Operated
•L-' Receiver, An (Howard E.

Rhodes) 257

"Efficient Push-Pull A. F. System, An

(Kendall Clough) 241

"Examination for Radio Servicemen, An

(J. B. V. Meacham) 405

"Experimenter's Armchair, The (Robert

S. Kruse) 380

LTEDERAL Radio Commission, A

r Well-Balanced .. 372

"Fellows of the Institufcou^he (Carl

Dreherj *»*uj^uj 51

"Few Radio Questions yfrSWreH, A .... 166

PAGE

Fight for Short- Wave Allocations, The ... 92
"Figure in Radio Progress, A (Edgar H.

Felix) 227

"Filters Needed in A. F. Amplifier? Are

(Keith Henney) 250

Fitting Receivers to the New Allocations . . 234

Frequency Control, More Discussion on. 373

"f^RID-LEAK, Grid-Condenser De-
VJ tection (Frederick EmmonsTer- 303

man)

"Grid-Leak Power Detection (Frederick
Emmons Terman) 382

I-fERE and There 18

-f 1 High-Frequency Allocations 297

"High-Power Output Tube, A (K. S.

Weaver) 329

"High-Voltage Metallic Rectifiers for
B-Power Units, New (J. George

Uzmann) 120

"Home Study Sheets, "Radio Broad-
cast's":

Alternating Current, Pt. Ill 28
Calibrating a Radio Wave-
meter 183

Effects of an Electric Current,

The 27

Experiments with a Wave-
meter 254

Fundamental Radio Theory . . 397

Inductance Standards 398

Plotting Curves 307

Plotting Power Tube Charac-
teristics 184

Resonance in Radio Circuits.

Pt. 1 107

Resonance in Radio Circuits,

Pt. II 108

Transmission Unit, The 253

"IMPORTANCE of Impedance Rela-

1 tions (C. T. Burke) 322

"Improved Knapp A-Power Unit, The. 43
*In the Radio Marketplace ....267, 331, 406

In the Visual Broadcasting Field 94

In the World of Broadcasting 298, 373

"Inexpensive Audio " Oscillator, An

(Edward Stanko) 186

"Isotone Screen-Grid "Super," The

(Dudley Walford) 174

*T7"NAPP A-Power Unit, The Im-

proved

*T ABORATORY Information Sheets,
t-^ "Radio Broadcast's":

Advantages of Dual Push Pull 276

Alternating-Current Ratings. . 132

A. C. Tubes 206

Analysis of Filter Circuits, An 274

Audio Amplifiers 204

Audio Transformer, The 56

Balancing Radio Receivers ... 56

Band-Pass Circuits 206

Bucking Coils in Dynamic

Loud Speakers 112

Circuit for Short-Wave and

Broadcast Reception 60

Effect of Room Acoustics. . 346

634466

1 '2i

INDEX— Continued

PAGE

Electrifying Battery-Operated

Sets 346

Filter Circuit Characteristics. 274
Formulas for Power Output . . 352
Frequency Characteristics of
Television Amplifier Devel-
oped by. the Bell Telephone

Laboratories 134

Heater Connections for A. C.

Tubes 272

Hook-Up for Short-Wave and

Broadcast Receivers, A .... 60
Importance of Bass Notes .... 348
Importance of Correct Fila-
ment Voltages 354

Mathematics of the Tuned

Circuit 348

Moving-Coil Loud Speakers . 58, 204
Neutralizing and C^u Bat-
ing R. F. Circuits . . ." 412

Obtaining Grid Bias frqm B-

Power Units 414

Power in Broadcasting Har-
monics 348

Power Output 58, 132. 206

Power Output Characteristics

of Vacuum Tubes 132

Resistance-Coupled Amplifiers

130, 132, 202

Shielding 204

Simple Two-Way Telephone

Set 414

Supplying Power Devices from

220 Volts A. C 130

Television 58, 60, 134

Test for a Faulty Push-Pull

Amplifier 352

Three Types of Graphs 272

Voltage Gain in Resistance-
Coupled Amplifiers 274

Wavelength-Kilocycle Chart . . 276
Where A. C. Hum Originates. . 276
'Laboratory, "Strays" from the (Keith
Henney):

Accuracy of the "Slide-Back"

Voltmeter 97

Accuracy of Variable Con-
densers 240, 301

Approved Amateur Intermedi-
ates 378

Cause of Winter Static 377

Correction, A 378

Dynamic Vs Magnetic Speak-
ers 23

Empirical Rules and Formulas 170
Experiments With the Pentode 377
"Gyps" Are Still At It, The. . 24
How Long Will the Engineers'

Job Last? 240

How Much Output Power Is

Needed? 169

How to Stabilize Rice Ampli-
fiers 24

How Useful Is a Tube? 240

Hum in the "Lab" Circuit

Receiver 97

Impedance of Standard Loud

Speakers 240

Importance of Reducing A. C.

Hum 301

Is a 112 Tube Needed in the

First Stage? 98

League of Nations to Broad-
cast 301

Life of A. C. Radio Tubes .... 378
More Data on Underground

Aerials 98

New High Voltage Rectifier

Tube 240

New Precision in Quartz Plates 24
New Radio Tubes in England 301
New Radio Tubes Needed. ... 24
New Regulations of Radio

Commission 240

New Trends in Radio Design 300

Novel Dynamic Baffle 378

Obtaining C Bias for A. F.

Amplifiers 169

Output Vs Voltage Amplifica-
tion . . 377

PAGE
1 ' Phantom ' ' Power Tubes .... 378

Portable Radio Sets 24

Power of Station Harmonics. . 239
Present Designs Do Not Con-
sider Economy 23

Rated Voltages Should Be Ap-
plied to Tubes 239

Receiving on 600 Meters with

Lab. Set 301

Regarding Power, Efficiency

and Energy 239

Regarding Series-Filament

Operation 23

Removing Noise in Shielded

Receher 301

Selectivity in the Browning-
Drake Set 301

Seven New Radio Booklets. . . 24
Some Interesting Formulas . . . 169
Task of Editing Radio Copy,

The 170

Test for Screen-Grid R. F.

Tubes, A 240

Three New Pamphlets Availa-
ble 239

Two New A. C. Tubes on the

Way 300

Underground Aerials 24

Vacuum Tubes as Fuses 377

What Is a Dynamic Speaker? 169

Letters from Readers 137, 354

""Loop-Operated Browning-Drake Re-
ceiver. A (J. H. Gockel) 171

*Loud Speaker, The Condenser Type

(Joseph Morgan) 369

*Loud Speakers— A JDebate 109

*]\ /TANUFACTURED Receivers,
1VJ. "Radio Broadcast's" Service

Data Sheets on (See Set Data Sheets)

41, 121, 177, 265, 327, 304
Manufacturers' Booklets

56, 128, 199, 270, 334
*March of Radio, The (E. H. F.)

16, 91. 163, 233, 296, 372
Market, The Real Size of the Radio ... 379
*Marketplace, In the Radio. . . .267, 331, 406
*Measurements on Broadcast Receivers

(L. M. Hull) 230

*Measurements on Dynamic Speakers

(Frank C. Jones) 316

*Measuring a Receiver's Performance

(Kenneth W. Jaryis) 167

Mexican Broadcasting Stations 358

Milestone in Television, A 17

*Modulator for the 1929 Short-Wave

Transmitter, A (Robert S. Kruse) ... 38
*More Data on the Sargent-Rayment

(Howard Barclay; 197

More Discussion on Frequency Control. 373
*Multiple-Receiver Antenna System, A

(V. D. Landon) 291

ATEED for Defining Television Prac-

1\ tices 18

New Allocation Repairs the Broadcast

Structure 16

*New A. C. Screen-Grid, Browning-
Drake Receiver, The (Glenn H.
Browning) 115

*New Apparatus 52

*New Apparatus and Its Applications

127, 194

*New Audio System for Dynamic
Speaker Reproduction, A (Frank C.
Jones; 29

*New Automatic Volume Control Sys-
tem (Charles Williamson) 299

*New High- Voltage Metallic Rectifiers
for B-Power Units (J. George Uz-
mann) 120

New Nation-wide Picture Transmission
Service 18

New Radio Service to Aviation, A 297

*New Trends in Radio Designs for 1929-
30 (Keith Henney) 14

*New Uses for Power Amplifiers (Fred
H. Canfield) 367

News of the Radio Industry 94, 236

Newspaper Has Radio Picture Transmitter 18

PAGE

* /^ORGANIZATION, An Unusual
W (Robert S. Kruse) 302

*"Our Readers Suggest ..."

47, 114, 192, 260, 324

*Phonograph-Radio Amplifiers (How-
ard E. Rhodes) 88

Picture Transmission Service. l\eiv Na-
tion-wide 18

Picture Transmitter, Neuispjpzr Has
Radio 18

*Power Amplifiers, New Uses for (Fred
H. Canfield) 367

*Power Rectifiers, Characteristics of
(Roger Wise) 393

*Power Supply for the "Vivetone 29,"
An Amplifier and (R. F. Goodwin) . . 45

*Practical Radio Service Records (John
S. Dunham) 392

*Problems in Synchronizing Television
Receiving Discs (Boyd Phelps> 123

*Production Testing with Oscillators
(Richard F. Shea) 387

Program Possibilities May Open New
Radio Market, A Study of 296

Progress in Short and Long Wave Radio 235

Progress in the Field of Aircraft Radio . . 93

*Push-Pull A. F. System, An Efficient
(Kendall Clough) 241

VARTZ Crystals (R. C. Hitch-

cock) 85

*Questions Answered, A Few Radio. . . . 166

J~>ACE for Television Publicity, The 17

-A. Radio Abroad 94

*" Radio Broadcast's" Home Study
Sheets (Keith Henney) (See Home)

27, 107, 183. 253, 307, 397
""Radio Broadcast's" Laboratory In-
formation Sheets (Howard E. Rhodes)
(See Laboratory)

56, 130, 202, 272, 346, 412
""Radio Broadcast's" Service Data
Sheets on Manufactured Receivers

(See Sets) 41, 121, 177, 265

*" Radio Broadcast's" Set Data Sheets

(See Sets) 327,403

*Radio Design for 1929-30. New Trends

in (Keith Henney) 14

Radio in Foreign Countries 19

Radio Industry, News of the 94

Radio Manufacturers, With the 165

Radio May Become the Cornerstone of the

Amusement Industries 91

Radio Picture Transmitter, Newspaper

Has .... 18

*Radio Service Records, Practical (John

S. Dunham) 375

Radio Service to Aviation, A New 297

*Radio Servicing, The Business Side of

(John S. Dunham) 236

Real Size of the Radio Market. The ... 379
*Real Versus Apparent Selectivity

(Kenneth W. Jarvis) 399

Reasons for the 300-Mile Chain Regu-
lation 92

Regenerative Decision, The 164

Regulation of Allocations Attempted .... 373
*Removing Nonsense from Short-Wave

Transmission (Robert S. Kruse) Ill

*Routine of Testing Receivers, The
(John S. Dunham) 375

•OARGENT-RAYMENT, More Data
O on the (Howard Barclay) 197

Schedule of the Best Short- Wave Programs 298

Schedule of Broadcast Television Trans-
missions 234

*Screen-Grid Browning-Drake, The
New A. C. (Glenn H. Browning) 115

*Screen-Grid Receiver, An A. C. Band-
Pass— The Master "Hi-Q 29" (Wil-
liam E. Barton) 117

*Screen-Grid Receiver, The "Chron-
ophase" (Bert E. Smith) 49

*Selectivity, Real Versus Apparent
(Kenneth W. Jarvis) 399

*Service Data Sheets on Manufactured
Receivers, "Radio Broadcast's" (See
Set Data Sheets) 41, 121, 177, 265

INDEX— Continued

PAGE

*Service Man Should Study, What the

(John S. Dunham) . . . . , 294

*Serviceman's Comer. The

101, 181, 255, 319, 389
*Servicing Home-Made Radio Receivers

(B. B. Alcom 179

'Servicing. The Business Side of Radio

(John S. Dunham i 236

Set Data Sheets. " Radio Broadcast's":
A. C.-66 Dayton Receiver.

The 121

Bosch Model 28 Receiver, The 404
Bremer-Tully 8-20 Receiver,

The 177

Browning-Drake Receiver.

The 266

Croslev Model 704-B Receiver,

The" 328

Crosley Model 705 Receiver,

The 328

Day-Fan 8-AC Power Set, The 403
Fada 50. 70, 71, 72 A. C. Elec-
tric Radio Receivers, The . . 122
Federal Type D (60 Cycle) Re-
ceiver 327

Freed-Eisemann Model NR-80 42
Freshman Model "G" Re-
ceivers 41

Freshman Model Q Receiver. 178
Freshman Model 2N-12 Re-
ceiver, The 403

King Model H. Receiver, The 404
Majestic Model 70-B Receiver,

The 328

Philco Electric Receiver. The. 265
Short-Wave Allocations, The Fight for . . 92

Short- Wave List 358

*Short-\Vave Receiver, A Two-Tube T.
R. F. (William Bostwick and W. T.

Thomas) 20

Short-Wave Stations 141

'Short-Wave Subjects, Armchair Chats

(Robert S. Krusei 188

'Short-Wave Superheterodyne, A

(Robert S. Kruse) 262

Short-Wave Transmission. Removing

Nonsense from i Robert S. Kruse) . . . Ill
'Short-Wave Transmitter, A Modulator

for the 1929 < Robert S. Kruse) 38

'Simple A. C. Operated Tube Tester, A

(The Laboratory Staff) 173

'"Skyscraper" Screen-Grid Receiver,

The (Clifford Dentoni 95

'Some Experiments with Band-Pass

Filters (Kendall Clough) 104

'Sound Motion Pictures (Carl Dreher)

32, 102, 182, 244, 314, 385
'"Strays" from the Laboratory (See

Laboratory) 23, 97, 169, 239, 300, 377

Study of Program Possibilities May

Open New Radio Markets, A 296

'Synchronizing Television Receiving
Discs, Problems in (Boyd Phelps) ... 123

T*ABLE of Wavelength Allocations . . 323

J. Television. A Milestone in 17

'Television Abroad? What Prospects of

(Lawrence W. Corbett) 11

'Television, Building Receivers for (Zeh

Bouck and James Millen) 35

Television Practices, Need for Defining . . 18

Television Publicity, The Race for 17

'Television Receiving Discs, Problems

in Synchronizing (Boyd Phelps) 123

PAGE

Telecision Transmissions 17

Television Transmissions, Schedule of

Broadcast 234

*Television. Unscrambling (Boyd

Phelps) 157

Test Set for the Radio Service Man. A

(B. B. Alcom i 99

'Testing Receivers, The Routine of

(John S. Dunham) 375

'Thermionic Milliammeter, A (G. F.

Lampkin) 325

300-Mile Chain Regulation, Reasons for

the 92

'Transmitting Amateur Television

(Boyd Phelps) 247

'Trouble Shooting in the Power Unit

(B. B. Alcorn) 264

'Tube-Tester, A Simple A. C. Operated

(The Laboratory Staff) 173

'Two-Tube T. R. F. Short-Wave Re-
ceiver, A (William Bostwick and W.

T. Thomas) 20

*T TNSCRAMBLING Television (Boyd

U Phelps) 157

'Unusual Organization, An (Robert S.
Kruse) 309

T/rISUAL Broadcasting Field, In the. 94
r ""Vivetone 29," An Amplifier and

Power Supply for the (R. F. Goodwin) 45
'Volume Control System, New Auto-
matic (Charles Williamson) 299

'Volume Control Systems 259

JI/G Y'S Attack on the Allocation Plan 163

rr WGY Protests New Allocation Plan 92

WRNY Television Transmissions 17

Wavelength Allocations, Table of 323

Well-Balanced Federal Radio Commis-
sion, A 372

'What is a Good Tube? 335

What is Public Interest? 17

'What Prospects of Television Abroad?

(Lawrence W. Corbett) 11

'What the Serviceman Should Study

(John S. Dunham) 294

'When the Set Stops Working (B. B.

Alcorn) 25

With the Broadcasting Stations. . .93, 164, 235

With the Radio Manufacturers 165

AUTHORS
Alcorn, B. B 25, 99. 179, 264

Barclay, Howard. . 197

Barton, William E 117

Bostwick, William 20

Bouck, Zeh 35

Browning, Glenn H.. . 115

Burke, C. T 322

Canfield, Fred H. . . 367

Clough, Kendall 104, 241

Corbett, Lawrence W 11

PAGE
Felix, Edgar H.

16, 91, 163, 227. 233, 296, 372, 388
Friauf, James B 188

Dall, William B 34

Denton, Clifford 95

Dreher, Carl. .32, 51, 102, 162, 182, 185, 244,

246, 252, 311, 314. 354, 385, 402

Dunham, John S 236, 294. 375, 392

Gockel, J. H

Goodwin, R. F. . .

171
45

Henney, Keith

14. 23, 97, 169. 239. 250, 300, 377

Hitchcock, R. C 85

Hull, L. M 230

Jarvis, Kenneth . . . . 167, 399

Jones, Frank C 29, 316

Jones, Frank H ... 313

Kruse, Robert S.

38, 111, 189, 262, 302, 309, 380

Laboratory Staff, The 173

Lampkin, G. F 325

Landon, V. D 291

Meacham, J. B. V 405

Millen. James 35

Morgan, Joseph 159, 369

Phelps, Boyd 123, 157, 250

Rhodes. Howard E 88. 116, 125, 257, 412

Shea, Richard F... 387

Smith, Bert E 49, 126

Stanko, Edward 186

Terman, Frederick Emmons. . . .303, 382
Thomas W. T. . . 20

Uzmann, J. George

120

Walford, Dudley 174

Weaver, K. S 329

Williamson, Charles 299

Wise, Roger 393

PORTRAITS

('Portraits in "The March of Radio")

'Aird, John

'Bowman, Charles A. .

'Cady, Walter G.
'Christiansen, Kay
Clark, Clarence R.

'Frigon, Augustin . . . .

296

235
164
402

296

'Gilmour, Hugh 93

Goldsmith. Alfred N. 51, 228

'Horton, J. W..

163

Kemp, G. S 156

Kyle, Colin 369

"Linden, Adolph F 372

'Manson, Donald 296

Raycroft, Louis B. F. 366

RADIO BROADCAST ADVERTISER

Hammarlund Hi-Q 29

Authorized Official Distributors

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ROYAL-EASTERN ELECTRICAL SUPPLY
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ROBERT s. KRUSE

Consultant and Technical Writer

103 Meadowbrook Road, West Hartford, Conn.
Telephone Hartford

ArelfourTubes Jus
Limping Along /

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SELF-ADJUSTING''/^^/

30 FRANKLIN ST., NEW YORK

Out-Selling All Other Kits!

Sargent-Rayment
Screen Grid Seven

Inventors of the

Infradyne

Manufactured by
Silver-Marshall, Inc.

A Station for Every Dial Degree

That is the average performance of the Sargent-Rayment Seven over the entire
dial. Under average conditions 100 stations per night can be heard with it.

The Sargent-Rayment Seven has an amplifier composed of four screen-grid
tubes. There is no regeneration. No "grid suppressors" or other counteracting
devices are used to stop oscillation, the oscillation is fundamentally eliminated
in the circuit design thus allowing every bit of tube amplification to be retained
for useful work. Under these conditions are we unreasonable when we claim
that this receiver has more power, more sensitivity and more selectivity than any-
thing else ever offered for sale? Watch the DX records go to smash this winter!

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Please send me at once full details regarding the Sargent-
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Address -•

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If you send this by AIR MAIL we will answer same way.

RADIO BROADCAST

NOVEMBER, 1928

WILLIS KINGSLEY WING, Editor

KEITH HENNEY EDGAR H. FELIX

Director of the Laboratory Contributing Editor

Vol.4X. No. i

Cover Design - - - From a Design by Haney Hoptyns Dunn
Frontispiece

Pictures Transmitted in One Minute at Radio World's Fair
What Prospects of Television Abroad? - Lawrence W. Corbett
New Trends in Radio Design for 1929-30 - - - Keith Henney
The March of Radio - An Editorial Interpretation

New Wavelength Allocation Plan Need for Defining Television Practice

Newspaper Has Radio Picture Transmitter
Commercial Broadcasting Increases
Here and There
Aircraft Radio
Radio in Foreign Countries

A Two-Tube T.R.F. Short- Wave Receiver

William BostwicJ{ and W. T. Thomas
"Strays" From the Laboratorv

Dynamic vs. Magnetic Speakers
Regarding Series- Filament Operation
Present Designs Do Not Consider Economy

What is Public Interest?
The Race for Television Publicity
WRNY Television Broadcast*
A Milestone in Television
New Nationwide Picture Service

How to Stabilize Rice Amplifiers
New Radio Tubes Needed

Keith Henney

The "Gyps" Are Still at It
New Precision in Quartz Plates
Seven New Radio Booklets
Portable Radio Sets
Underground Aerials

B. B. Akorn

When the Set Stops Working

"Radio Broadcast's" Home Study Sheets - -

No. 9. The Effect of an Electric Current No. 10. Alternating Current — Part HI

A New Audio System for Dynamic Speaker Reproduction

Franl( C. Jones
As the Broadcaster Sees It Carl Dreher

Sound Motion Pictures — Part II Commercial Publications

A and B Power from the D.C. Lines -William B. Doll

Building Receivers for Television Zeh Bouci( and James Millen
A Modulator for the 1929 Short- Wave Transmitter

Robert S. Kru.se
"Radio Broadcast's" Service Data Sheet on Manufactured Re-

ceivers

No. ii. Freshman Model "G" Receivers

No. 11. Freed Eisemann Model NR-8o

The Improved Knapp A-Power Unit

An Amplifier and Power Supply for the "Vivetone 29"

R. F. Goodwin
"Our Readers Suggest —

Short-Wave Plug-in Coils Improving Capacity Feed-Back Circuits

Receiving Without an Aerial Tuning-m With a Distant Loud Speaker

Polarity Indicators Home-made I. F. Transformers

Reducing Hum in the Detector Prolonging the Life of the 171 A

Reducing Hum in A.C. Sets Special Soldering Irons for Difficult Jobs

The "Chronophase" Screen-Grid Receiver - Bert E. Smith

The Fellows of the Institute - Carl Dreher

New Apparatus - - - Useful Information on New Products
Manufacturers' Booklets
"Radio Broadcast's" Laboratory Information Sheets - -

No. ijj. Balancing Radio Receivers No. 158. Hook-up for Short-Wave and

No. 134. The Audio Transformer Broadcast Receivers

No. 135. Television No. ajo. Circuit for Short-wave and Broad

No. 336. Moving-Coil Loud Speaker* cast Reception

No. »j7. Power Output No. 340. Television

The contents of this magazine is indexed in The Readers' Guide
to Periodical Literature, which is on file at all public libraries.

IO
II
14

34
35

45
47

49
5i
52
54
56

AMONG OTHER THINGS. . .

A FUNDAMENTAL part of the policy of this magazine
is the careful scrutiny of everything appearing in its
pages, from the smallest advertisement to the most intricate of
technical articles. We have always felt that our first duty to the
reader was to protect him from misleading statements and in-
accuracies of whatever sort. This rr.agaiine is not, however, the
expression of one man's opinion, ncr even of the group respon-
sible for editing and publishing it. We have frequently given
space to articles on technical subjects in which the authors came
to conclusions at variance with our own opinions. Almost in-
variably after the appearance of the magazine we would be del-
uged with letters and telephone calls demanding to know why
we "said" this or that. The editorial expressions of this maga-
zine, whether general or technical are always definitely identi-
fied. Articles from other sources are also indentified, and for
the opinions expressed, the author is alone responsible. We wel-
come the opportunity of printing controversial articles.

'"PO JUDGE from reports which reach us, the motion-picture
JL industry is in nearly as complete a turmoil as that in which
radio found itself some years ago. The cause of the trouble is,
of course, the application of synchronized and non-synchronized
sound accompaniment to the "feature picture." This develop-
ment is bringing into play practically all of the experience that
broadcasters have so laboriously accumulated in the past few
years, and is drawing into the movie field many broadcasters
and other engineers who have developed apparatus and its
uses for this work. In this connection, the pages of Carl Dreher's
department, "As the Broadcaster Sees It," are well worth
watching, for Mr. Dreher is including much first-hand and
practical information on sound motion picture work. Much
of this work is being done by expert radio service men.

ANEW ZEALAND radio distributor writes us he is in-
terested in communicating with American radio manu-
facturers who wish New Zealand distribution for kits or com-
plete sets, either a.c. or d.c. operated. Manufacturers who are
interested may communicate with the editor.

"pOR those to whom the news has not yet traveled, we
J- repeat the announcement made in this column last month :
the bound volume of RADIO BROADCAST Laboratory Information
Sheets Nos. 1-190 is now available at $i. Order from your
newsdealer or directly from the Circulation Department of
Doubleday, Doran.

'"HO JUDGE from the comments in many letters, the Home
JL Study Sheets are increasing in popularity. It may have
escaped the attention of many who are following these Sheets
that we are quite willing to examine the answers to the problems
in each Study Sheet. These answers will be promptly examined
and returned with our comments.

THE December issue will contain an interesting article on
band-pass filters, an interesting and practical article on
television, more good data for the service man and professional
set builder, instructions on how to grind quartz crystals, a
number- of important constructional articles — and our regular
departments.

— WILLIS KINGSLEY WING.

DOUBLEDAT, DORA7\[ ^ COMPACT, IXC., Garden Qity9

MAGAZINES

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Pictures Transmitted in One Minute at Radio World's Fair

This radio-photo receiver was demonstrated by the Westinghouse Electric and Manufacturing Company for the first time
in the New Madison Square Garden, New York, during the week of the radio show. The apparatus is capable of converting
electrical impulses into a complete photograph in less than one minute. Because of the difficulty in securing a wavelength
assignment Lie demonstration was carried on with the use of wires. However, the engineers state that the equipment
functions equally well by radio within the limits of fading and static. The chief advantage of this new apparatus is that
it reduces the time required for the transmission of a photograph from five minutes to one minute. The size of the repro-
duced photograph is five by eight inches

DEVICE PUTS MOVIES ON THE AIR
This apparatus is the heart of the television motion picture transmitter developed by West-
inghouse. It is used at KDKA for the broadcasting of radio motion- picture programs

By LAWRENCE W. CORBETT

IN DISCUSSING the progress of television the technical press of
the United States and Great Britain appears to take turn in de-
ploring the lack of initiative of its own countrymen and in
praising that of its rivals across the Atlantic. Yet, if we take the opinion
of no less an authority than Sir Oliver Lodge, we must admit that the
eading exponents of the art in both countries are equally up against a
stone wail and that the limitation of the apparatus they are using at

present will not permit further
improvement.

But Sir Oliver Lodge, and other
equally famous scientists whose
opinions incline in his direction,
are by no means pessimistic of
the ultimate success of television.
His criticism applies only to that
apparatus now commonly used by
many of those who claim recog-
nition in the art. Mechanical con-
trivances, Sir Oliver believes, are
limited by certain physical restric-
tions which it will be inordinately
impossible to surmount. To use
his own words:

Cathode rays or moving elec-
trons are the only things likely
to be sufficiently docile and con-
trollable to be used as the agents
for television. No material things
are likely to be able to move
quickly enough, but electrons re-
spond so instantaneously that, if
devices can be invented for utiliz-
ing them, the theoretical difficul-
ties with the required rapidity of
motion would begin to disappear
both from the sender and the re-
ceiver, especially as photoelectric
response is almost infinitely rapid.

A. Campbell Swinton, whose
early apparatus was described in
an article on television which
appeared in RADIO BROADCAST
for July, 1928, by R. P. Clarkson,
appears to have realized the limita-
tions that mechanical equipment
would impose upon television, as

The British Situation

BY NORMAN EDWARDS
Managing Editor, Popular Wirekss and Modern Wireless

FOR months past the question of television has been agitating the minds
of the British public, and day by day the Press in all parts of Great Britain
has been persisting in dinning into the public ear that "television is here."

The Baird International Television Development Co., backed by British
capital, invested for the development of television in England, now definitely
has promised a television service by the end of the current year, despite the fact
that the British Broadcasting Corporation, uhich has a monopoly on broad-
casting in England, has refused to cooperate with Baird and bis associates be-
cause, in the opinion of the Chief Engineer of the B. B. C., and his advisers,
the Baird system is not developed sufficiently to warrant it being utilised as a
public service: and the Chief Engineer and bis associates in the British Post
Office having investigated the Baird system believe it to be in a state of experi-
mental infancy and unfitted for offering a means of service to the public.

Furthermore, the Wireless Telegraphy Act, which governs the use of radio in
England, legally has been interpreted to cover television, and as the British
Post Office refused to licence the Baird system as a public utility service, the
question now arises in the public mind as to what Baird will do.

The Baird people definitely seem to believe that they can find a legal flaw in
the Act of Parliament which governs the use of radio in Great Britain, and pro-
pose 'o start a service, without receiving the permission of the British Post Office.
But it is believed that the Post Office will take steps to prevent Baird giiing an
unauthorised service. The position at the moment is complicated.

The reason why the British Post Office and the B. B. C. will not cooperate with
Baird is not due to any prejudice but simply to the technical fact that the experts
concerned do not consider Baird' s system likely to be successful in satisfying
the public demand for a television service. Baird still adheres to the mechanical
system which experts have pointed out repeatedly — experts which include Sir
Oliver Lodge; Dr. Lee de Forest; Captain Eckersley, the Chief Engineer of the
B. B. C., and A. A. Campbell .Swinton, F. R. S. — shows no likelihood of being
developed in such a way as to provide a commercially possible television service,
and in fact, not even a service which would warrant the authorities in England
granting the Baird Company facilities for exploiting it to the public.

I can say in conclusion that Mr. Corbett's article sums up both technically
and legally, with excellent succinctness, the television situation in Great Britain.

far back as thirty years ago, and to have recommended the utiliza-
tion of cathode rays. As stated in the July article, the proposed equipment
originally devised by Mr. Swinton offered possibilities which many of
our so-called "advanced" present-day systems do not.

At the time of writing this article (early in August and in London)
much is being said about Baird's statement that he will market in
September a home television receiver which will cost $125.00 The in-
ventor has told the writer that
•••••••^•^•^^^^^^^^^^^av, he is already well in production

with these receivers and that
there will be no shortage when
the instruments are released. In
fact, the writer has already seen
a finished model in the Baird
laboratories. The cabinet houses
both the television receiver and
a electrodynamic loud speaker of
American pattern. An eight-inch
diameter circular glass screen is
to the left of the cabinet, and
through this screen, but on a
smaller-sized square, is seen the
actual imagine.

When questioned as the avail-
ability of service to purchasers of
these sets (bearing in mind that
the B. B. C. has refused to
cooperate with Baird at the pres-
ent time) the inventor informed
the writer that he will transmit
pictures from a 4-kw. (input)
transmitter on the roof of his
laboratories in the heart of Lon-
don. These transmissions, it is
planned at the time of writing, will
take place on 200 meters (1500
kc.) Mr. Baird has no license for
this transmitter, since, says he,
one is not required for television
transmissions.

Here is raised an interesting
point. An English editor of a
group of radio publications who
has studied the British Telegraphy
Act very thoroughly tells the
writer that by no possible inter-

RADIO BROADCAST

NOVEMBER, 1928

pretation of the law may Baird "televise,"

which in itself constitutes transmission of intelli-

gence, unless he obtains a license. And he has

been refused the necessary license! It has been

prophesied that Baird will be enjoined by law

from transmitting as soon as he commences to do

so! Surely this attitude on the part of the

authorities can be due to noth-

ing more or less than lack of

faith by the British Post

Office engineers (under

whose scrutiny radio is

microscopically preserved) in

Baird 's present methods.

Baird, it must be admitted,

has made remarkable strides in

the advancement of the art

but now, it is felt, he has come

to the end of his tether, and is

being criticized severely for

placing on the market appa-

ratus representing a germ of an idea still in an

embryonic stage of undevelopment.

It is argued that Marconi had no more to offer
the public in those early days when he first
fought to establish his ideas about wireless, hut
then, he appealed to a far more critical public
(and then only to highly-trained engineers) than
Baird is doing. Moreover, Baird, aided by color-
ful and exaggerated reports in a general press
that knows nothing more about his invention
than the fans who will be expected to invest in
his televisors, is meeting with considerably less
opposition than did Marconi.

COLOR TELEVISION

white) television, however, affords several times
as much detail as color television.

The following explanation of the principles of
color television was dictated by Mr. Baird es-
pecially for RADIO BROADCAST. Except for the
changes outlined below, the equipment is similar
to that used by Baird in his monochrome experi-
ments and already described in
this magazine on several occa-
sions :

Green

FIG

TELEVISION, the latest develop-
^-"' ment of Baird, was described in the London
daily papers in glowing terms. The writer was
given a personal demonstration of this new child
of television, by Baird, and was impressed only to
the extent that is due to a radical development
yet in its cradle days. A man's head, covered
alternately with a blue or red cloth, afforded the
subject to be transmitted in the demonstration.
It was possible to see when the subject opened
or closed his mouth, put out his tongue, and
possibly when he rolled his eyes, and to see the
different colors of the head coverings, but to say
that the features were recognizable would highly
exaggerate the matter. Monochrome (black and

I Hi. 2

BAIRD S EXPLANATION

HE transmitting machine
consists of a disc perforated
with three sets of holes ar-
ranged in spirals and set round
the periphery of the disc. See
Fig. i.

One spiral is covered with
red filters, the second spiral is
covered with green filters, and
the third spiral with blue filters,
so that each spiral lets through only light of one
color. As the disc revolves, a spot of red light first
traverses the object being transmitted, this red
light being thrown back from the object and effect-
ing a photo-electric cell. The red light, having
completed this traversal of the object, the green
spiral next comes into operation and traverses
the object with a spot of green light. This is then
followed by the traversal of the object with a
spot of blue light from the spiral covered with
blue filters. Thus, the object is traversed first
by green, then by red, and then by blue illumina-
tion and three images are sent out to the receiver,
the first image showing only the red parts of the
object, the second image showing only the
green parts, and the third image showing only
the blue parts.

At the receiving station a similar disc to that
of the transmitter revolves exactly in step with
the transmitting disc, and behind this disc in
line with the eye of the observer are two glow
discharge lamps — a neon lamp, and a lamp filled
with helium and mercury vapor. The neon lamp
supplies the red constituents of the light and
the helium and mercury vapor the blue and
green. The eye of the observer sees first a red
image, then a blue image, and then a green
image, the images being presented so rapidly
that persistence of vision causes them to blend
and the observer to see a composite image made
up of these three colors.

Now red, green, and blue constitute the three
primary colors from which all other colors are
made up: for example, purple is a combination
of red and blue, yellow is a combination of green
and red, and in the same way all other colors
are made up of various proportions of these three
primaries, red, green, and blue. The process
used in color television is exactly similar to the
three-color process used in color cinematography.
The application of color television will natur-
ally be considerable, but it is not proposed by the
Baird Television Company to put color machines
on the market for some time, as this apparatus
is still in the experimental stage. The first tele-
vision machines to be marketed will be simply
monochrome, showing one color, and these
machines were exhibited at the Radio Exhi-
bition held at Olympia, on September 22nd
last, and will be placed on the market at a
price of £25.

The image appears on a glass screen approxi-
mately eight inches in diameter and the televisor
includes a loud speaker of the electrodynamic
type. A special receiving device has been de-
signed which enables both vision and speech to
be received from the same aerial and at the
same time.

As three separate images must be sent in color
television the speed of transmission should be
increased three times. In practice, however, it
is found that this increase of speed is unnecessary
as each of the three images contains in itself quite
a large proportion of the visual image received
and it is not necessary to transmit at more than
twice the speed of normal television to obtain
a satisfactory blend of the images and colors.

CAMPBELL SWINTON'S EXPERIMENTS

I^HE favorable opinion tendered toward the
^ suggestion of Oliver Lodge and others who
suggest the utilization of electrons as a basis
for experiment in the development of television
leads one to expect that newer experimenters in
the field will turn their attention to such systems
as that avocated by Campbell Swinton. His
experiments have been carried to a much
farther stage than Mr. Clarkson explained in
RADIO BROADCAST recently.

It was only shortly after Braun, in 180.7, in-
troduced the cathode-ray oscillograph that it oc-
curred to Swinton to work on the cathode-ray
principle in an endeavor to make practical some
system of television. He found in experiments
that the cathode-ray beam could be deflected
both magnetically and electrostatically with re-
markable precision. With two similar cathode-
ray beams simultaneously controlled and de-
flected by electric or magnetic forces due to
identical currents Swinton expected to obtain
absolute synchronism in the motions of the
beams with maximum accuracy, irrespective of
the speed. He planned to use one of Braun's os-
cillographs at the transmitting end and another
one at the receiving end, the beams to be
synchronously and simultaneously deflected by
the varying fields of two electromagnets placed
at right angles to each other, and energized by
the same two a. c. currents of widely different
frequencies. In this manner the moving ex-
tremities of the two beams would sweep over the
surfaces at the transmitting and receiving ends
with remarkable rapidity and synchronism, so
rapidly, in fact, as to take advantage of the well-
known phenomenon of persistence of vision.
• To produce the required picture at the receiving
end it was only necessary that the rapidly scan-
ning extremity of the cathode-ray beam be
impinged on a sensitive fluorescent screen. The
beam, of course, would be caused to vary in in-
tensity by the varying signals from the trans-
mitter, thus producing the necessary graduations
of light and shade to produce a picture. Swinton's
real difficulty lay in devising a system which
would efficiently accomplish the variations in

I — | 1000~ ]|
Per Sec:

"•• Triode
Oscillators

FIG. 3

NOVEMBER, 1928

WHAT PROSPECTS OF TELEVISION ABROAD

BAIRD DEMONSTRATES DAYLIGHT TELEVISION

In this picture Jack Buchanan, the popular musical comedy star, is shown silling in front of the new
Baird daylight television camera Mr. Baird is standing in the center, and his assistant is adjusting

the lense of the television camera

In describing the operation of his apparatus,
Swinton writes as follows:

At both ends the two cathode-ray beams im-
pinge on screens, which they are caused by the
defecting systems to sweep rhythmically and in
complete synchronization in parallel lines back-
wards and forwards from end to end. In the trans-
mitter the screen is composed of a very large
number of minute photo-electric cells each of
which are activated more or less by the amount
of illumination it receives from the image thrown
upon the whole screen by the lens. The end of the
transmitting cathode beam explores each of these
cells in turn, and as to whether it finds it illumi-
nated and thus activated or not, an electrical
impulse of varying intensity, proportional to the
amount of local illumination, is transmitted to
the neighboring gauze grid. The varying electric
current thus originated after amplification and
conversion into wireless waves is transmitted to
the receiver, where, after further amplification
and detection, it varies the strength of the re-
ceiver cathode-ray beam, which, in turn affects
the brightness of that particular portion of the
fluorescent screen on which the end of the cath-
ode beam is at that instant impinging. Thus,
on the receiving fluorescent screen a replica of
the picture thrown by the lens on the transmit-
ting screen is reproduced.

Very probably with modern knowledge and
arrangements the transmitting method in which
selenium is used might be got to work, though
perhaps it would be too sluggish for showing
rapid movements in the picture transmitted.

A CRITICISM OF MODERN METHODS

/COMMENTING in a paper on the advances
^ made in television, whose mechanical
devices formed the nucleus of the apparatus, Mr.
Swinton has said: —

What has been effected mechanically, more
especially in America, shows what can be done
by vast expenditure, labor, and elaboration. As
experimental shows they were no doubt magni-

ficent, but having regard to the apparent im-
possibility of either improving or amplifying to
any sufficient extent, one doubts whether they
can lead to anything really worth having along
their own lines. Surely it would be better policy
if those who can afford the time and money
would abandon mechanical devices and expend
their labors on what appear likely to prove the
ultimately more promising methods in which
ihe only moving parts are imponderable elec-
trons.

In a letter which appeared in the London
Times Mr. Swinton expressed other views on tele-
vision. Excerpts from the letter follow:

To the Editor of the Times: —

The telegram on the progress of television in
your July 14 issue leads me to think some com-
ment should be made on the many very absurd
prognostications that have appeared on this im-
portant subject.

It is well known that all methods of television
are based on the same principle as is the repro-
duction of pictures in the Press, wherein the pic-
ture in each case is composed of a mosaic of
minute dots so small and so closely packed to-
gether that the individual dots are not recogniz-
able as such by the unaided human eye. Let us
take as an illustration a well-reproduced news-
paper half-tone 10 x 16 inches in size. This con-
tains more than 250,000 dots.

Now on the same principle that requires that
cinematograph film pictures have to succeed one
another at the rate of 16 a second, so as to give
the illusion of continuous motion, for the pur-
pose of successful television each of the thousands
of dots has to be registered in its proper place
and with its proper strength no fewer than 16
times a second. Thus to transmit the picture
referred to would require registering the dots at
a rate of 4,000,000 a second.

Such achievements are obviously entirely
beyond the possible capacity of any mechanism
with material moving parts and this view, which
I have personally been inculcating in scientific
circles for many years, has recently been thor-
oughly endorsed by no less an authority than Sir
Oliver Lodge, himself a notable pioneer inventor
in wireless telegraphy, who has recently written

two articles on the subject. I n these he entirely
agrees with my view that nothing of this order
can ever be hoped for from material mechanism,
and that the only way in which it can ever be
accomplished is by doing away entirely with
material moving parts and utilizing the vastly
superior agency of electrons, those infinitesimal
and imponderable unit particles of negative
electricity which are the most mobile things
known to science.

A. A. CAMPBELL SWINTON.

OTHER EUROPEAN INVENTORS

NOT a great deal is heard of the effort of
European inventors on the Continent, al-
though from time to time small items of intel-
ligence do come through. In France, M. Belin,
in conjunction with M. Holweck, has succeeded
in transmitting shadows. The apparatus of M.
Belin is unique in that the transmitter makes use
of two mirrors vibrating at right angles to one
another, the combined action of which enables
the subject transmitted to be explored by a
potassium photo-electric cell. At the receiving end
there is a fluorescent screen traversed by a
cathode ray. Thus we get a combination of the
use of the mechanical and the electron!

Another Frenchman, M. Dauvillier, has also
succeeded in transmitting shadows, but admits
that an irrcrease of one thousandfold in sensi-
tivity will be necessary before his apparatus is
perfect.

Other scientists who are bending their efforts
in an endeavor to be first in the race are Mihaly,
Korn, Nesper, and Muller. The latter has been
experimenting for some time in an endeavor to
produce metal foils of extreme thinness, and has
produced some of gold a hundred times thinner
than heretofore, so thin, in fact, that printed let-
ters can be read through six layers. Varying
intensities of light passing through these layers
of foil have been found to alter a current of
electricity passing through. The use of this dis-
covery may lead to the production of a very
cheap television equipment, it is hoped.

New Trends j

A THE present writing (Sep-
tember) several distinct
trends in the progress of
radio may be observed by any one
who gets about even a little bit.
These trends are often first evi-
denced in home-made receivers and
in the kit sets put out by wide-
awake manufacturers and are later
seen in the sets built by the large
complete-set people. Sometimes,
of course, the set manufacturer
is in advance of the others, but
in general the tendency has been
for this branch of the industry
to follow the leader. Whether
this is due to lack of engineering
initiative, or to hesitancy to adopt
something new, or to patent arrangements, is diffi-
cult to say. It is true, however, that among the li-
censees of the R.C.A. there have been few major
new ideas that have seen the light of the dealers'
shelves. These people are still making t.r.f. sets
or neutrodynes. Small improvements have been
made, but they are in the nature of refinements
of existing circuits and apparatus.

What are these new trends, and what do they
mean to the future of the radio business — say
the 1929-30 season?

THE DYNAMIC SPEAKER

VA/E HAVE already discussed in this maga-
* * zine the moving-coil or dynamic speaker.
After considerable effort to find an unbiased
source of loud speaker information, we discov-
ered Joseph Morgan of the International Resist-
ance Company, who wrote the article "All
About Loud Speakers" in the August RADIO
BROADCAST.

There is no doubt that the present season is
going to be a dynamic speaker season. Nearly
every set manufacturer of note will have a model
or two equipped with this newest milestone on
the way toward more perfect re-
production. There are many such
speakers now on the market; some
of them are the Magnavox, Jensen,
Peerless, Rola, Newcombe-Hawley,
Marco, Radiola 105, Karrand, etc.

Why is the moving coil speaker
superior to our present cone and
horn types' Briefly, because it pre-
sents to the final power tube a nearly
pure resistance rather than a com-
plicated combination of inductance,
capacity, and resistance whose fre-
quency characteristic is anything but
a straight line. The dynamic speaker
has a moving system that can move
through large distances, up to a
quarter inch, which means that There are
plenty of low-frequency power can form \,ere
be put into it with the certainty by another

for 1929-30

that sound energy will come out, and that
there will be no clatter of armature against
pole pieces. No other type of speaker has been
tested in the Laboratory which reproduced fun-
damental tones below 100 cycles. They emit
sound, to be sure, but it is not like the original.

Will 1929 See These New Develop-
ments in Everyday Use?

1. THE DYNAMIC SPEAKER

2. THE BAND SELECTOR

3. THE ONE-STAGE AUDIO

4. THE FIVE-ELEMENT TUBE

The dynamic speaker, then, is here, and in
1929 will take its place in the best commercial
sets and in the homes of the most critical home
set builders and engineers. It entails several
hardships on the constructor. The filter in his
power supply must be better than is necesaary
with other speakers that do not reproduce tones

FIG. I

no tubes in the Jones band selector which is shown in schematic
All the selecting is done by one set of apparatus, the amplifying
, The tuned circuits are coupled together by the small inductance, LI

below 100 cycles. He must put the
speaker in the middle of a rather
large and awkward and solid board
not less than three feet squaie
for best results. His amplifier, to
utilize the advantages of this kind
of speaker, must be very good. Be-
cause the speaker is more efficient
— at least some of them are — he
can get along with less power,
but at present it is not safe to
economize at this point. The con-
structor had better plan to use
I7i'sor better in his final stage.
The day of the 199 tube loud
speaker is not yet here. On the
other hand, we do not believe
it necessary for the home listener
to go to 250-type power output tubes, although
the reserve power possible with such an ampli-
fier as was described in the July RADIO
BROADCAST (page 141), is something to strive
for as the ultimate.

In the laboratory of Dr. John P. Minton, the
well-known acoustical engineer, we heard a
lo-inch Peerless dynamic speaker which was
mounted in a three-foot baffle of not too solid
construction. The speaker was operated from an
amplifier employing a single lyi-type tube in the
output stage, and the results were very satisfac-
tory. The signals were made louder than could
be tolerated comfortably in a small apartment,
and they were considerably "up" from the out-
put of a W.E. 560 AW. Persons who invariably
talk louder than the radio when the latter is
turned on would have considerable trouble in
preventing one enjoying a symphony concert
from a local station with such a speaker con-
nected in the output circuit.

At the present time the Laboratory staff is
busy measuring the characteristics of a number
of the newer speakers of this type and when the
datum is available in its final form it will be pub-
lished in RADIO BROADCAST.

"MIL BAM)-I'\SS R.F.

ANOTHER very distinct trend
is the band selector business.
In December, 1927, Dr. F. K.
Vreeland read a paper before the
I. R. E. on his ideas of what a good
radio-frequency amplifier should be.
He had two suggestions. One was
to stagger the three tuning con-
densers of a t.r.f. set slightly, that
is, tune two of the condensers
slightly above and below the exact
resonance point. This, according
to Dr. Vreeland, would tend to
broaden the top of the response
curve and to steepen its sides. This
would make it possible to receive
the high audio frequencies that are

NEW TRENDS IN RADIO DESIGN FOR 1929-30

so badly clipped at present, and yet to prevent
tro?s-talk from adjacent radio channels.

Dr. Vreeland's other suggestion has occasioned
a lot of discussion. It consists in using in the re-
ceiver a band selector composed of — this is our
interpretation, not Dr. Vreeland's — two tuned
circuits coupled closely enough so that the re-
sonance curve of such a system has a flat top. a
top with a dip in it, or a sharply peaked top de-
pending upon the constants of the circuit.
According to the discussion, this is not a new
idea — we do not know that Dr.i Vreeland said
it was — but the fact is that in the old spark
days, the bane of radio inspectors was the closely
coupled antenna-transmitter system which
emitted a broad wave.

The effects of staggering are two: (i) to
broaden the top of the response curve and there-
by to improve fidelity; and (2) to reduce the r.f.
gain somewhat. With deliberate staggering it
should be possible to use very highly selective
circuits — "low loss," if you will. Then the gain
per stage ought to be somewhat greater. The
gain in fidelity, however, will not be noticeable
on the average amplifier and loud speaker. On
the other hand, with a flat audio amplifier, a
good transmitting station, and a dynamic
speaker, the higher frequencies as heard should
be materially improved.

We must now record a visit to the Technidyne
Laboratory, where we again had the pleasure of
chatting with Mr. Joseph Jones who is business
manager of this organization, of which Mr.
Lester Jones is the engineer. This was not our
first meeting or conversation, and only con-
vinced us the more that this is an organization
which will be heard from more and more in the
next few years.

The Technidyne group has a band selector,
too, but it differs in several respects. The Jones
group has in addition a self-shielded coil, a
loop of similar characteristics, and an untuned
r.f. amplifier of considerable gain. How these
work into a modern receiver will be indicated
below. The coil has two windings, one inside the
other. The high potential ends of the coil are in-
side, so that one can grasp hold of the coil, or
wrap a short-circuited turn of heavy copper
wire about it, without destroying signals or
even detuning the set. In our opinion this is an
extremely useful invention and has resulted in
a very valuable patent. So long as coils are
used in receivers, there must be means of keep-
ing their respective fields from getting out of
bounds. One method is to encase the coil in a
metallic can — shielding it — and another is to
use a self-shielded coil.

The Jones band selector will probably be used
in several receivers, notably the Sparton. It
works out as follows. A band selector is made
up of several coils and condensers and encased
in a container. This has but one wire coming
from it, the connection to the following unit,
the r.f. amplifier, which connects to the detector
and the audio system. This amplifier is untuned,
has five stages in it, with the gain varying from
}ooo to 15,000 from the short to the long waves
to offset the lack of coupling on these waves be-
tween set and antenna. When an antenna is
attached to the amplifier input, the mix-up of
signals is worse than anything the Radio Com-
mission ever imagined.

Phis circuit has several obvious advantages.
In manufacture there are three belts on which
the selector, the r. f. amplifier, and the a.f. am-
plifier are placed for inspection. This is to be
contrasted to factories in which the completed
receiver is placed upon a single belt. If anything
goes wrong with a selector unil. it is removed;
the same thing happens with an r.f. or a.f. am-

F1G. 2

The circuit diagram of the Vreeland band selector; note the inductance, X3, which couples together the
two tuned circuits composed of AI and X-i and their associated tuning, condensers

plifier. When any good selector is attached to
any good r.f. and a.f. amplifier units a good re-
ceiver results.

In service a similar occurrence takes place.
The service man takes with him an extra selector
and an r.f. amplifier. If the customer's selector
is out of order, a new unit is slipped into place —
the entire set is not placed out of commission.

The Hammarlund-Roberts engineers have
incorporated the band selector idea in their
1920 Master receiver already described in this
magazine (October, page 341). In this receiver
both the primaries and the secondaries of the
r.f. transformers are tuned, the two circuits
being coupled together by the mutual inductance
existing between the two coils. This is very loose
mechanical coupling. The Vreeland and the Jones
system use other types of coupling, it being
possible, of course, to couple two tuned circuits
together with mutual or self-inductance, or with

FIG 3

The hand selector used in the Hammarlund-
Roberts " Hi-Q 29" receiver. In this case the tuned
circuits are coupled together by the mutual induc-
tance existing between primary and secondary

capacity or resistance or combinations of all of
these.

Here, then, is the second trend toward band
selectors, electrical contrivances which cut out
of the broadcast spectrum a swath of the desired
width. Such circuits have been suggested as
a relief in the present overcrowded ether
conditions. We believe, however, that another
year will see the band selector the rage
in set construction. The Jones system is a pre-
selector, that is, the signals are first selected and
then amplified. In the Vreeland and the Master
Hi-Q circuits amplification and selection go on
at the same time.

THE ABBREVIATED AUDIO

THE third noticeable trend is toward the
elimination of the first stage of audio, and
working the power amplifier by the detector.
Let us see what this means. First of all it means
that the detector must supply a much higher
output a.f. voltage and therefore must be sup-
plied with a higher input modulated r.f. voltage.
The Jones system does not use the usual first stage
of audio. The detector is a C-bias affair with input
r f. voltages of the order of 10 to 1 5 volts. A new
R. C.A. super has also been put on the market

which uses a.c. tubes and has only one audio
stage.

The elimination of the first stage of a.f. reduces
cost, makes a set simpler to construct and should
reduce not only tube noises and the tendency to-
ward severe microphonics, but eliminate con-
siderable a.c. hum, which is a great advantage
when the dynamic speaker is used with a good
amplifier.

One well-known physicist-radio engineer states
that in his opinion the proper place for the loud
speaker is in the detector circuit. Whether the
set of the future will have no audio amplification
at all cannot be debated at present.

A NEW TUBE IN THE OFFING

IN ENGLAND there is considerable talk of the
' new special-purpose tube with five elements,
the Pentode. This is a power valve built along
lines similar to our present screen-grid tube. The
idea is to get much greater output power with
given input voltages. It is a tube with a large
amplification factor and a high plate impedance;
with our present low-impedance speakers it may
require new coupling devices. A number of arti-
cles have appeared in Wireless World (England)
recently on this tube, which lead us to believe
that the Pentode will do much toward making
unnecessary all the r.f. amplification that elimi-
nating one audio stage demands at present.

WHAT ARE WE COMING TQ?

HERE, then, are the trends in sight. The first,
the dynamic speaker, is here now. It is one
more step toward better reproduction, greater
fidelity of voice and music. The second, the band
selector, is another step toward fidelity, with the
possibility of an increase in selectivity. In the
hands of Lester Jones the problem of selectivity
has been separated' from that of amplification —
a feat we predicted months ago. The super-
heterodyne is such a circuit, although not to
the degree the Jones system is. The third trend
is the elimination of the first stage audio. In the
Jones system this is done by using greater r.f.
amplification and detectors more heavily biased
than those of the present. In the super-hetero-
dyne the amplification is at intermediate fre-
quencies. The Pentode tube may make it possible
to eliminate some of the additional r.f. gain
now necessary, with an obvious advantage from
the standpoint of cost and simplicity.

It looks as though it is never safe to predict
that the time" has come when there is nothing
new under the radio sun. The home constructor,
however, need not feel it unwise to construct a
set at present on the suspicion that next year's
circuits will make obsolete his present gear.
Receivers built to-day according to recognized
engineering principles and equipped with good
amplifiers and speakers will be standards of com-
parison for some time to come.

IMF.W.V

( I1KKHVI RADIO hVFI\ I \

New Allocation Repairs the Broadcast Structure

THE Federal Radio Commission has, at
last, bestirred itself. It has formulated an
orderly plan of allocation, based upon con-
servative engineering estimates of the capacity
of the broadcasting band. When the plan is in
smooth operation, it will increase the program
service obtainable with any radio receiver cap-
able of reaching out beyond neighborhood locals.

We hesitate to praise any constructive step
announced by the Commission because, up to
this time, it has always reversed itself before
promised reforms have been put into operation.
It proposed to eliminate all stations persistently
wandering from their channels, but backwatered
before the echo of its brave statements had died
out. It called a host of stations before it to prove
they were operating in the public interest, neces-
sity and convenience, with a grand fanfare to the
effect that many would thus be weeded out, but
the actual result of the hearings was negligible.
From past experience, we cannot avoid fearing
a complete reversal of form and a repudiation of
the meritorious broadcast allocation plan. We
hope that public approval of the announced plans
will be so forcibly expressed that, this time, there
will be no turning back.

The plan itself is a normal and sensible classi-
fication of channels into local, regional, and na-
tional groupings. Forty channels, eight per zone,
are nationally cleared for exclusive service by
stations of 5000 watts or more. Only one station
is in operation on each channel at a time, al-
though time division may be employed in the
case of stations not prepared to operate on a full-
time basis, or to permit division of channel
facilities among the states according to popula-
tion, as required by the Davis Amendment.
This exclusive service band lies between 1 190
and 640 kilocycles.

Four channels, 1460 to 1490 inclus-
ive, are devoted to stations of a maxi-
mum power of 5000 watts, to be
shared by two zones, presumably on
opposite coasts of the country, thus
providing good regional service.

Thirty-five channels, ranging
throughout the dials from 1430 to 580
kilocycles, are assigned to stations
having a maximum power of 1000
watts. These channels will be used in
not less than two nor more than three
zones simultaneously for night broad-
casting, although in isolated in-
stances, the maximum will, for the
time being, be exceeded when no seri-
ous interference is caused. These are
the so-called "regional channels."

Five additional channels, ranging
throughout the band, for stations up
to 1000 watts, will be duplicated in
every zone, thus providing medium
power, local, and limited regional
service.

I inally, six channels at the lower
end of the dials, will be thickly
populated with loo-watt broad-
casters, accommodating a great
number of stations providing strictly
local service.

This, in brief, is the skeleton of the allocation
plan, as it was first announced by the Commis-
sion. Undoubtedly, it will be modified. Such
dubious features as limiting the maximum power
of permanently licensed stations to 25,000 watts,
on the exclusive channels, is a useless curtail-
ment of the service of these channels. Did it
serve to increase the number of stations which
might be assigned to a channel, it could be justi-
fied on that ground. It amounts only to an order
compelling stations to render less than their best
possible service. Apparently, it is a concession
to Judge Robinson, who wanted to put a 5000-
watt power limit on broadcasting, thereby re-
ducing the total area of the United States served
by high-grade broadcasting to considerably less
than ten per cent.

EFFECT OF THE DAVIS AMENDMENT

HpHE Davis Amendment requires equal dis-
* tribution of channels among the zones, and
their distribution within zones according to
population. As a consequence of the varying
areas of the zones, more than half of the desirable
regional and national channels necessarily are
assigned to the Northeastern quarter of the
country, comprising the first, second and part
of the fourth zones. The Western half of the
country, the fifth zone, has only one-fifth of the
total number of cleared channels. The Southern
zone, comprising an area of about a fourth of the
country, is given only one third the number of
preferred channels assigned the Northeastern
quarter of the country.

The Commission naturally has found these
silly requirements of the pernicious Davis
Amendment a serious handicap in its work and
has not overlooked any opportunity to impress

MARCONI'S NEW 6-KW, 2OOO-.METER TRANSMITTER

This wireltss telegraph station at (irytoiken. S.ndh (ii'urfia. main-
tains a daily telegraph sen-ice with the Falkland Islands which in
turn communicate with Montevideo

the public with the effect of the provisions of the
Amendment in unnecessarily depriving the South
and West of broadcasting stations. However, had
the Commission acted promptly upon the various
allocation plans offered it from the very day of
its organization (of which one, submitted by a ]
member of the staff of RADIO BROADCAST, pro- I
vided almost precisely the structure at last
agreed upon), the Amendment never would have
been passed. The year and a half of dilly dallying
which preceded the adoption of the present plan
has inflicted a more or less permanent handicap
upon the broadcast structure in the form of the
Davis Amendment.

To the broadcast listener, the faithful adoption
of the present plan means clear reception on most
of the positions of the dial. Forty channels are
completely cleared, providing for eight stations
in simultaneous operation per zone. Thirty-four
channels are partly cleared for regional service,
offering a standard of reception about equal to
that heretofore found on the so-called cleared
channels. The number of stations reasonably
clear of heterodynes, is vastly increased and
chain programs will no longer crowd the few
choice dial positions.

By taking into consideration the channels
which will be used simultaneously by more than
one zone, the total number of stations in the
regional class, operating simultaneously at night,
will be 125, or 25 per zone. These regional sta-
tions will give satisfactory service for moderate
distances beyond the so-called "high-grade
service range" and should be entirely free of
heterodynes within the high-grade service area.

The six channels devoted to local service may
be duplicated at fairly short distances, but again
the Davis Amendment acts to curtail the poten-
tial service of these channels. The
number of stations per zone upon
these channels must be equalized al-
though, for example, there could
otherwise be about twenty times as
many local communities using these
channels in the fifth zone as in the
first, without having any greater con-
gestion than is found in the first, so
marked are the differences in the
areas of these zones.

Another curious consequence of the
Davis Amendment is the somewhat
diverse effect it has upon the two
centers of broadcast congestion, New
York and Chicago. Because New York
has such a predominant proportion of
the population of its zone, it suffers a
somewhat smaller curtailment of as-
signments than Chicago. Casual ob-
servation of the Chicago situation
shows a heavy mortality in time on
the air of its all-too-numerous 5000-
watt stations, but the actual effect
will not be nearly as drastic as it
appears. Many of Chicago's 5000
waiters have been operating on ex-
tensive time division in the past and
others are merely call letters rather
than actual stations. Practically
no licenses will be cancelled any-

NOVEMBER, 1928

WHAT IS PUBLIC INTEREST?

where and, by liberal use of time division and
limited application of power cuts, the new struc-
ture will work a minimum hardship upon station
owners.

In spite of the dire predictions of opponents to
a plan based upon engineering considerations,
the plan supports the contention of engineers
that allocation, based upon sound technical eval-
uation of the capacity of the broadcast band,
does not require heavy mortality of stations.

PLAN BETTER THAN HOPED

C"OR two years, we have urged in these columns
* that the number of stations on the air simul-
taneously be reduced to 200 or 225; the plan
exceeds our fondest hopes because it places only
165 stations in simultaneous operation on 74
channels. The balance of the band is reserved
for strictly local services. Receivers in good loca-
tions will bring in a parade of un heterodyned
stations and good programs will again have al-
most nationwide audiences. The wall of local—
stations, in congested centers, will be partly
levelled. Every section will profit by lessened
heterodyning. The South and West are seriously
restricted by enforced disuse of available chan-
nels. In the South, this is no immediate hardship,
as there are fewer stations in operation but, on
the Pacific Coast, a great many needless holes in
the ether can be credited to the Davis Amend-
ment.

We predict one unexpected result of the new
broadcasting structure: the restoration of popu-
larity of long-distance listening. Radio reception
has arrived at a respectable degree of quality of
reproduction. Cleared channels will bring back a
limited amount of dial twisting and, within a
year or two, long distance will again be a desired
quality in a radio receiver. Long distance is the
magic of radio, just as speed is the zest of motor-
ing. Beauty in appearance and tone will always
predominate as a sales appeal, but the flash of
distance will rise again as improved conditions
make its enjoyment possible. This repeats in
radio the cycle of automobile sales appeal: first,
speed, then an era of emphasis on comfort and
beauty and, finally, these characteristics com-
bined, as they are in the products of to-day.

What Is Public Interest?

CJIS G. CALDWELL, as attorney for the
Commission, on August 25, issued a de-
tailed interpretation of the so-called public-
interest, convenience and necessity provisions
of the Radio Act. Inasmuch as the Commission
must, sooner or later, prove in the courts that
the new allocation plan is justified by these con-
siderations, that interpretation is virtually the
Commission's plan of defense. The statement
points out particularly that the Davis Amend-
ment, although calling for equalization of powers
and stations in each district, does not set aside
the public interest, convenience, and necessity
clause and that, therefore, all new stations,
authorized in under-quota districts, must pass
qualification standards imposed by these four,
all-important words. It further points out that
renewal of existing licenses is not incumbent
upon the Federal Radio Commission, unless it
finds that public interest, convenience, and neces-
sity are served; that the issuance of a license is
not to be regarded as a finding beyond the dura-
tion of the period covered by said license; re-
licensing in the past, indicating that the station
has met the test of public convenience, does not,
however, bind the Commission to continue past
mistakes, should any have been made inadver-
tently; that public interest, convenience, and
necessity cannot possibly be defined and must
be judged by individual situations and condi-

THE STAGE OF A TELEVISION DRAMA

This picture shows bow WGY broadcast the first television drama, " The Queen's Messenger." Three tele-
vision cameras were used — one for each of the two players and the third for the "props" — and by a twist
of the knob the director could bring the desired camera into the circuit. Also, a television receiver was in-
stalled in the studio to enable the director to check the image

tions; that there is demand for a variety of ser-
vices, including high-power service, covering
large territories, and low-power service for local
interest; that the broadcasting of phonograph
records as a considerable part of a station's ser-
vice is not a public service unless special records
are developed for broadcasting only; that adver-
tising should be incidental to a real service ren-
dered by a program; that stations of 500 watts
power or more should not be located in thickly
inhabited communities; that very low-power
stations should not be permitted in very large
cities; such channels being more usefully em-
ployed in smaller towns; that the character and
financial responsibility of applicants for licenses
are important considerations; that broadcasting
time should not be used to air discussions of a
private nature; that stations, not operating on"
a regular schedule, do not serve the public; that
a broadcaster, who is not sufficiently concerned
with public interest to equip his transmitter with
adequate frequency control or check thereon, is
not entitled to a broadcasting license. These
considerations were the basis upon which the
practical application of the broadcast allocation
plan were founded when the Commissioners
made their individual station assignments to
national, regional, and local channels.

The Race for Television Publicity

THE race for television publicity continues.
The latest to score is the Westinghouse
Company, which demonstrated a sixty-line
television scanner and reproducer. This is ten
lines better than the television elephant which
the Bell Laboratories built some years ago. In-
stead of scanning a living subject, light was
passed to the photo-electric cell through a motion-
picture film. This was proclaimed in the news-
papers as a radical invention, although the first
broadcasting of radio movies, as we recall it, was
demonstrated to members of Harding's cabinet
seven years ago. by C. Francis Jenkins. With
amazing ingenuousness, the Westinghouse pub-
licity stated that Mr. Conrad began his television
researches only three months ago.

The uselessness of the device from a practical
viewpoint could be gleaned from a single state-
ment in the publicity to the effect that the fre-

quencies used to transmit the sixty-line picture
lay between 500 and 60,000 cycles, so that a total
of 150,000 cycles of ether space would be re-
quired to radiate the signal by the conventional,
double-side-band method. An extra jooo-cycle
wave was used for synchronizing purposes.

The usual statement was made that the device
would be marketed by the Radio Corporation of
America when ready for public consumption.
Mr. H. P. Davis, Vice-President of the Westing-
house Company, also stated to the press that the
device would soon be ready for the home user. -
So are steam yachts!

WRNY Television Transmissions

STATION WRNY began its transmissions on
August 21. Its television signal is now
heard from the fifth to the tenth minute of
each hour that the station is on the air. A 48-line
picture is transmitted on the broadcast band, but
its channel width is restricted by sending only
7.5 pictures a second instead of sixteen. This
does not improve the quality of the transmission.

A Milestone in Television

THE first time that remote-control televi-
sion broadcasting has ever been under-
taken was the occasion of Governor Smith's
acceptance speech on August 27. On this occa-
sion, WGY installed its portable television trans-
mitter, which makes a 24-line picture, at the
State House in Albany. The television signal
was transmitted through eighteen miles of wire
and then radiated by WGY, the General Electric
station at Schenectady.

The television pick-up equipment, erected near
the microphone, consisted of three units, two
tripod-mounted, photo-electric cells in boxes, a
light source, and a scanning device. The cells
were placed at the left of the Governor, within
three feet of his face, and the light source between
the cells. A looo-watt lamp was used to play on
the Governor's face, the intensity of the light
being broken up by a scanning disc.

The General Electric people did not issue any
applesauce publicity about milestones in history
at the time. When they issue a statement that
home television is practical, we will believe it.

RADIO BROADCAST

NOVEMBER, 1928

A MODERN RADIO INSTALLATION ABOARD AN ITALIAN LINER

New Nationwide Picture
Transmission Service

A HEADLINE in the New York Times, an-
nouncing a nationwide television service,
proved upon analysis to be the announce-
ment of a proposed extension of commercial still-
picture transmission by the R. C. A. A jooo-cycle
signal, transmitted over a special carrier, is used
for synchronization. The image is thrown upon
a ground-glass screen and there photographed.
The device was produced in the laboratories of
the Westinghouse Company and may be used
by the R. C. A. in competition with the A. T. &
T. system. This is neither television nor broad-
casting. Otherwise, the headline was correct.

Need for Defining Television
Practices

SINCE many broadcasting stations appear
to be groping toward television, it is desir-
able to reduce to a minimum the number
of scanning disc apertures and their different
combinations, and to agree upon the direction
of disc spiral and disc speeds to be used. These
factors are being determined in a very unscientific
and haphazard fashion. Each individual television
transmitter, being put into operation, seems to
work according to the whim of its builder, with
the result that any television receiver now built
is capable of receiving from only a single trans-
mitter, although it may be within range of two
or three signal sources.

The variables are not confined merely to the
number of holes in the disc. Some transmitters
scan their subjects in such a manner that the
first sweep of the disc makes the top line of the
picture; others start at the bottom. Some require
the receiving disc to run clockwise; others
counter-clockwise. Still others, intending that
universal or direct-current motors be used for
driving receiving discs, are not operating at
closely regulated speeds so that standard syn-
chronous motors may also be used for reception.

It is too early to propose standards for these
factors because standards imply agreement upon
practical constants. Television is altogether too
crude to be standardized. It would have been no
more ridiculous, in 1904, to standardized upon
28 x 3 inch automobile tires, than to decide to-
day upon 24 or 48 line scanning discs for tele-
vision. But it is worth agreeing upon temporary
standards of (i) disc speed, (2) direction of spi-
rals, (3) common multiple for number of holes,
and (4) direction of disc rotation.

Newspaper Has Radio Picture
Transmitter

THE Edinburgh Scotsman, a leading news-
paper, claims the distinction of being the
first newspaper in the world to own and
operate 'its own photo-telegraphic service for the
regular transmission of news pictures. The time
of sending pictures from London to Edinburgh
has been cut from eight hours to eight minutes.
The costliness of the several systems in use for
commercial picture transmission in the United
States have precluded their routine use by news-
papers. The A. T. & T. system is well organized,
but newspapers complain that they have to wait
so long between the filing of pictures and the time
they are actually sent that airplane delivery is
more rapid and desirable. The R. C. A. trans-
oceanic service is useful only in the case of pic-
tures of extraordinary interest because trans-
atlantic transmission of pictures is difficult.

Commercial Broadcasting Increases

A VERY healthy trend in the strengthening
of the economic foundation of broadcast-
ing is indicated by the growing percent-
age of national advertisers using commercial,
goodwill broadcasting, as reported by the Asso-
ciation of National Advertisers. The figures rep-
resent investigation of the activities of 352
leading advertisers. For that number, the per-
centage using radio grew from 1 1.6 per cent, in

1027 to 14.5 per cent, in 1928 and the actual
number from 41 to 51. Broadcasting is the only
one of the eight classifications, into which na-
tional advertising falls, that has shown a mar-
ked increase in the number of users. The greater
the number of national advertisers, seeking to
please the public by goodwill programs, the
greater the competition for public attention,
and consequently the higher the program stand-
ards. Better commercial programs also mean fur-
ther improvement in standards of non-commer-
cial programs and a very happy outlook for the
radio listener. Another field which is developing
is broadcast advertising by local stations. The
peak of direct advertising by the local broad-
caster is passing and more intelligent and ser-
viceable use of the radio announcement is being
made by the smaller stations.

Here and There

A NEW chain is again announced on the
^* Pacific Coast, comprising KJR, Seattle;
HEX, Portland, KGA, Spokane; KYA, San Francis-
co; and KMTR, Los Angeles. Rival chains to the
N. B. C. Pacific Coast network which did not
materialize have been announced before. Per-
hais this new one will.

\AR. OSWALD SCHUETTE, professional
* " * radio agitator, employed by the Radio Pro-
tective Association, offered a petition to the
Commission that the licenses of WEAF, wjz,

WGY, WRC, KOA, KGO, KDKA, KYW, KFKX, WBZ

and WB/.A be revoked on the grounds that the
companies operating these stations constitute a
radio monopoly. Louis G. Caldwell, counsel for
the Commission, pointed out that, until the com-
panies called are found guilty of monopoly in
the courts, licenses cannot be revoked on that
ground; that there being no point-to-point com-
munication, and therefore no competition be-
tween cable, wire, telegraph or telephone sys-
tems involved, the Commission is powerless,
under Sections XI II or XV, to take the silly
action proposed by Mr. Schuette and his excited
associates. This proposal is even more drastic
than Judge Robinson's suggestion that the power
of these stations be cut to 5000 watts. It might
prove a healthy lesson to carry out such a threat,
just to witness the storm of listener protest
against the disruption of broadcasting service
from favorite stations.

AS EVERYONE perforce knows, radio has
**• come to its own as a political medium. The
Democrats are spending about half a million for
radio, using six half-hour periods a week, includ-
ing a liberal appeal to the women's audience.
They are also spending $100,000 for individual
programs over independent stations. Entertain-
ment is being broadcast in connection with politi-
cal features. The Republicans are using the net-
work three times a week, the entire country being
covered once a week by the inclusion of the
Pacific network. Also, 43 half-hour programs are
being sent over the Columbia network, thus in-
suring the listener of no rest from political
blasting.

Aircrajt Radio

CAPTAIN S. C. HOOPER pointed out the
importance of using the 5Oo-kilocycle
distress frequency on aircraft making
over-water (lights. This enables flyers to establish
communication with ships at sea and shore sta-
tions, while utilizing the high frequencies, per-
mitting long-distance transmission, often pre-

NOVEMBER, 1928

RADIO IN FOREIGN COUNTRIES

eludes nearby reception. For example, the
Greater Rockford, which made an amazing land-
ing somewhere between Newfoundland and
Greenland, was heard successfully in Wisconsin.
But none of the shore stations in Canada or
Greenland could hear its signals because of skip-
distance effect. Had it been possible to take com-
pass readings in Greenland or Canada, the
whereabouts of the place would have been much
better known and the rescue would have been
accomplished very much sooner.

NEW YORK STATE plans an aviation
weather service and is establishing twenty
weather bureau stations which will make reports
to Gustav Lundgren, meteorologist, at Albany.
The data will be compiled and telegraphed to
every airport in the state, as well as broadcast
twice daily through N. B. C. stations.

A DIRECT, high-power service between San
Francisco and Tokic, the first Occident to
orient service to be directly connected by mod-
ern, high-speed telegraphy, was announced by
the R. C. A. Messages to Japan heretofore have
been relayed through Hawaii.

THE Federal Radio Commission authorized
the R. C. A. to establish direct communica-
tion with Liberia, although a channel for that
purpose already has been allocated the Firestone
Company which, by the way, did not oppose the
R. C. A.'s application. If the Commission is
holding to its promulgated principle, a single
radio link is not sufficient to handle all the traffic
between Liberia and the United States and the
duplicate services are warranted by the amount
of message traffic to be handled.

A NOTH ER pair of licenses granted for services
** of doubtful value are the permits issued to
the Universal Wireless Communication Com-
pany for two io,ooo-watt stations, one for New
York and one for Chicago, to operate an overland
radio-telegraph service. Filling channels with
unnecessary services means later denials to
essential services.

HpHE International Telephone and Telegraph
* Company has made an agreement with the
Spanish and General Corporation to build a radio-
telegraph station in the Azores for transatlantic
communication. This will distribute North- and

AN ELECTRODYNAMIC SPEAKER OF DE LUXE DESIGN

Three dynamic units are used in the loud speaker shown above. The baffle on which the units are mounted
is of box construction and measures 45 by 5 j feet. The designer of the speaker, Mr. Mampe, of Palisade,

N. f ., is shown in the picture

REAR VIEW OF GIANT SPEAKER

South-American traffic to European countries
where the Mackay Companies have no direct
communication through their agreement with
Eastern Cables, Ltd.

HpHE call letters of the ship and planes of the
*• Byrd Expedition are WFBT, WFA, WFD, WFE,
KI-K and, for the planes, WFC, WFB and WFF.
The wavelengths in meters of the channels to be
used will be 91.3, 68.1, 53.1, 45.6, 34.06, 26.5,
22.8, 17.95, and 13.72. Eavesdropping upon the
affairs of the expedition will be possible all over
the United States. The expedition will carry 20
transmitters and 26 receiving sets and a most
comprehensive line of accessories and parts to
keep the installations in operation for two years.

Radio in Foreign Countries

A' THE Berlin Radio Show, there were 350
exhibitors, including the Army and Navy,
Lufthansa and the German Postal Ad-
ministration. The most interesting television
device demonstrated was the invention of one
Mihaly, which gave the shadowy outline of the
person spoken to on the telephone.

HpHE Department of Commerce report for
* June shows a marked increase in United
States exports of radio equipment. The greatest
growth was in transmitting sets and parts,
which, compared with June, 1927, exports of
$5,806, rose to just short of $50,000. Receiving
sets rose from $174,433 to $228,983. Radio im-
ports were more than $5000.

THE Radio Corporation has made an agree-
ment for interchange of patent rights and for
the sale of radio equipment with the State Elec-
trical Trust of Leningrad.

A PUBLICITY statement from Russia re-
** ports that there are 67 broadcasting stations
now in operation in that country, serving 250,000
listeners. Because of the great area involved,
Russia will naturally require many broadcasting
stations, although the number of receivers in use

is still discouragingly small. Increasing public in-
terest promises considerable growth in the future.

THE British Radio Union, a £30,000,000 con-
cern, will acquire all the ordinary shares of
the Eastern, Eastern Extension, and Western
Telegraph Companies and all the ordinary and
preference shares and debentures, if any, of the
Marconi Wireless Telegraph Company. This
is the practical consummation of the British
cable merger plan.

TH E British Board of Trade has passed a law,
compelling Class 2 ships, freighters with more
than fifty in the crew, to carry automatic dis-
tress-signal alarms. Class 3 ships, with less than
fifty in the crew, are unaffected, while Class I
ships, passenger liners and other vessels with
more than 200 persons on board are permitted
to displace one wireless operator with the auto-
matic alarm.

DR. MclNTYRE of the Ministry of Health
of the British Board of Trade and Drs. H.C.
Case and Philip Morton are experimenting,
aboard the S. S. Mauritania, to develop an inter-
national medical chest, making possible a code,
directing medical treatment at sea, which will be
understandable in any language. This promises
to be an aid to smaller ships which have no
physician aboard.

THE National Electrical Manufacturers' As-
sociation estimates that, by cutting the total
number of its meetings in half and holding the
annual meeting in the fall instead of in the spring,
it will save its members, through reduced time,
traveling and other expenses, approximately
$300,000. Attendance at some trade association
meetings of the radio industry really makes it
doubtful whether such expenses should be
charged to personal amusements or as a legiti-
mate business expense. The radio industry might
employ an accountant to determine the financial
loss it suffers by supporting two trade organiza-
tions and then follow the wise example of NEMA
and reduce the number to one.

— E. H. F.

FIG. 5. FRONT VIEW OF RECEIVER

A Two-Tube T*R«F. Short-wave Receiver

By WILLIAM BOSTWICK and W. T. THOMAS

A RECEIVER to operate satisfactorily on
short waves must fulfill several important
requirements, and in explaining the design
of theshort-wavereceiverdescribed in this article,
it will be well to consider these one by one, so
that the reader may have a clear idea of the va-
rious phases of the problem at hand.

In the first place the receiver must be com-
pletely free from body capacity, and arranged
so that its coils pick up no energy themselves.
This means complete housing in good metallic
shields.

The receiver should be equipped with a re-
generative detector the control of which must be
smooth and easy. Capacity control of regenera-
tion has been selected as best meeting these
conditions.

Any receiver placed before the public for
construction, should, in the opinion of the
authors, be a non-radiating device.

The receiver must be selective yet must not
cut side bands, and it must cover the required
wavelength range.

Lastly mentioned but far from last in order of
importance, the set must have high overall
voltage amplification, yet keep the amplification
of noise at a minimum.

THE DESIGN OF THE UNIT

THE above resume of what a short-wave set
should be, is, of course, general, and outlines
the things to be expected from the correct re-
ceiver for this job. The set that we propose to
develop in this paper is one that fully meets this
rigid set of requirements, and at a figure that
will not strain the average pocketbook.

The set, as seen in the photographs and
diagrams, is composed almost entirely of stand-
ard commercial parts. The tuning range of
the receiver is from 15 to 140 meters, effected by
means of three sels of easily interchangeable
coils. This range covers to-day's domestic and
foreign short-wave broadcasting, and the more
important amateur bands. No audio amplifier is
included in the set, which should preferably be
used in conjunction with two stages of high-

FOR the first time we are glad to be able
to present a short-wave receiver using
a screen-grid amplifier which is tuned. Up
to the present time, designers of such receivers
have been content to use the screen-grid tube
as a sort of blocking tube which prevents
oscillations from the detector getting into the
antenna, and which gives a certain amount of
amplification, it is true, but not an amount
comparable to what is possible when the
amplifier input is tuned.

In the Laboratory the receiver worked
beautifully without any a./, amplifier, the
signals from the detector being as loud as
those from an ordinary short-wave receiver
using one stage of audio. It is true that
such a receiver as this has an additional tun-
ing control, and that for maximum signal
strength and selectivity, both the amplifier
and the detector circuits must be tuned, but it
is believed that it is worth it.

The writer recommends that 180 volts be
used on the screen-grid plate and 60 volts on
the screening grid of this tube. In the Labora-
tory the receiver worked properly with only
i ;$ and 45 volts, respectivdy, on the plate arid
grid. — THE EDITOR.

grade audio, or better still, with a power am-
plifier using a push-pull circuit. The set re-
quires the use of a 6-volt storage battery or an
"A" power unit, and three different B poten-
tials, namely, the plate voltage for the screen-
grid tube, the screen-grid voltage, and the
detector plate voltage. The writers have found
that 180, 60, and 45 volts, respectively, work out
very well.

Impedance coupling is employed in the r.f.
stage, as this is the only practical method of
matching the plate impedance of the r. f. tube
at short wavelengths. A screen-grid tube is
employed on account of its high voltage ampli-

fication, and also because it is the only tube
that can supply stability to a short-wave r. f.
amplifier. A ZOOA detector tube should be used.
Perhaps a word as to results would not be out
of place at this point. During the short time
that this set has been in operation in our labora-
tory it has completely and easily covered the
world. Phone reception has been effected with
points ranging from Alaska to New Zealand and
from England to Russia and Java, not to men-
tion many other foreign stations at lesser
distances, a low gain two-stage audio amplifier
being used on the output of the set. Quite often
signals from jsw, the short-wave broadcasting
station of the British Broadcasting Co., in
Chelmsford, England, are strong enough to give
loud speaker operation directly off the detector
output. For those interested in amateur c.w.
reception it will suffice to say that Australian
amateurs have been heard in Ithaca, N. Y., at
3 P.M. with the phones on the table, and this
also right off the detector, using no audio.

CONSTRUCTION

IT WILL be noticed that in the design of this
' receiver one of the shield cans called for is of
rather large dimensions, necessitated by the
placing of both the r. f. and detector tubes in the
same compartment, the overall width parallel
with the panel being 13". In some localities
difficulty may be experienced in obtaining a
shield of this size ready made, and for this reason
it was thought advisable to include a description
of the shields, for the benefit of those who wish
to make their own. Let it be said, however, that
unless one has had some experience in work of
this nature, he had better take the drawings to
a good tinsmith and have him make them, in
which case it will be well to impress upon him the
importance of having a close fit around the base;
when the two halves are put together, the gap
should not exceed a'z" at anv point if a really
good job is done.

The material may be either half hard brass or
copper and should be slightly less than A>"- Half
hard brass 0.0225" was used by the writers.

NOVEMBER, 1918

A TWO-TUBE T. R. F. SHORT- WAVE RECEIVER

Aluminum may be used equally well but should
be roughly three times the thickness mentioned.

Upon referring to drawings in Fig. 2 it will be
noted that the developments and bending lines
are given, the lettering corresponding with the
assembly sketches, all dimensions being given in
inches and all bends are at right angles and
should be quite sharp. A wooden block roughly
6"x 4"x 75" was carefully squared up and was
found very useful as were also a pair of large
wood clamps and a mallet.

It is advisable to commence with piece B
in Fig. 2, being careful to bend exactly on the
lines. Next the side pieces, GI and G2, are cut
out and bent along the dotted lines, these form-
ing the sides of the cover can, being soldered
along the two diagonal edges and also soldered
inside the top piece, B. The rectangular slot
shown in the cover cans are best cut after the
soldering is completed.

Next piece A is cut out and bent along the
dotted lines, care being taken that the lips on
the portion fitting against the panel are bent so
as to just allow room to slide the top piece in
from above.

The large can is made in precisely the same
manner except that the two rectangular slots
are provided in the cover can to accommodate the
two condensers. After the shields are completed,
three small V-shaped notches are cut in both
upper and lower portions in order to allow cer-
tain wires to go to the terminal strip and the
jack, as can be seen from the pictures.

The next step is the mounting of the shields
and the panel on the baseboard. The lower por-
tions of the shields are screwed to the base using
round-headed screws, being sure to place the
small shield at the extreme left of the board, and
the large one about \" from the right-hand ex-
tremity of the smaller, as shown in the pictures.
The panel mounts directly to the baseboard
with four i" wood screws. The drilling of the
panel is very simple and for that reason no layout
is shown. A |" hole is drilled \" from the right-
hand end of the panel to accommodate the fila-
ment-control jack.

The three variable condensers, Ci C2 C3 in
Fig. i, are mounted symmetrically on the

panel as shown in Fig. 5, holes being drilled
through both the panel and the upright shield
fronts; the templates furnished with the conden-
sers giving the exact hole positions and sizes.
The holes for the shafts of the two end conden-

FIG. 3. DETAIL OF COIL ASSEMBLY

sers are 3!" from the respective panel edges, and
3J" down from the top of the panel. The third
and larger condenser, C3, used for regeneration
control, is mounted exactly between the other
two and at the same distance from the top edge
of the panel.

THE COILS USED

THE next job is the making of the coils for
the various wavelength ranges of the r.f.
tuner, together with their mounts. All coils are
3" in diameter and are of the so-called air
wound type. The size of the wire is No. 16 and the
winding is spaced the diameter of the wire.
\\ indings of this type can be purchased by the
inch from most radio stores. There are three of
these coils to be made to cover the 15-140
meter range, having three, eight, and fifteen
turns respectively. The general form of these
may be seen in the photographs. The first step in
construction is to cut three strips of -fa" bakelite,
23" by J". Drill through all three of these strips

6 '
U>

~~Y

J.
-f.

|-> 7>i >j*-6-->1

: ,_ 13".

4"k-

FIG. 2. DETAIL OF SHIELD COMPARTMENTS

two holes exactly 2" apart and on the center line
of the strips, and of a diameter suitable to take
the male portion of the General Radio coil mount
plugs. The placing of the strips and the method
of holding the coil may be seen from the Fig. 3.
This procedure is followed in making the other
two coils.

The next thing to be done is to remove the
primary coil and its supports from the mounting
base of the Aero coil set. A piece of Bakelite
jj" x ij" has this primary mounted on it at a
point |" from an end in exactly the same man-
ner as it was originally mounted on the Aero
base. Drill two holes in the new base exactly 2"
apart and symmetrically placed with respect
to the base. These are to take the female portion
of the General Radio plugs. Having completed
this piece, mount it in the first shield as shown in
the Fig. 4, taking care to raise it enough so that
the plugs extending through it do not touch the
metal shield. To this base is now attached, as
the constructor sees fit, a rigid vertical rod of
copper or brass extending to a height of 6|"
above-.the shield base. This is the "signal artery"
to the set, and is shown in the photographs and
the diagram in Fig. 3. A connection is made at
the base between this and the nearest female
General Radio mount in any convenient manner.
Care must be taken not to permit this upright
rod to extend through the mounting far enough
to come in contact with the shield base. Now
placing the small ij-volt biasing battery as
shown in Fig. 4, and wiring according to Fig. 2,
we are finished with the first can. It might be
well to mention at this point that both shield
bases are connected together by soldering a
short "jumper" wire to each and that the
ground, A minus, B minus leads are common and
connect directly to base shields.

The base for the Aero coils can now be
mounted in the large shield base by means of
wood screws and the feet supplied for the pur-
pose. Care must again be taken to prevent the
coil plugs from touching the shield base when the
plugs are fully inserted. The location of this
mounting is very important. Its center is placed
6j" from the right-hand edge of the large shield
base and 3" from the back edge of the same.

The set of Aero coils that are to be used in the
detector circuit, with the mount just mentioned,
have to undergo a little treatment before they
can be put into service. Due to the addition of
the r. f. tube to these coils, the wavelength range
is raised and we are forced to remove some wire.
From the smallest coil one half turn is to be
removed from the grid end (end farthest from
the tickler coil). To do this unsolder the wire
from the lug, slip the wire through the guide
bars until you reach a point directly above
the base, then run the wire directly down
diametrically across the coil to the lug where
it was at first, cut off the excess and solder.
In the same manner one and one half turns must
be removed from the medium coil and three turns
from the larger.

The detector socket may now be mounted in
the rear right-hand corner of the large ba'e
shield, so that the tube pin points toward the
panel. Fig. 4 shows how much clearance should
be left for the sides of the shield cover. The r. f.
socket is also mounted in this base and is located
in the rear left-hand corner, placed so that the
pin is paraHel to the panel and pointing toward
the first shield. This socket is raised up on a
small wooden block until the control grid cap of
the tube is exactly 6J" above the shield base.
The mounting positions of the two condensers,
C6 and Ce, and the two r. f. chokes, L3 and L5,
are also shown. The 2o-ohm rheostat, Ri, on the
screen-grid tube should have one lug soldered
directly to the plus filament lug on the r.f. tube

RADIO BROADCAST

NOVEMBER, 1928

socket, and the lo-ohm rheostat, Rj, is likewise
fastened to the lug on the detector tube socket.
The location of the two fixed condensers, €4 .C?,
and Cs, is also shown.

This places all the apparatus except the grid
leak and condenser, whose locations are readily
observable. The only point to be noted is the
raising of the grid-leak mount from the base
shield by the thickness of a $" wooden block;
these parts should be placed as near the detector
tube as possible. The terminal strip at the back
of the set and the filament-control jack are next
mounted as shown.

The rheostats are adjusted once, to give the
correct tube voltages, and thereafter are left
alone. Using a 6-volt storage A battery, the
respective settings were found to be as follows:
The r.f. rheostat, Ri, was turned on one third
from the full off position and the detector rheo-
stat, Rj. was set one half from the full off posi-
tion.

Should the constructor prefer fixed filament
resistors may be used for Ri and Rj, the size
required for the screen-grid tube being the same
as for the ux-i2o power tube: the detector takes
the usual J-ampere (four ohm) type.

In making the wiring to the terminal strip
care should be taken to make sure that the
notches provided to allow these wires to pass
through the shields have their sides well rounded
with a file so that they will not cut through the
insulation on the wires.

Having finished the preceding operations,
there are only two or three things more to be
done before putting the set on the air When the
small shield cover is slipped in place it will be
found that the vertical rod will hit the top of the
cover and accordingly a J" hole must be drilled
in the cover to accommodate this rod. It will be
found best to drill a smaller hole first to serve as
a guide as to the position of the rod. Similarly a
\" hole must be made in the larger shield to
allow the cap of the screen-grid tube to protrude.
Great care must be taken to prevent either of
these coming in contact with the shield covers
and a small wrapping of insulating tape or a
small piece of rubber tubing is necessary to
insulate these points. A removable connection
must now be provided to join the top of the rod
and the cap of the screen-grid tube when the
shields are in place. This can be made out of
bus wire as shown in the photographs, care being
taken that a snug connection is made when it is
sprung into place.

FIG. 4. LAYOUT OF PARTS ON BASEBOARD

OPERATING DATA

TO PLACE the set in operation it is but neces-
sary to connect it to antenna and ground, as
well as the required d.c. potentials, and feed the
output into a two-stage audio amplifier, to give
the necessary volume for consistent loud-speaker
operation. It will be found that in all probability
the dials on the tuned stages will not agree
particularly well, as it is almost impossible to
match coils and condensers at the frequencies
with which we are dealing. The small coils will be
found to cover 14-38.5 meters; the intermediate,
35-81 meters; and the large coils 80-140 meters.
These are of necessity approximate values. It is
understood that corresponding pairs of coils are
to be used simultaneously in the coil mounts;
the home-built coils in the r.f. stage and the
commercial coils in the detector stage In chang-
ing coils care must be taken to handle them only
by their bases as the fragile windings and frames
are easily injured.

After adjusting the filament rheostats and
applying the necessary voltages, we are ready
to commence operations The set is operated
just like any tuned r f set using detector re-
generation, except that the tuning is extremely
sharp.

The right-hand dial is the station selector, and
the left tunes the r.f. stage (which must be kept
in resonance with the detector circuit, for

FIG. I. COMPLETE SCHEMATIC DIAGRAM

maximum energy transfer to the detector), and
the center dial controls regeneration. It will
be well to permit the detector to oscillate until
a station carrier-wave is heard, then bring the
r.f. stage into resonance with the detector. This
may de-tune the detector slightly, but a simple
readjustment will care for this. The regenera-
tion control may now be backed off and the
detector should slide smoothly out of oscillation;
if it does not, a reduction in detector plate
voltage is advisable. The broadcaster's modula-
tion should now come through satisfactorily.

And now a word as to the antennae for use on
short waves. The difference between high or low,
short or long is not very marked. In general a
single wire about 60 feet long placed as high as
possible will answer well.

LIST OF PARTS

THE parts used in the writer's receiver follow.
Parts of equal electrical and mechanical
characteristics may be used in place of those
mentioned in the list, of course.

Cj, Q — 2 Amsco S.L.F. variable condensers,
Type 1213

Cj — I Amsco S.L.F. variable condenser, type
1223

€4, CT — 2 Tobe fixed condensers, o.ooi mfd.

C6, Ct — 2 Tobe by-pass condensers, i.o mfd.

C8 — I Tobe fixed condenser, o.oooi mfd.

Li, La, L< — ' Set Aero short-wave coils and
mount

L», Li — 2 Short-wave r.f. chokes

Ri — i rheostat or fixed resistor, 20 ohms

Rz — I Tobe grid leak, and mount, 10 meg-
ohms

Ra — i rheostat, 10 ohms, or fixed resistor,
ohms

1 ij-volt C battery (flashlight type)

2 Amsco full-floating tube sockets

I Two-circuit filament-control jack

1 7-contact terminal strip
6 General Radio plugs

2 General Radio receptacles
i Bakelite Panel, 7" x 21"

1 Soft wood baseboard, 9" x 20" x J"

2 Shield cans as shown or stock for same

3 Marco vernier dials

Miscellaenous wood and machine screws, wire,
etc., piece of scrap bakelite i}v x 5!" and 9
pieces i" x 2j" x ,',,".

The last two items are for coil mounts and base.
To put the receiver into operation the following

are needed.
i Screen-grid tube
i Detector tube (2OOA type)
6-volt storage battery or A supply
Source of B power (see text)
2-stage audio amplifier

'•Dynamtc rt.

^Magnetic

Speakers

WE ARE being asked
our opinion of the
"dynamic" speaker at
least once a day, and
our present ideas are as follows. In the
first place, all loud speakers are dynamic,
in the sense that some of their parts move.
This includes the old horn type, the newer
cone type, and the newest type in which
the coil moves with respect to the field.
So the reader had best look carefully into
the claims of speaker advertising, or the
claims of over-the-counter salesmen to
make certain that the speaker he is buy-
ing is the type he wants.

Considering the question of fidelity of
response only, not the question of effi-
ciency, the moving-coil speaker compared
to a good cone will "prove in" only on
frequencies below 100 cycles. Compared
to the old stand-by, the W. E. 5<jo-AW, a
good dynamic speaker unit on a baffle
3' x 5' and one inch thick (a baffle }'
square would be equally effective) repro-
duced notes below 100 cycles while the
cone reproduced nothing below that fre-
quency. It is true that tones come out of
the cone when loo-cycle and lower notes
are put on it, but these are not true tones,
but complicated harmonics of the funda-
mental tones generated in the speaker
itself. So when one listens to a good
orchestra with a good amplifier and good
cone from a good station, the viola he
hears is a synthetic viola made up of
harmonics generated partially in the
speaker and partially in the ear.

On frequencies above 100 cycles the cone as
tested in the Laboratory was about as good as
the best of the dynamics. Of course, measure-
ments show that the cone was "down" at fre-
quencies as low as 200 cycles compared to the
moving-coil speaker, but the ear is deceived and
satisfied easily and reception coming from the
54O-AW is, in our opinion, good enough for the
average household. The Balsa-wood speaker,
already described in this magazine, is somewhat
better than the cones, but suffers from mechanical
troubles.

The moving-coil speaker, then, is better than
the good cones only on frequencies below 100
cycles. It must be mounted in a rather large
and solid board, not in a small and beautiful
cabinet. In the latter housing the lower tones will
be lost, and certain frequencies will be badly re-
inforced due to cabinet resonance; these tones
will "boom."

Because frequencies below loo cycles are re-
produced— some moving-coil speakers go down
to 50 cycles easily — the filter of one's power-
supply unit must be such that
no I2o-cycle or 6o-cycle hum
gets through. This means that
a. c. tubes are almost out of the
question — and our experience
has been that when one places
a socket-operated moving-coil
speaker in the middle of a
large baffle board, the hum is
pretty bad. If the speaker is a
d. c. unit, it does not produce
a hum of its own, but if it has
a small built-in rectifier the
user may expect some hum —
if a large baffle is used.

For one who wants the best,
the moving-coil speaker in a
baffle about three feet square —
larger if possible — will give it.
But with this combination one
must have an almost perfect

trays'

amplifier, one which is flat from 50 to 5000
cycles — there are very few of them — and a per-
fectly quiet power-supply unit, and there are
very few of them, especially if a. c. tubes are
used. And there are few stations transmitting
programs that will cause one to note the differ-
ence between an excellent moving-coil speaker
and a good cone. Even the few stations that
transmit down to 100 cycles do not have many
programs which make one wish for a better
amplifier and loud speaker. A recent release
from the Freshman Company states there is
little music below 120 cycles. \Ve venture to say
that the majority of home-made receivers using
good parts is ahead of the majority of broadcast-
ing stations when one considers fidelity.

SEVERAL of the readers
of RADIO BROADCAST have
written for information on the
problem of series-filament
operation. Are there other readers who would
like to have an article on this subject? Some data

Regarding

Series-Filament

Operation

was published in RADIO BROADCAST in the
June, 1927, issue and an article appeared
in Radio Engineering in the June, 1928,
issue.

The advantage is as follows:

It is possible to build a quieter receiver
for a.c. operation than by the use of a.c.
tubes. With excellent amplifiers and dy-
namic speakers on large baffle boards
this is important. With the ordinary
amplifier and loud speaker it is not im-
portant as the average a.c.-operated set
is quiet enough.

The disadvantage of series-filament
operation is that small tubes, such as the
199, must be used. This fact makes it
somewhat more tricky to design the cir-
cuit as when one tube goes bad the others
may follow unless precautions are taken
to prevent such difficulty. Also, under
some conditions there may be a greater
tendency for the receiver to motor-
boat.

Until the UX-2OIB tubes, which re-
quire only one-eighth ampere for the fila-
ment and which are now sold in Canada,
are released, the igg-type tubes must
be employed. These tubes have a
rather low gain, and are quite micro-
phonic. Some other tube manufactur-
ers, Sonatron, for example, make an
eighth-ampere tube but these have not
been submitted to our Laboratory for
test and we can therefore neither
^r recommend nor discourage their use at
the present time.

Who wants technical data on series-
filament operation?

Vresent 'Designs
Do Not Consider

Economy

.TRANSMISSION UNITS
cj ro rb A *-* >
o en o cji o (ji o

s Aw rage moving Co

'/ N/> LtM.11

*^^\

^.-GoodCon
/

e Speaker

\
\

100 1000

FREQUENCY IN CYCLES PER SECOND

AVERAGE CHARACTERISTIC CURVES OF DYNAMIC AND

ELECTROMAGNETIC SPEAKERS

ECONOMY in radio receiver
operation has been out of the
question for several years. In
the early days tubes were ex-
pensive, they required high filament currents,
and anyway who would think of investing a
hundred dollars in a radio and spending twenty
dollars a year on upkeep? Nowadays the criterion
seems to be how expensive a radio can I build or
buy? Multi-tube sets, sets that take power — some
of them take lots of it — from the lamp socket,
sets in which tubes don't seem to last as long as
in the old days are the rule to-day.

There is one notable exception to this seeming
connivance between radio apparatus manufac-
turers and the power companies. This is the
Eveready set which requires a plate current of
only 8 milliamperes. This receiver is very clearly
within the pocketbook requirements of the rural
listener, or the listener who desires not to awe
his neighbors by relating the tales of his loo-watt
leceiver. This Eveready prod-
uct uses high-mu tubes, and,
of course, the output will not
be called a public nuisance. It
is a receiver we are glad to see
designed and sold.

How many readers of RADIO
BROADCAST would like to build
such a set?

And while we are asking for
readers' opinions, how many
would like to see the contents
of the magazine marked with
the Dewey decimal system for
article classification? A reader
brings up the point that it is
difficult to index and to file the
contents of the magazine at
present, and that each month
he must mark the articles he
wants to file according to this

5000 10,000

well-known system. Such a marking might take
place in the editorial office before the magazine
is put on the presses — but it takes work and
time, and it is our present opinion that there are
other things which the readers would sooner see
in these pages than a classification of material

How to
Stabilize Rice
^Amplifiers

FIG. I FIG.

Kinks on stabilising a Rice circuit

THERE are several methods
of keeping a Rice neutralized
amplifier from breaking into
oscillations at a very high
frequency determined by the leakage induc-
tance of the input coil and the tube capacity in
parallel with the neutralizing capacity. One
method is to insert a choke coil in the center-tap
connection of the amplifier input coil as shown
in Fig. i. This method is used in the R. B. Lab.
Circuit. The choke may be replaced by a resis-
tance as Mr. Knowles pointed out in his June,
1928, article. This method is illustrated in
Fig. 2.

Another, and less expensive, method is to wind
a turn or two of wire about the two ends of a
solenoid coil in such a direction that no loss is
introduced at broadcast frequencies but a large
loss at the frequency at which the circuit tends
to oscillate. The direction of winding is opposite
to the direction in which the tuning inductance
is wound.

If a few ohms of resistance is inserted in this
winding either by using a concentrated resistor,
or by using resistance wire — iron picture wire
purchased in hardware stores will do — the
parasitic oscillations will not be produced and
everything will be lovely. This system is shown
in Fig. 3.

RADIO BROADCAST

but a dollar or two and which was worth nothing
at all. Another example of the way some people
acquire experience was brought to our attention
recently.

In a certain store on " Radio Row," New York
City, a transformer was being sold for $1.00 to
which could be added a dry
rectifier unit, like the Elkon,
and a condenser. This equip-
ment, it was said, would pro-
vide the purchaser with an ex-
cellent A-power unit at small
cost. The idea is perfectly good,
the transformer is good, the
Elkon rectifier units and con-
densers were good. Where was
the catch?

The transformer delivered
only 7 volts, and after passing
through the rectifier and filter
2 the potential was reduced to 3.8

volts at the output. The com-
bination was absolutely worth-
less unless one were going to
use it on a dry-battery tube set — but nothing
was said about the voltage obtainable when
the purchaser paid out his money.

NOVEMBER, 1928

be obtained from the Superintendent of Docu-
ments for five cents, contains in few words a
most readable summary of the entire problem of
the transmission and reception of radio broad-
casting.

Variable

1(adio

Sets

^ew Precision
in Quartz
Plates

WE HAVE a letter from
Carl Zeiss, Inc. — whose name
is known to every user of
photographic, microscopic, or
telescopic equipment throughout the world —
stating the firm is now building glass discs for
measuring purposes with an accuracy of 1 0 0'0 „ 0
of an inch and asking if there is
need for quartz plates of this
accuracy. At the present time
quartz plates for frequency mea-
surement and control are sold by
but few companies. We should
say that Carl Zeiss, Inc. might
sell a number of quartz plates
accurately ground.

Seven

Booklets

Tubes
Needed

THERE are three Radio-
irons we should like to see re-
leased in the United States.
One is the much heralded
eighth-ampere general-purpose tube similar in its
characteristics to our 201 A. It is known as the

UX-2OIB.

Another is a power tube, called the UX-I2IB,
which is designed for use in the last stage. It con-
sumes a filament current of one-eighth ampere,
uses a C bias of i<> s volts with a plate potential
of 13; volts and can deliver 12; milliwatts of
undistorted audio-frequency power, an increase
of almost three times the power handling ability
'of the 2oiA-type tube. The third tube is the
wx-25 which is a better J-ampere tube than
the 201 A but not as good as our 1 12-type tube.

The electrical characteristics of
the three tubes described above will
be found in a table on this page.

THE follow-
ing booklets
are more than
worthy of

mention — they should be read
by all readers who like to "keep
up."

Through Electrical Eyes, by
John Mills. A reprint of an ad-
dress given at Atlantic City
Nov. 26, 1927, on the physics and
chemistry involved in television.
Mr. Mills is Director of Publications of the
liell Telephone Laboratories.

The Electrad Truvolt Divider Manual, Electrad,
Inc.

The Amperile Blue Book, Radiall Company.

The Polymet Manual of Engineering Data,
Poly met Mfg. Corp.

The Gateway to Better Radio, Clarostat Mfg.
Co.

The Boston Post Book on Television by Henry
M. Lane.

Radio Facts and Principles. The testimony of
John V. L. Hogan before Federal Radio Com-
mission, July 23, [i)28. This booklet, which may

FIG. 3

Extra coil and resis-
tor improves Rice Set

.\ RECENT copy of the
Wireless Trader (England)
devoted sixteen full pages to
describing 66 models of porta-
ble sets manufactured in that country by no less
than 61 manufacturers. Only the most rudi-
mentary arithmetic is necessary to count the
number of portable sets made in this country;
in spite of the fact that there seems to be a
rather consistent demand for them, to judge by
the letters that come to the Editors.

WE ARE finding it difficult
Underground to know what stand to take

cderials on the underground antenna

business. At present the vote
among readers of these pages seems about half
and half; some do and others don't like them.
A reader in Ontario writes: "have been using
ground antenna for the past five years with ex-
cellent results. It consists of 50 feet of well-
insulated cable attached to an old galvanized
water tank three feet in earth. Am able to get
British Honduras, Jamaica, and others on home-
made three-tube regenerative set."

Another in Texas writes: "My experience with
underground antennas is that they are not worth
planting, and we don't have rocky ground, noth-
ing but plain silt soil."

The following letter is from
a reader in Bangor, Maine:

While I can't give you any
signal-static ratio, measurements,
comparing using an underground
antenna with the usual types, 1
have formed my own opinion
along these lines. 1 installed a
much-advertised device, accord-
ing to directions, and operated it
with a lyrman "70" screen-grid
super-heterodyne, and I want to
say that, as an eliminator of
static and power-line distur-
bances, it proved to be a dismal
failure. As an antenna it was
very efficient, but it didn't even
begin to live up to the claims
made for it.

PEOPLE who do
business with
"gyps" must ex-
pect to be "gyp-
ped." Not long ago we disclosed an
interesting output device which cost

The "rrpt"
are still
at it

Designation

amp.
voltage

bias

V-

UX-201B

121B

wx-25

. 125 amps.

.125 "
.25

90
135
135
90

4.5
9
16.5
4.5

2.5
3.0
6.0
3.5

IHlm
10000
5000
8000

8
8

5
7.9

725

800
1000
1000

15
55
125
130

If — Filament current u. - Amplification factor
Ip = Plate current in mils Gm ~ Mutual conductance
Rp = Internal resistance of tube P — Output power in milliwatts

After reading the letters we
are inclined to believe that the
underground antennas are fine
for those who like them and fierce for those
who don't.

The trouble lies, not with the antennas, but
with the advertising. If such advertising states
that signals will be as loud as signals obtained
with high clear antennas of the conventional
sort but devoid of static, the copy should not be
trusted too implicitly. On the other hand, if the
advertisement states that the static will be re-
duced and that reception in geneal will be clearer
it seems a fair gamble.

The underground antenna is not new. It is
very old. It never gave as strong signals as a
free wire high above earth, but under certain
conditions it may give reception
freer of static than the elevated wire.
We often disconnect the antenna
from our own receiver during bad
weather in order to receive local sta- .
tions with a minimum of static. This
makes it necessary to make the set
somewhat more sensitive, more
sharply tuned, and more critical
of adjustment. — Ktrm HENNEY.

A Service Man's Experience

By B. B. ALCORN

FROM the standpoint of the radio dealer or
service man, the problems arising from the
service and repairing of radio sets are
most complex, as each new advance in radio de-
sign creates new problems in repair, and almost
daily some new radio ailment presents itself for
correction. To cover all the problems of radio
servicing is beyond the scope of this article, but
there is room for considerable
discussion of the ordinary service
methods from the point of view
of the man in the field who is
meeting and solving these prob-
lems daily.

The troubles encountered in
radio servicing may be divided
into about ten major divisions,
as follows:

1. Defective tubes

2. Defective batteries

3. Open circuits

4. Defective parts

5. Defective antennas

6. Defective grounds

7. Use of harmful gadgets

8. Misconnections

9. Short-circuits

10. Defective arresters
Under each of these majordivi-
sions are numerous subdivisions.
It is the purpose of this article
to cover the first three of these divisions, particu-
larly the third, giving actual cases which have
come before the writer. Subsequently articles will
deal with service problems under the remainder
of these divisions. Many of the cases cited are of
an unusual nature, such as are not encountered
in the ordinary day's work, but it is such peculiar
problems which illustrate the repair man's tech-
nique better than the rank and file of radio ail-
ments.

TUBES AND BATTERIES

THE location and treatment of troubles under
major divisions I and 2 require little explana-
tion. In the case of batteries a good voltmeter
is all that is necessary for locating the trouble;
for testing tubes, a tube tester or a set of tubes
known to be good may be used. Defective tubes
or batteries should, of course, be replaced with
good ones. At this point, however, it is suggested
that a very simple and accurate method of deter-
mining which tube or tubes are weak is to insert
a 0-50 milliammeter in series with the minus B
lead. First take the total reading, then remove
the tubes one at a time and note the current drop
at each removal. The tube (not counting the
detector) whose removal gives the smallest drop
In current is the weak tube — assuming, of course,
that all the tubes are supplied with the same B
and C voltages. A recheck on this method can be
made by replacing the tubes one by one and
noting the gain per tube.

OPEN CIRCUITS

UNDER the heading of open circuits the serv-
ice problems are very numerous, and may
apply to any part of the receiver or accessory
apparatus. The methods of locating these

troubles are as numerous as the troubles them-
selves; the best method, perhaps, is the use of
a set checking device, of which a large number
of types are available. While the manufactured
set checkers are all good, their cost in most cases
is prohibitive, especially when a dealer or service
shop has a number of service men to provide
with outfits. The writer has designed a set

A RADIO SERVICE MAN AT WORK

checker which is not too costly and which is
built around the meters which every service man
who takes his job seriously should already pos-
sess. This checker will show short circuits, open
circuits, and the approximate condition of tubes.
Since, however, this article aims only to tell of
some of the cases encountered in radio servicing,
the construction of this tester will be given in a
future issue.

The usual symptom of an open is a loss of
volume or total lack of signal. In most instances
of sets having three dials, the trouble, if it is an
r.f. stage, will isolate itself by the fact that one

RADIO servicing problems, as they
present themselves to the man in the
field, is the subject dealt with in this article.
Mr. Alcorn, who has a thriving service busi-
ness of his own in Long Island, divides these
problems into ten major groups, three of
which he discusses in detail; the remainder
will be treated in subsequent articles. If the
great number of service men and professional
set builders need any one thing, it is to ex-
change information with one another, and
we hope, in these articles and a special page
for service men which we shall soon inaugur-
ate, to help in that direction. We will be de-
lighted to hear from service men about the
subjects they would like to have Mr. Alcorn
discuss. Short illustrated contributions, not
longer than 500 words, dealing with "kinks"
in service work, are invited and if accepted,
will be paid for at our regular rates.

— THE EDITOR.

of the dials will have very little effect on the
tuning. Again it may show itself by a zero read-
ing of the voltmeter connected between the plate
terminal of the tube socket and B minus, which of
course means an open primary in an r.f. coil. A
simple test in such a case is to place the fingers,
or a 2-megohm grid leak, across the primary
terminals of the suspected transformer. If the
set is inoperative because of an
open-circuited primary the fingers
or the grid leak will, 1 have found,
cause the set to function, though
not at its maximum efficiency.
The fingers may also be used to
determine whether or not the
open is in the audio amplifier by
touching the grid lead to the
detector tube. If the audio
system is o.k. there will generally
be produced a low howl from the
loud speaker.

Open circuits in the r.f. ampli-
fier are among the most difficult
of troubles to locate. In the case
of open grid suppressors in mod-
ern a.c. sets, for example, the
writer has found that few if any
of the set checks will show this
trouble, which manifests itself by
lack of volume — a condition
which might be attributed to a
number of other causes. A quick test for faults
in these resistors, which are usually of the wire-
wound type, is to touch the stator plates of the
variable condensers with the point of an ordin-
ary lead pencil. If the suppressor is open there
will then be a noticeable increase in volume; if
not, the volume will decrease.

An unusual open was recently encountered by
the writer in a Thermiodyne T F6 which for a time
was very baffling. The set check showed that all
the coils and connections were perfect, yet a jar
would either cause the signals to disappear or
come in more strongly. It certainly looked like
an open or loose connection, and that was what
it turned out to be, but in a very unusual place—
the gang condenser. The condenser was of stand-
ard design, that is, the stator plates were wedged
into aluminum supports, and it was found that
the plates in the second condenser had become
loose where they fitted in the stator support.
Some ingenuity was necessary in remedying this
trouble, as it was impossible to get a new gang
condenser for some time, and difficulties in sol-
dering aluminum made it impractical to solder
the plates in place. As a solution a magneto file,
such as is used to clean breaker points, was
ground to a chisel edge, and a wedge driven under
the condenser to break the force of pounding
upon it. Then with the improvised chisel, which
fitted easily between the stator plates, a burr was
struck on each side of the loose plates, causing
the metal of the plate supporter to grip them
tightly. The job was reassembled, and the con-
tact formed between the plates and the supporter
was found to be sufficient to make the set operate
without fading regardless of the amount of jog-
ging-

At the shop we recently ran into another very

RADIO BROADCAST

NOVEMBER, 1928

peculiar open in the r.f. amplifier of a Radi-
ola 18. This particular open had all the symp-
toms of a short-circuit. When the call for service
came in over the phone the owner stated that the
set had been performing satisfactorily when
suddenly it had died down, come in strong again,
and then stopped entirely. It looked as though
the trouble came from a defective uv-227, and
the only equipment taken on the call was a good
tube. Upon arrival at the owner's home all the
tubes were found to be lighting properly, but
no signals came through. It was then noted that
the power-pack was beginning to smoke; the set
was therefore brought back to the shop, as it was
thought that the power supply was at fault. Up-
on removal from the cabinet, one section of the
voltage-divider resistance was found to be so
hot that it could not be touched. Nevertheless,
when the power unit was given the manufac-
turer's test it proved to be in good condition,
and when connected to another receiver of the
same type it worked without heating.

Next the receiver was given a continuity test
and strange to say, came through with flying
colors. There seemed nothing to which to attri-
bute the heating of the voltage divider except a
short-circuit. However, as a final test a set
checker was used on the receiver, and when
plugged into the third r.f. socket showed an open
in the plate circuit. This was located in the
primary of the third r.f. coil, and when remedied
the set performed better than ever, according
to its owner. In my opinion the heating of the
voltage divider was caused by the feeding back
to the power unit of the current that was sup-
posed to flow through the defective primary.
The symptoms were certainly those of a short-
circuit and not an open. The reason that the
open did not show up in the continuity test was
that the break was so minute that the high volt-
age used in the test — we used 400 volts d.c. from
a B-power unit that we happened to have in the
shop — jumped the break and thus gave a reading
on the meter. Fig. i shows the Radiola 18 circuit.

An easy method of localizing opens in the r.f.
side of almost any receiver is the well-known one
of disconnecting the antenna and connecting it
successively to the grids of each of the r.f. tubes
up to and including the detector. This method
was tried without success in a recent case of an
Atwater Kent Model 35 which had an open of
such microscopic size that the circuit continuity
seemed to be o.k. when tested. Still only two

near-by and powerful stations would come
through, and their signals came in accompanied
by a hissing, frying sound. It was necesary in
this case to resort to a voltmeter and low-voltage
battery to locate the open in an r.f. coil. The coil
had to be rewound.

Another case occurred recently in which an
open that appeared to be in the set turned out
to be in a vacuum tube. The tube in question was
used in the audio, and the first diagnosis was a
burnt-out audio transformer. However, test of
the transformers showed them to be in good con-
dition. When a new tube was used in place of
one of the audio tubes the receiver functioned
perfectly. The defective tube was tested in the
shop and showed no plate current, although it
appeared to be in good condition. A section of the
base was then cut off and showed an open circuit
in the plate lead. This is the first case of this sort
that we have encountered.

A curious case of coincidence once occurred
when two sets in different parts of the town,
identical in installation, came to us at the same
time with exactly the same defect. The first diag-
nosis was an open circuit, but both sets were
found to be in good condition, as were the an-
tennas. Still neither of the receivers gave any
signals, or at best very weak signals. The owner
of one of the receivers was asked whether he had
noticed anything unusual about his receiver pre-
vious to its breakdown, and recalled that about
a week previous to the breakdown the set had
stopped altogether and had started up again
when he had opened a window. The window was
the one through which the lead-in wire entered
the house. An inspection of the lead-in wire
showed that it was broken inside the insulation
about a foot from the lead-in strip, and that when
the window was raised it released the tension
on the wire and allowed the two broken ends to
meet and form a contact. However, after a week
of constant tension the insulation stretched so
much that it kept the two ends permanently
separated.

A unique case of an open in an accessory oc-
curred in a Zenith six-tube receiver equipped with
a B-power unit, trickle charger, battery and re-
lay. The combination had been giving good
service for a long while when the service man was
called in on the complaint that the set was very
noisy. The set was turned on when the service
man arrived, and a test showed it to be o.k. The
man waited about half an hour, as the owner

stated that the trouble did not always show up
at first, but nothing happened. It seemed to be
a case of local disturbance. The service man
returned. He had hardly reached the shop, how-
ever, when the owner called by telephone to say
that the receiver was "acting up" again. He was
requested to leave the set going until the service
man arrived. Upon arrival the service man found
the loud speaker giving out a continuous buzz
that the removal of the antenna, ground and
three r.f. tubes did not diminish to any apprecia-
ble extent. The trouble turned out to be in the
relay, the spring of which had somehow increased
its tension to such an extent that it caused arcing
at the contacts; this caused a noise that fed
through to the set. It was necessary for the set
to remain in operation from thirty to fifty min-
utes before this happened, the cause being the
heating of the relay.

Before concluding it is well to call the atten-
tion of all service men to the importance of con-
sulting the owner of a defective set as to any
unusual effects he has noticed in its operation —
just as a doctor inquires as to the symptoms of a
patient. Many times the hints dropped by the
set owner are invaluable in localizing the trouble
and making an exhaustive test unnecessary.
Another thing that all service men should ask is
whether any attempt has been made to remedy
the set before he arrived. In many cases an in-
expert attempt to remedy a minor trouble has
resulted in serious breakdown. A recent case
occurred where the only trouble was the removal
of the attachment plug of a light-socket operated
receiver. The owner, who knew nothing of radio,
attempted to remedy the receiver by changing
six connections on the power unit. He said noth-
ing of what he had done, and a service fee of
ten dollars was charged against him for the tests
which were necessary. If he had been asked or
had told of his own attempt, the repair could
have been made in a few minutes.

The foregoing are among the most unusual
cases of open circuits encountered by the writer,
and it is quite certain that equally peculiar cases
will turn up in the future. It is hoped that space
will permit to pass these on to the readers of
RADIO BROADCAST. The next article in this series
will be devoted to troubles arising from defective
parts — a category that includes a wide variety
of radio "griefs." The article will also include a
description of the construction and use of the set
tester referred to in this article.

ux-

171

9 135

THIS SCHEMATIC DIAGRAM SHOWS THE R. F., DETECTOR AND A. F. CIRCUITS OF THE RADIOLA |8.

O
B+

Powe

NOVEMBER, 1928

RADIO BROADCAST

O No. 9

RADIO BROADCAST'S HOME-STUDY SHEETS

The Effects of an Electric Current

November, 1928

CUPPOSE we have a source of electricity, a battery for example.
° How can we be made aware of its presence? To demonstrate the
effects of the electric current, which are (A) chemical, (B) heating, and
(C) magnetic, we shall need the following apparatus:

LIST OF APPARATUS

1. Six-volt storage battery or three dry-cells;

2. Mariner's compass costing about $1.00;

3. One-half pound of bell wire;

4. Cop per- sulphate crystals, about 25 cents worth;

5. Glass tumbler;

6. Rheostat, 30 ohms;

7. Two Morse Eureka spring clips;

8. Two Fahnestock clips;

9. Brass angles, screws, bakelite strip, etc., for mounting compass;
10. Metals, such as brass screws, zinc battery case, iron, etc., for

chemical experiment.

PROCEDURE

A. To demonstrate the magnetic effect of the electric current:

1. Wind about 30 turns of bell wire into a coil aboutjsix inches in
diameter. The exact size of wire, number of turns, and size or form of
coil are not important. At about the 10th and 20th turns make a twist
in the wire and scrape the insulation from it so that the Eureka clip
may be attached to these points.

2. Mount the compass on a supporting stand in the center of the
coil as shown in Fig. 2.

3. Connect the rheostat, coil, battery, and clip as shown in Fig. 1.

4. Place this home-made assembly, known as a galvanometer, with
respect to the earth's North and South poles so that when looking down
on the device the compass needle is parallel with the coil.

5. Note the effect on compass needle when: (a) the clip is placed on
various turns of wire and the circuit is closed by moving rheostat arm,
(b) the battery connections are reversed, and (c) the voltage is varied
by using only one, two or three cells of the

battery with rheostat in same position.

6. Remove compass from its support and
place it over, and then under, one of the wires
connecting the battery and coil. Close circuit
to coil. Note effect on needle when (a) position
of compass is changed, (b) battery connections
are reversed, (c) strength of current through
wire is changed with rheostat arm and (d) dis-
tance of compass from wire is increased.

7. Note what is said about electro-magnetism
on pages 100-105 of the Signal Corps book,
"Principles Underlying Radio Communica-
tion." ($1.00 Government Printing Office).

B. To demonstrate chemical effect oj the electric
current:

1. Place a few crystals of copper sulphate
in a glass container, such as a tumbler, and
cover with water. Attach a large copper lug, a
washer or a penny with a hole in it to a wire

and connect it to one terminal of the battery. PIC I

To the other terminal of the battery attach a
wire to which may be connected, by means of
an Eureka clip, with the various metals such as

FIG. 3. DEMONSTRATING ELECTROLYSIS

it with water and throw it away. If some of the acid gets on the hands or
clothing, cover acid at once with ammonia or borax solution.

C. To demonstrate the heating effect of the electric current:

1. If you have not already accidentally demonstrated the heating
effect of the electric current in experiment A by turning the rheostat too
near the "all out" position, do so now, taking care not to let the current
flow through the last few turns of the rheo-
stat long enough to burn them off.

-s/WVW\AAA/ — I

30 ohms

2-6VoHs

a steel screw driver, iron screws, brass strip, nickel-plated binding posts
or washers, part of the zinc case of a dry cell, etc.

2. Note effect upon these metals when they are submerged in the
copper-sulphate solution and current is passing through the circuit.

3. Reverse battery connections in each case and note result.

4. If a voltmeter, reading not over 5 volts, is handy, connect its term-
inals to the two wires from the copper-sulphate container. Note the
voltage developed by the cell and its polarity when various metals are
used. Use galvanometer already described if voltmeter is not available.

5. Replace copper-sulphate solution with a small amount of the elec-
trolyte from the storage battery. Remove electrolyte from battery with
a hydrometer or medicine dropper, taking care that no acid touches
anything but the glass container. Repeat experiment above. Do not
return electrolyte to the battery, since it is now quite impure. Dilute

FIG. 2. DEMONSTRATING MAGNETIC FORCE

DISCUSSION

A. The fact that the compass needle moves
when it is near a wire, or a coil, carrying an
electric current indicates that a magnetic field
surrounds such conductors. When the current
is turned off this field no longer exists and the
needle is then influenced only by the earth's
magnetism. The stronger the current, or the
greater the number of turns of wire, the
greater is this electric field, and for this reason
it will exert an influence on a compass at a
greater distance, or at a given distance the
needle will be caused to swing a greater num-
ber of degrees. When the direction of the cur-
rent flow through the wire or the coil changes,
the needle changes its direction too. The gal-
vanometer, therefore, is a sensitive detector
not only of the existence of current but of its
strength and direction too. Measuring instru-
ments, such as voltmeters and ammeters, util-
ize the principle demonstrated here although
in a much more exact and precise manner.

B. When two dissimilar metals are placed in a solution — copper and
zinc in a copper-sulphate solution, for example — and a current is passed
through the circuit, various things happen. The solution may change
color, bubbles may be seen coming from one of the metals, or one piece
of metal, known as the electrode, may have a deposit on it. This is the
principle which underlies the electroplating of metals. Storage and dry
cells are commercial applications of the fact demonstrated in paragraph
B4, that two dissimilar metals in a proper solution develop a voltage.

C. The fact that a wire gets hot when sufficient current flows through
it demonstrates another of the effects of the electric current. The amount
of heat generated per unit of time depends upon the strength of the cur-
rent and the resistance of the wire. This effect is made use of in electric
irons, water boilers, toasters, etc. Some wires when heated change in
length appreciably. If such a wire is stretched between two fixed
points, the extent to which it sags may be used to determine the amount
of current flowing. A device using such a principle is known as a " hot-
wire ammeter" and such meters are used to measure antenna current,
or for measuring current in circuits where an electromagnetic type of
meter cannot be used.

QUESTIONS

1. Suppose current flows out of a battery from the positive terminal
through the circuit and into the battery at the negative terminal. Can
you determine a law relating the direction of current flow and the move-
ment of the compass needle?

2. Do you know the "right-hand rule" for determining direction of
current flow and needle-swing?

3. Can you explain what happens in the process of electroplating?

4. Why is a voltage developed in the copper-sulphate experiment?

5. What determines the polarity and magnitude of the voltage?

6. What are the bubbles appearing at one of the metals in the acid
solution?

7. What generates the heat in a wire when current passes through
it?

8. Do you know the law applying to -resistance, heat, and current?

9. Why is manganin or nichrome wire used instead of copper where
considerable heat is to be developed?

NOTE, The answers to these questions will be found in the Signal Corps
book.

RADIO BROADCAST

NOVEMBER, 1928

No. 10

RADIO BROADCAST'S HOME STUDY SHEETS

Alternating Current

November, 1928

Part 111

'90°

1HOA

Time

UOME-STUDY Sheets 7 and 8
* * gave some of the properties of
a.c. circuits, and of the effects of
an inductance upon such circuits.
Since radio circuits are made up
largely of inductances and capaci-
ties, it remains to study the effect
of a condenser upon an a.c. circuii.
In many ways the effect of a coil
and a condenser are opposite, for
example, the larger the inductance
the more it opposes the flow of a.c.
current, but the larger the con-
denser the less it opposes the flow
of a.c. current. The higher the fre-
quency of the a.c. voltage, the less
current will flow through a given
inductance and the more current will
flow through a condenser. When a
combination of coil, condenser, and

frequency is chosen so that a condition called resonance occurs, the
effects of the inductance and condensr are exactly equal and opposite,
so that their opposition to the flow of a.c. currents cancel each other.

CAPACITY REACTANCE

The opposition to the flow of a.c. currents offered by a condenser is
inversely proportional to its capacity, the property of a condenser which
tends to prevent any change in the voltage of a circuit

When current flows into a condenser, from a battery, for example,
a voltage is built up across the plates. When th-s voltage equals the
voltage of the battery no more electricity flows into the condenser, and
we say it is charged. If the battery is removed, and a wire connected
across the plates of a condenser, a spark jumps and the quantity of
electricity stored there rushes through the wire. The condenser is now
said to be discharged. When an a.c voltage is impressed on a condenser,
the quantity or charge flows into it until the condenser is at the same
voltage as the charging voltage. At this point, the voltage of the a.c.
circuit changes polarity; that is, it is now in the opposite direction. The
condenser, however, is still at its original polarity, and tends to dis-
charge into the line and to maintain the voltage of the line. A condenser,
then, helps to maintain the voltage of a circuit constant, and by so doing
it resembles a reservoir which is filled up when the voltage is high, and
is allowed to discharge when the voltage is low.

When the condenser is charged, it is surrounded by an electrostatic
field just as a coil carrying a current is surrounded by an electromag-
netic field.

The opposition which a condenser offers to the flow of current is
known as its Reactance, and is measured in ohms just as resistance
or inductive reactance is. This capacity reactance is inversely propor-
tional to the capacity and the frequency of the circuit. The abbreviation
and formula for capacity reactance are,

270'

FIG.

6.28 X C X I

fc.

FIG. 2

Thus, doubling the capacity or the frequency, halves the reactance.

Since the voltage across a condenser does not rise to its maximum

value as soon as current flows into it, there is a lag between the times

of maximum cur-
rent and maximum
voltage. However,
as we express the
current phase with
respect to the volt-
age phase, we say
the current into a
condenser leads the
voltage across it.
As a matter of fact,
the maximum volt-
age is not reached
until the current
has gone t hrough
90 degrees, or one
fourth of its cycle.
In other words, the
current in a capa-
city circuit is said
to lead the volt-
age by 90 degrees.

This is illustrated in Fig. 1 in which the maximum value of voltane is
reached 90 degrees after the maximum current. It is also illustrated in
Fig. 2 in which are two arms rotating at the same speed but 90 degrees
apart. In the case of an inductive circuit the maximum value of the
voltage is ahead of the maximum value of the current; in the capacity
case the voltage is behind the current.

Since the maximum values of current and voltage are 90 degrees
apart, we must take this fact into account when we desire to know the
instantaneous voltage or value of the current. If the voltage is at the
60-degree phase, the current is at the 150-degree phase. This angle of
90° is called the angle of lead, or the phase angle between the voltage
and current. The instantaneous value of the current is given by

i - I sin ($ + 90°)^

EXAMPLE. What is the instantaneous current in a capacilive circuit
when the voltage is at the 60-degree phase if the maximum current is
10 amperes.-' The answer may be calculated as follows:
i - 10 sin (60 + 90°)

- 10 sin 150

- 10 x .5

- 5 amperes

Note: The sin of angles greater
than 90° may be found from the
expression, sin (90° + A°) = sin
(180° — A°)

Fig. 3 is a vector diagram of the
above example. It is drawn to scale
so that the various lengths of lines
represent the various values of cur-
rent and voltage.

CURRENT IN CAPACITIVE CIRCUIT

The current in a resistance circuit
is given by Ohm's Law; the current
is equal to the voltage divided by
£

the resistance, i.e., I = ~
K

The current in an inductive circuit
is equal to the voltage divided by

the inductive reactance, i.e. I = y-

Similarly, the current in a capacitive circuit is equal to the voltage
divided by the capacitive reactance, i.e., I = ^-

I = E 6.28 \ f X C

and if the voltage is effective, maximum or instantaneous, the current
has corresponding values.

IMPEDANCE

Suppose a circuit has resistance and capacity, resistance and induc-
tance, or a combina-
tion of all three
factors, each of
which is tending to
oppose the flow of
a.c. current. What
is the resultant op-
position or imped-
ance? In a resistance
or reactive circuit
we can add the
several values to get
the final resultant
-remembering that
the effect of a capa-
city is opposite to
that of an induct-
ance so their react-
ances must be sub-
tracted— but when
resistance and react-
ance are combined
we cannot add them
algebraically. They
must be added vectorially, that is according to the formula

Z"- = R* + X=

X=4
FIG.

Z = i/R' + X'

= i/R2 + (XL — Xc)2

This can be done by arithmetic, or by graphical means.
Example. What is the impedance when R - 3 ohms, X = 4 ohms?

Z! = 3' + V

2. = 1/9 + 16

This expression may remind the experimenter of one of the first
laws in geometry he learned, namely, the "square on the hypotenuse of
a right-angled triangle is equal to the sum of the squares on the two
sides." If, therefore, in Fig. 3, we lay off a line equal in length to three
units and label it R, and make another line perpendicular to it equal to
4 and call it X, the length of the line that closes the triangle will be
equal to Z.

The reactance, X, in this problem can be pure a inductive reactance
of 4 ohms or a capacity reactance of 4 ohms, or a combination of in-
ductive and capacitive reactance such that the resujtant obtained by
subtracting them equals 4 ohms. For example, if XL = 8 ohms,
Xc — 4 ohms then

X = (XL — Xc) - (8 — 4) = 4

or if XL = 4 ohms, and Xr = 8 ohms, then

X- Oil \ ' - (4 — 8) = — 4

When, however, this value of minus 4 ohms is squared it becomes a
positive quantity equal to 16 and may be fitted directly into the equa-
tion to determine the impedance.

PROBLEMS

1 . Plot the reactance of a 1 -mfd. condenser as the frequency is varied
from 100 cycles to KI.(KK) cycles. What is the reactance of a 0.001 -mfd.
condenser at 10 kc.? At KKX) kc.?

2. Make a vector diagram for the following condition and solve by
means of the formulas above. In a capacitive circuit the instantaneous
voltage at the 30-degree phase is 5 volts; what is the instantaneous
current if the effective current is 10 amixTes?

3. What is the reactance at 1000 cycles in a circuit which has a
0.25-henry inductance and a 0.01 -mfd. condenser? If the condenser
has a capacity of 0.001 mfd.?

PLOTTING TRANSFORMER CURVES IN A RADIO LABORATORY

A New Audio System for
Speaker

namic

IN ORDER to reproduce radio programs in
sufficient volume to fill a room from a loud-
speaker, some type of audio amplifying sys-
tem following the detector tube is necessary.
Immediately difficulties are inevitable from
this necessity, since to obtain good reproduction
it is necessary to amplify equally all tones or
frequencies up to at least 5000 cycles and down
to less than 100 cycles per second. There
are several types of audio amplifiers in ^^^
widespread use which must first be con-
sidered briefly before going on to a de-
scription of a new audio system designed
to overcome many of the defects of
present systems.

By FRANK C. JONES

Consulting Engineer, Cray & Danielson Mfg. Co.

value of effective tube input capacity. This effect
is generally overlooked, although it is actually
very important, as can be seen from the follow-
ing. The input capacity of a tube is obtained
from the formula,

RESISTANCE AND TRANSFORMER COUPLING

UIRST let us consider the usual re-
sistance-coupled amplifier where the
plate is connected to the next tube
through a coupling condenser and the
plate and grid potentials are fed in
through resistances. If the plate re- «»».
sistor, RI, in Fig. i, is equal to or
greater than the plate impedance of
the tube, the tube will operate over a straight
part of its characteristic curve, so that
the voltage impressed on the first grid circuit may
be amplified without distortion. Now if the coup-
ling condenser, C, is made so large that its re-
actance is small in comparison to the grid re-
sistor value, the loss in amplification will not be
great. However, a large condenser takes a longer
period to discharge, so that if a large a.c. voltage
should be impressed across it — enough to swing
the grid of the next tube positive — the tube will
block and become inoperative. Using a small
value of grid resistor, R2, or a small coupling
condenser, the low frequencies will be discrimin-
ated against enough to cause the response curve
to fall off rapidly. Another drawback to resist-
ance-coupled amplifiers is the low gain obtained
when using ordinary 201 A or 226 tubes. The use of
high-mu tubes will overcome this, but then the
high frequencies may be lost due to the larger

CJ"HE question as to whether each piece oj apparatus
•*• in a radio receiver should be perfected, or whether
better results would be obtained if the entire system were
designed to have a "flat characteristic," has been the cause
of considerable discussion.. The advantage of the latter
method is that a higher over-all amplification may be
secured. The Remler amplifying system employs two
audio-frequency stages, each of which produces some
distortion, but the defects of one compensate those of the
other. — THE EDITOR

Where Cgf = grid-filament capacity of tube.

Cgp = grid-plate capacity of tube.

rp = plate resistance of tube.

T-, = coupling resistance.
A typical example is

6+ 12

30x250,000 1

2 50,000+ 1 00,000 J

I = 270 mmfd.

This value is enough to practically short-circuit
the higher audio frequencies. The higher the
amplification constant of the tube, the greater
this capacity will be.

The latter effect is present in impedance cou-
pled amplifiers to even a greater extent than in
other systems because the distributed capacity
of the coupling choke coils adds to this capacity.
The actual tube input capacity may be less
but the other capacities generally more than
make up for it. The low frequencies may be lost

since the impedance at the lowest frequency
desired must be several times greater than the
tube impedance. The principle of resonance may
be used to overcome this loss at low frequencies
but not the loss at the upper audio frequencies.
Another factor for either of the above systems
is the relatively low gain, so that with 2OIA or
226 tubes about three stages would have to pre-
cede a type 250 power tube. This
.QQ^ means a four-stage amplifier, which is
uneconomical besides having wonderful
possibilities of singing or motor-boating.
The next amplifier to be considered
is the transformer-coupled type. This type
of amplifier has been developed about
as far as it can within practical limits
using special alloy cores and special
windings. By keeping the turn ratio
fairly low, 2 or 3 to i, the response curve
can be made to cover a frequency range of
from 70 or 80 cycles up to 5000 or 6000
cycles without much droop at either end.
mta^ The advantage of transformer coupling
lies in the fact that low-mu tubes may
be used and good gain per stage may
still be obtained. Herein also lies a disadvantage,
since with the new dynamic loud speakers and
type 2 50. power tubes, a greater amount of gain is
required. Let us consider the requirements set
up by the use of a good dynamic loud speaker.

LOUD SPEAKERS AND POWER TUBES

A NYONE who has listened to a good dynamic
loud speaker will admit that this type is
very much better than the magnetic drive types.
The improvement is so great that this coming
year will see a rise in popularity probably as great
as the magnetic-cone speakers had over the old
horn-type speakers of a few years ago. The older
type of cone speakers using a magnetic drive
unit, are incapable of reproducting low notes or
very high notes either. The magnetic type of
speaker that will actually reproduce tones below
200 or 300 cycles per second is very rare and none
will give a true response below 100 cycles. The

RADIO BROADCAST

NOVEMBER, 1928

author has had the opportunity of running re-
sponse curves on about three dozen different
types of loud speakers and it was found that the
response below the limits mentioned is only ap-
parent; that is, the response is not actually the
real low tones but consists of higher harmonics.
For example, one very excellent magnetic-type
speaker at 60 cycles gave a good apparent re-
sponse but when the wave was
analyzed it was found to con-
sist of less than 5 per cent
fundamental tones of 60 cycles.
All of the "noise" consisted of
higher harmonics which would
mean very serious distortion of
the low frequencies in music
reproduction. On the other
hand, a good dynamic loud
speaker in a large baffle-board
or large cabinet is capable of
true response down to 30 or
40 cycles and up to between 5000 and 7500
cycles per second. Below the cut-off of the
baffleboard or cabinet the response is similar to
a magnetic-type speaker in that it consists nearly
entirely of harmonics. A dynamic speaker re-
quires an amplifier of large power rating if low
frequencies are to be reproduced without over-
loading. A good loud speaker mounted in a large
cabinet, or in a large baffleboard; or better yet,
in a wall, will take nearly the full output of a
type 250 power tube for even good room volume.
A good deal of power is necessary for the low
notes, and the better the frequency response
range of a loud speaker, the greater must be the
power output of the audio amplifier for the
same apparent volume. A type 250 power tube
or its equivalent must be used to prevent over-
loading on the very low notes such as may be
transmitted from an organ, a bass viol or tuba,
or even a bass voice. To hear the bass notes in
their proper relation to the higher notes from a
radio receiver is to have real enjoyment from
good radio programs.

To project equal volumes of sound into the
air at 1000 and 100 cycles it requires a much
greater movement of the sound-producing dia-
phragm at the lower frequency. The dynamic
speaker has the ability to reproduce these low
notes -partly because the movement of its dia-
phragm may be much greater without mechan-
ical rattling than is possible in the magnetic
type. These large movements, in turn, imply
that considerable electrical power is available to
drive the moving parts of the speaker, and
this in turn means that a power
tube with considerable output
must be used. The 2$o-type tube
has sufficient output for this
purpose.

nearly negligible. Now consider the case where a
pair of transformers having an effective turn ratio
of 3 to i are used. The voltage available to swing
the grid of the power tube can be figured from
£ = 1x3x8x3 = 24 volts, which is not suffi-
cient. This figure of 24 volts shows that the detec-
tor would have to put out nearly I volt across the
primary of the first audio transformer to obtain
a 70-volt power tube grid swing,
and this would be an enormous
overload on the detector. The
above values of voltage are all
in terms of maximum a.c. volt-
age, not average values, and
the first amplifier tube was
assumed to have an effective
gain of 8, since the mu of a
201 A tube is about 8.5 or more.

FIG. I This means that the trans-

formers will have to have a
much larger turn ratio than 3
to i unless more than two stages are to be used.
More than two stages are rather undesirable from
a cost and space required basis as well as because
of more tube noise and motor-boating possibilities.
Increasing the turn ratio means that the prim-
ary windings will have to be diminished in size
or the secondaries increased. Decreasing the
primary turn lowers the primary impedance to
low notes so that the response to these notes will
be poor. Increasing the secondary turn increases
the secondary capacity to ground or distributed
capacity to such an extent that the higher fre-
quencies are lost. This is very important because
brilliant reproduction depends on the presence
of frequencies up to 5000 or 6000 cycles per
second.

The only way seems to be to cut down on the
primary number of turns, since even resonating
the distributed and shunt capacities with the
transformer leakage reactance does not make a
good high-frequency response when a large
secondary winding is used. By using a large
core of very high-grade material, a nickel-steel
alloy, the impedance presented to the tube can
be increased so that it is still several times that
of the tube even at the low frequencies. The im-
pedance increases with frequency, so that most
of the voltage generated by the tube appears
across the transformer primary for higher notes.
Since the primary impedance is less for lower
frequencies, the proportion of voltage drop across
the tube impedance is greater, causing a droop
in the response curve. It was found by laboratory
tests that a very good transformer of as high as

THE IMPORTANCE OF THE TURN
RATIO

TO WORK a type 250 power
tube to its full capacity the
audio transformers preceding it
should have a rather large turn
ratio in order to prevent overload
of the detector tube. A detector
tube produces higher harmonics
of any audio tone, and these are a
source of distortion, since they
change the character of the re-
produced tone. The greater the
output required of the detector
tube, the more apparent this effect
becomes, so it is desirable to limit
the detector tube output to less
than } of a volt. With outputs of
less than J of a volt the ordinary
2OIA detector tube distortion is

1000
300
100

/>
4

60
50

Over

all eka

r-rirCt-

ISfl

C./M

-o

ll/

«r

185
lfiz

•stage I

-an*/

-rm

•-

-"'

«•"

143
12
10
8
6
4
2

-*
' -

— -

Fint-staoe It

anx/

«rm

'.-

"^.

38 8 8 8

FREQUENCY.CYCLES PER SECOND

FIG. 2. RESPONSE CURVES OBTAINED
WITH HIGH-RATIO TRANSFORMERS

6'. to i ratio could be designed with a frequency
response as shown by the curve of Fig. 2.
There is a drop on the low frequencies even when
using a H2.\ tube having a plate impedance of
about 5000 ohms. Using a 201 A tube having a
plate impedance of about 10,000 ohms, there is a
very bad drop at both the high and low frequen-
cies, giving the effect of a peaked amplifier. Using
a i I2A tube, such a transformer would be fine for
the second stage but would be hopeless for con-
necting to a detector having a high plate imped-
ance. That problem and also the one of compen-
sating for the low frequency droop of the second
stage were solved as in the manner described be-
low.

A RESONATED PRIMARY

OOR several years the different telephone re-
peaters and many broadcast station ampli-
fiers have used the principle of resonance in the
transformer primaries to bring up the low-fre-
quency response. There is no reason why this
arrangement should not be used in radio receiving
set amplifiers, so a first-stage transformer was
designed with primary resonated at about 30
cycles per second. Fig. 3 shows a complete audio
amplifier with the first transformer primary wind-
ing resonated by means of condenser Ci. Re-
sonance has the effect of lowering the total plate
circuit impedance to that of the a.c. resistance
of the circuit only. The action is as follows: At
resonance the current is increased through the
detector plate circuit load, since the impedance
at resonance is equal to the resistance component
only. This means that the voltage drop across the
condenser and across the inductance of the
primary winding will be increased due to the
increased current flow. The increased voltage
drop across the primary, causes a larger voltage
to be generated across the secondary, with
a rise in the response curve. The amount of
riseat resonancedependson the circuit constants,
that is, the plate impedance of the detector tube,
the value of the resistance, RI, and the relative
sizes of the condenser, Ci, and the primary in-
ductance of the transformer. The resistance, RI,
may be made part of the resonant circuit by
making it relatively small, or it may be made
high, on the order of 100,000 ohms, so that it
tends to isolate the resonant part of the audio
circuit from the plate supply unit. In this case it
confines the audio frequencies to the path
through condenser Ci and the primary winding
to the filament. By keeping RI high in value,
the loss due to it at higher frequencies is re-
duced. Since the audio-frequency
path in this circuit is isolated from
the plate-supply unit, the chances
of audio regeneration or motor-
boating are practically eliminated.
This removes a bugbear which is
generally very troublesome in high-
gain amplifier systems.

An auto-transformer connection
is used in the first stage, since the
effective turn ratio is much greater
for the same total number of turns.
The response characteristic of such
a resonated transformer is shown
by the lower curve of Fig. 2.
The resultant characteristic for
the two transformers is shown on
the same curve sheet and it is flat
down to 50 cycles per second. Cer-
tainly no ordinary transformer-
coupled amplifier could be de-
signed which would amplify so
evenly all frequencies as will this
special combination. The total
gain using a M2A tube and a
type 250 power tube is about

A NEW AUDIO SYSTEM FOR DYNAMIC SPEAKER REPRODUCTION

57 TU. This does not include the detector audio
gain.

The use of the same resonating idea in the
second transformer is unnecessary, since we are
interested only in obtaining a perfectly flat
characteristic with a very large voltage gain.
One difficulty is that the d.c. voltage drop
through a shunt resistance would be enormous
unless small values were used. For example, 3
milliamperes through 100,000 ohms would cause
a 300 volt drop. The heating effect would be
troublesome also since nearly one watt would
have to be dissipated in the form of heat. If
low values — less than 30,000 ohms — were used,
the audio-frequency loss through the shunt
resistance would be a large percentage of the
gain of the transformer. In that case an ordinary
low-ratio, high-quality transformer might as
well have been used.

THE OUTPUT TRANSFORMER

IN CONNECTION with the complete amplifier
* system, the last, and perhaps the most im-
portant link in the chain, is the output device.
It was found that the best output transformers
on the market were quite unsatisfactory below
200 cycles per second when the full plate current
of a type 250 power tube was flowing through
the primary. Core saturation takes place and
even with a good-sized air-gap in the core few
notes, below 200 cycles reach the loud speaker.
However, an output transformer is quite neces-
sary with a dynamic loud speaker in order to
match properly the load to the power tube. By
keeping the d.c. current out of the output trans-
former by means of a choke coil, Li, and conden-
ser, Q, as shown in Fig. 3, the transformer may
be made very excellent for even the very low
frequencies. The condenser, Q, may be made to
resonate with the transformer primary to com-
pensate for the loss due to the shunt choke coil,
1-2, and series condenser reactance on the low
frequencies. Another advantage is that the audio-
frequency path is through the condenser and
transformer primary back to filament instead of
through the power-supply unit. This prevents
audio feedback to preceding stages.

1 he result is a very stable high-gain amplifying
system which has more gain than even an audio
amplifier using a screen-grid tube in the first
stage. The use of a screen-grid tube, even as a
space-charge amplifier, means that impedance
coupling should be used between that tube and
the power tube, which gives, roughly figuring,
J X 3 X 5»X 1X4X3=100 volts
across the output device. This as-
sumes a {-volt detector output, a 3
to I transformer, a gain of 50 in
the screen-grid tube, a gain of
4 in the power tube and that f of
this voltage appears across theout-
put device and -J across the power
tube plate resistance. For the
amplifier described, using 45 and
65 to i ratio transformers and
tubes giving a gain of 8 and 4,
the total voltage appearing across
the output would be 1X42 X8X
6J X 4 X" 1 = 1 56. The ordinary
amplifier using 3 to i transformers
and tubes giving the same gains
of 8 and 4 would give an output
of {X3X8X3X4Xf = 48.

Ratio V,:\

A MODIFICATION FOR A.C. OPERA-
TION

TTHIS amplifier having such an
' excellent frequency character-
istic should be used preferably
with d.c. filament tubes in the
r.f. stages, detector and first

.135 , I *«0

FIG. 3. SCHEMATIC DIAGRAM OF COMPLETE REMLER AMPLIFIER

audio stage, since the amplification is so
high at 60 cycles. However, experiments with
special amplifiers cutting off very sharply
just above 60 cycles per second, have shown
that the hum when using a.c. tubes may
be made nearly negligible. The reason for cutting
off above 60 cycles is apparent when it is re-
membered that ordinary loud speakers are excel-
lent harmonic producers on low frequencies.
The use of raw a.c. on the filaments of the tubes
causes a oo-cycle component to be impressed
in the grid circuits with the result that if the am-
plifier system is good as low as 60 cycles, this
frequency will be amplified and reproduced by
the speaker. The speaker, in case it is a cone or
small horn will reproduce it as 120 cycle and
higher tones. The point is that by not amplifying
the 60 cycle component, practically all of the
composite a.c. hum in the loud speaker is
eliminated.

Immediately the idea of resonating the prim-
ary of one of the transformers at about 80 cycles,
was used to make the complete amplifier cut off
sharply just above 60 cycles per second. The cut-
off is a great many times sharper below resonance
than when the transformer itself is made to have
a fairly good audio response. The audio voltage
drop across the resonating condenser increases
as the frequency becomes less, giving a very
sharp cut-off below resonance. By designing the
circuit constants properly, one transformer may
be made to compensate for the other as shown in
Fig. 4 from about 5000 or 6000 cycles down
to at least 80 cycles per second. This makes an
ideal arrangement for receiving sets using a.c.

300
100

•3
r>

bO
40
30

14
12
10
8
6
4
2
n

Over-all ckar

icterit,

ticfi

rr amplifier

FRANSFORMERS WITH SHARP CUT-OFF ABOVE 60 CYC

Detector: Rp 2000 ohms
First audio tube RP- 5000 ohmsjp 6 ma
Power tube CX-371 A

,ES"

-^_ First-stage tra

ns/or

>irr

— *i

---'

^-Second'^0

je tran

form

frh'

ltl<

'"-*^J

•

S S 8

ooo
ooo

o
3

FREOUENCY.CYCLES PER SECOND

FIG. 4. RESPONSE CURVES OBTAINED
WITH 35 TO I RATIO TRANSFORMERS

tubes, since the low notes are well reproduced
when a good dynamic speaker is used. Nearly
05 per cent, of the very low notes such as those
from an organ, as played over the radio, are
between 80 and 200 cycles per second. Very few
people realize this and most people will quite
willingly swear that they hear 30- to jo-cycle
notes in their radio sets when in nearly all
cases their loud speakers and amplifiers will
not reproduce anything below 100 cycles per
second.

Another advantage of cutting off sharply a
little below 100 cycles is that a smaller power
tube may be used, such as a 171 tube. The ap-
parent room volume of sound with the a.c.
system will be about the same as with the first
d.c. amplifier system developed, since in this
case the lowest note reproduced will be about 70
cycles as against 25 or 30 cycles in the d.c. sys-
tem. This makes more power available for the
tones which are reproduced. Incidentally, the
cost of manufacturing such transformers is less.
Most a.c. set manufacturers use audio trans-
formers which will not pass 6o-cycle signals, in
order to minimize a.c. hum and in so doing gen-
erally lose in efficiency up to 300 or 400 cycles.
I his is bound to happen because it is not possible
to obtain a really sharp low-frequency cut-off
using ordinary transformers. The resonant prim-
ary principle should go far towards solving the
a.c. hum problem in a.c. receivers.

It will be noticed that only the transformers
working out of the detector tube in both the d.c.
and a.c. systems have resonated primaries. This
arrangement keeps the d.c. plate current out of
the primaries and so prevents
core saturation and generation
of harmonics due to this effect.
By the use of very large cores
of nickel-steel alloy, the chances
of core saturation from d.c. are
practically eliminated. If small
cores are used and large primaries,
that is, a large number of turns,
core saturation may take place
with bad distortion effects. The
transformers described are built
along ample lines to overcome
the possibility of trouble from
core satuation.

The possibilities of audio ampli-
fier systems using the low-fre-
quency resonated primary prin-
ciple are many and it is quite
likely that this scheme will be
widely used to make better audio
amplifier systems. It is the en-
gineer's problem to forever strive
towards perfection, never reaching
it but always advancing; and this
design, it is hoped, is a step in
that direction.

AS I HI

AS I hk WkS IT

Sound Motion Pictures — Part II

IN OUR October issue the processes of syn-
chronous sound and movie-picture record-
ing and reproduction were described in a
general way. In successive articles some of the
points which were only briefly touched on in
the first paper will receive further discussion.
I do not expect to present any original material
in this series nor to assume a professional tone.
The object is merely to bring together data
from widely scattered sources which are not
readily accessible to the many radio broadcast
operators, motion-picture projectionists, and
others who are interested in sound-movie tech-
nique.

One portion of the reproducing equipment
which was merely mentioned in the October
article is the photo-electric cell used in film
systems. This variety of electron tube has been
likened to an electric eye; its function is to trans-
form optical variations into proportionate elec-
tric currents. In the art of sound reproduction
from films, as well as in television, the photo-
electric cell plays a part analogous to that of
the microphone in telephony. The microphone,
however, takes speech or music directly from
the air, whereas in motion picture work the
photocell receives light which has been modu-
lated by a film record. In this application, there-
fore, the r61e of the photo-cell is more strictly
parallel to that of a phonograph pick-up, which
takes mechanical vibration from a rotating
disc and transforms it into corresponding elec-
tric pulsations.

Fig. i shows a schematic view of a photo-
electric cell. The bulb is made of quartz or glass,
coated on the inside with silver, except for a
window a half inch or so in diameter which re-
mains clear for the passage of the light which
is to act on the cell. Opposite the window a layer
of the active material of the cell is deposited on
the silver. This substance is usually a com-
pound of the so-called alkali metals, potas-
sium, rubidium, or caesium, which have similar
chemical and physical properties. They re-
semble silver in appearance and are quite soft.
Some of their compounds, such as potassium
hydride, have marked photo-electric qualities;
that is, light causes them to emit electrons much
as heat makes the filament of a vacuum tube
do the same thing. THe next step, referring to
Fig. I again, is to place an anode in the tube to
pick up the electrons as they are released. If
this anode is placed in the path of the light com-
ing in through the window it is made in the
form of a wire loop but it may also be a
small metal plate displaced a little to let the
light pass. The final step is to connect between
the photo-electric cathode and the anode a
circuit for coupling the cell to the input of
an amplifier and for keeping a positive poten-
tial on the anode so that it collects the emit-
ted electrons more effectively. Frequently a gas,
such as argon, is used to increase ionization and
thus to make the cell more sensitive. Even so
the space current in such devices amounts to
only a few microamperes.

The photo-electric cell is liable to the same
difficulty as a carbon telephone transmitter — it
has its inherent noises arising from internal
turbulences and the input must be high enough,
even at its lowest levels, to override the noise.

The exciting lamps used are on this account of
the high-intensity type, so that the amount of
light passing through the film during low volume
portions will still meet marginal requirements.
In some other respects the photo-electric cell
is superior to the best grades of microphones.
It has no natural period of its own and any lag
which it may introduce would have to be mea-
sured in billionths of a second; the response, to
all intents and purposes, is instantaneous. Sel-
enium, the substance which was used as the
original intermediary between light and elec-
tricity, did not change its resistance immediately
under the influence of light.

As the energy output of the photo-electric cell,
although accurately proportional to the light
entering it, is so small, it must be used, even
for measurement purposes, with vacuum-tube
amplifiers. This combination was described in
the Physical Review as long ago as 1917, and
there may be earlier citations. Figs. 3 and 4
show two methods of coupling a photo-cell to
the input of a vacuum tube. The transformer
method seems to be preferred in commercial
sound motion-picture systems.

Inasmuch as our interest in photo-electric
cells is in connection with motion-picture pro-
jection, two special points may be taken up
here. One is the method of getting the light
through the film to the cell, as shown in Fig. 2.
The lamp in this case has a straight filament at
right angles to the paper, so that it is shown
in the diagram as a dot. This source is brought
approximately to focus on a parallel horizontal
slit about 1.5 mils high, and the image of the
slit projected onto the film, giving a light rec-
tangle a little narrower than the sound track
(about 80 mils) and about a mil high. The gates
and aperture plates about the film are not shown.
The light which gets through the film then
spreads out and goes through the window of the
photo-electric cell. Of course if the cell is dis-
placed, so that only part of the light gets
through, there will be a corresponding loss in

Anode

--Silver
Photo-electric Cathode

Filament-

mint-

Lamp

Slit

Film r

W-—-J >U-Afr

__-/ Photo

F v

U Cell

Optical';

System

FIGS. I AND 2

These drawings show how photo-electric cells
are used in sound motion-picture apparatus

volume, so the whole system must be properly
lined up. If the filament of the exciting lamp sags,
the image cannot be accurately centered on the
slit, and a loss of volume results. In this case,
as well as when the glass of the high intensity
lamp begins to darken, the lamp must be re-
placed. However, a loss in volume may also re-
sult from a photo-cell losing its emitting quali-
ties. A microammeter is useful in checking
this.

The method of fading from one projector to
another, insofar as it affects the photo-cells, is
another point of interest. This is shown in Fig.
5. A potentiometer with a neutral midpoint is
the essential part of the diagram. As the trans-
formers pass only the alternating component of
the photo-cell current, the operation of the po-
tentiometer will be noiseless. It will be noticed
that there is considerable similarity between
this circuit and microphone mixers in broadcast-
ing.

Commercial Publications

NATIONAL RADIO TUBE COMPANY. This concern,

whose address is 3420 Eighteenth Street, San
Francisco, is circulating an offer to rebuild
burned out, or otherwise defective transmitting
tubes, for broadcast stations. They claim means
of re-processing tubes so that they will be as
good as new at about half the cost of a
replacement.

GENERAL RADIO COMPANY. The General Radio
Experimenter continues to present material of
interest to broadcasters. The May issue contains
an article on "An Artificial Cable Box." Such a
device makes it possible to simulate the behavior
of actual telephone lines and cables in the
laboratory. The circuit used is shown in Fig. 6.
Here the loop resistance of the section, 4R, and
the shunt capacitance C, are made the same as
the equivalent quantities in the type of line
whose behavior is to be studied.

Inasmuch as an actual cable has its constants
distributed along its length, an artificial cable
simulates the behavior of a real one more closely
when it is built up of a considerable number of
uniform sections.

A type of cable box with the designation 321-
C, marketed by the General Radio Company, is
the electrical equivalent of 32 miles of standard
paper telephone cable in the usual wire gauges.
Within a cabinet 15 by 8 by 55 inches there are
seven units of the following electrical lengths,
respectively: 19-8-4-2-1-0.5-0.5 miles. These
are controlled by telephone key switches so that
any combination may be secured, making it
possible to get any length up to 32 miles in half-
mile steps.

The 1 6-, 8-, and 4-mile sections are not
lumped, but built up of 2-mile units, and the
smaller lengths are similarly split up. The re-
sistance elements, wound non-inductively, are
calibrated 100.25 percent., while the rolled paper
condensers have a precision of 0.5 per cent. The
maximum potential which may be safely applied
to the box is 300 volts.

Boxes of this type are supplied to represent
16, 10, and 22 gauge non-loaded paper cable
The constants at the mean speech frequency,
796 cycles, are given as follows:

CIRCUITS FOR PHOTO-ELECTRIC CELLS

Gauge Resistance per

Loop Mile

16 B & S 42.2 ohms
10 83.2

22 lyi.

Capacity per

Mile
0.062 mfd

.062

.073

In cables of this type the inductance may be
neglected, but General Radio also designs
special artificial circuits to simulate loaded lines,
in which case an appropriate amount of in-
ductance is included in each loop mile, or open
wire lines.

The same issue of the Experimenter describes a
6oo,ooo-ohm potentiometer suitable for use in
the grid circuit of an amplifier tube as a gain con-
trol. The circuit, which is familiar to most broad-
cast engineers, is shown in Fig. 7. Such a poten-
tiometer requires a high resistance in order that
it may not draw appreciable current from the
preceding element. This particular design,
known as the Type 452, covers 30 TU in 1 5 steps
of 2 TU each.

The August issue of the General Radio Com-
pany's bulletin contains a discussion of electrical
filters, which are divided into four groups: (i)
Low-pass filters which cut off all frequencies
above a certain value; (2) High-pass filters which
cut off all frequencies below a certain value: (3)
Band-elimination filters which block all fre-
quencies between certain limits; and (4) Band-
pass filters which block all frequencies on either
side of a section which is permitted to get
through.

Classes I, 2, and 4 have been discussed pre-
viously in this department. Type 3 is new in this
respect, and as it is not described in the General
Radio article I might point out that an audio
rejector of the sort shown in Fig. 8 is an ele-
mentary form of band elimination filter. In
parallel with the speech coil of a loud speaker,
for example, it may be used to smooth out a peak
in the response. The effect of the resistance in
series with the capacitive and inductive elements
is to broaden the resonance curve and to limit the
effect of the device. If the resistance is very low
practically no current of the resonant frequency
will get past the shunt circuit, but by adjusting
the resistance one may balance the by-passing
effect of the shunt circuit against the undesired
peak in the instrument under treatment and get
a flat over-all characteristic. Compare this use
of the resistance element with its similar function
in a line equalizer (Fig. 9) which is also a form of
filter.

Coming back from this digression to the Gen-
eral Radio discussion, we note there sketches of
sections of high-pass and low-pass filters, here
reproduced as Figs. 10 and 1 1, respectively. The

Tit. 6 Input

Fif . 8 ~ A C

Cell No. 1

Transformer

Fig. 4

Transformer
Fig. 5

PHOTO-ELECTRIC CELL CIRCUITS USED IN SOUND MOTION PICTURES

types of filters regularly marketed by General
Radio are of these classes, designed for im-
pedances of 600 and 6000 ohms, and with cut-off
frequencies of 500, 1000, and 2000 cycles. That
is, one may order from stock a 6ooo-ohm filter
of the high-pass type, for example, to cut off
everything below 1000 cycles.

It may be shown that, given the iterative im-
pedance Z and the cut-off frequency F for which
a filter is to be designed, the value of the ele-
ments is given for a high-pass filter section by

0.07958
FZ

O.07958Z

farads

henries

while for a low-pass filter we have similarly

o.3i83Z
Li= — = — henries

In each case F is in cycles per second, while Z
is in ohms.

The September issue of the Experimenter
carries as the leading article, "Notes on Group
Address Systems," by C. T. Burke. Since papers
on public address technique have appeared in
this department of RADIO BROADCAST Mr. Burke's
discussion will not be abstracted here, but it

Line

To Amplifier

HCi

-II

Input

Output Fig. 10

V V VVVV

Input S

5 '.. C2 -

S Output

Fig. 11

TjC2

n 1

—o

L. P. Filur

A GROUP OF INTERESTING FILTER CIRCUITS

should be interesting as a general treatment for
broadcast operators who have not devoted atten-
tion to the subject. A schematic diagram is
included.

This issue also contains a description of the
Type 426-A thermionic voltmeter, including a
diagram of the bridge circuit employed with the
vacuum tube. The range is 0-3 volts, and under
the usual conditions of use the calibration is
maintained to within 0.5 per cent, for about
1000 hours, and is good over the whole audio-
frequency range. Even at 20 kilocycles there is
only a 2 per cent, error, and less than 3 percent,
at 300 kilocycles. At broadcast and higher fre-
quencies the calibration is no longer valid. The
entire instrument, including the battery, is self-
contained.

Another item in the September issue concerns
the Type 532 Station Frequency Meter, the en-
tire scale of which covers only 0.3 per cent, of the
frequency of the designated station, the variable
condenser being connected across a larger fixed
capacity. This gives ten scale divisions per
kilocycle. The resonant frequency is read by an
ingenious method, which consists in connecting
across the main condenser a small auxiliary
capacity to shift the peak of the meter from
one side of the station peak to the other. The
frequency meter is adjusted until the indicating
galvanometer does not change its reading when
the button connecting this condenser is pressed.
This gives a more accurate setting than an at-
tempt to find the exact peak by the maximum
galvanometer reading. The accuracy of this
meter is certified as within 500 cycles with a
temperature variation of not over 5 degrees
Fahrenheit from the temperature specified in
the calibration. The guarantee is for six months,
after which the instrument must be recali-
brated.

The General Radio Experimenter is a very
commendable publication, to my mind, and
broadcast technicians should get their names
upon its mailing list. It is sent free on application
to the company's offices at Cambridge, Mass. It
is a commercial publication, but the discussions
are severely free from advertising blather and
generally contain as much general theory on the
subject as specific description of the General
Radio Company product. A commercial publica-
tion which is scientifically and informatively
written is better than a sensational medium
whose ultimate commercial aims are less frankly
revealed, and if you fail to extract a few dollars'
worth of data from the Experimenter it is your
fault.

A and

from the D* C. Lines

WHERE 1 10 volts d.c. is the house-light-
ing current supplied by the local power
company, there is opportunity for sup-
planting storage or dry batteries for both fila-
ment and plate circuits in a radio set by means
of comparatively simple and inexpensive devices.
These have been described in the columns of
radio periodicals and are fairly well understood
by those who "roll their own" in d.c. dis-
tricts.

There are a few pitfalls in this sort of appara-
tus which have been touched on in the form of
warnings, but practicable solutions do not seem

Samp fuse-'-O.

3 amp fuse

20,000 Ohms I 500,000 Ohms
K- B B+45 B+lOO

FIG. 1

to be generally known. The purpose of this article
is to indicate means for polishing off some of these
rough edges.

First let us state briefly the sort of A battery
substitute used. In simplest form it is a bank of
electric light bulbs, wired in parallel, in series
with the filament circuit of the radio set and the
light socket. The total wattage of the bulbs is
adjusted to the current consumption of the set,
the basis being about 30 watts per quarter-am-
pere tube. This is a little in excess of actual
needs, but the excess may be bypassed around
the set through a 30- or jo-ohm rheostat, R, in
Fig. i, the varying of which offers a form of fila-
ment control to compensate line voltage varia-
tions. This resistance is across the current source,
and consequently the current into the set in-
creases as the resistance is increased. The bank
of bulbs may be one or several reading lamps
which in the evening would be lighted anyway,
and, as a result, the set current costs the user
nothing additional except for daytime opera-
tion.

HOW TO REDUCE COMMUTATOR HUM

A CHOKE coil, capable of carrying about 2
*» amperes, and capacities across the line, al-
though not always necessary, are frequently
used as a filter to curtail commutator hum which
is present in the d.c. supply voltage. See Fig. i.
The writer has found that various types of
5-volt tubes operate with complete satisfaction
at 45 volts. His ear at least can detect no differ-
ence between a set operating at 4} and one
operating at 5 volts. An inexpensive and compact
dry battery of 4} volts is everywhere available
in the form of a C battery. If the voltage at the
set is adjusted to 4.;. and a 4' -\o]t C battery is
placed across the A substitute, the plus to plus
and the minus to minus, the hum, if there be
any in the speaker, will decrease to a negligible
amount. In normal operation there will be no
current drawn from the battery and its life will
consequently be long.

By WILLIAM B. DALL

This little battery has another important
function if used as indicated, which cannot
be performed by high-capacity condensers. It
will absorb what otherwise might be a consider-
able voltage rise and so protect the tubes in the
set against burn-out.

The danger in the use of the d.c. A battery
substitute described, lies in the fact that if a
tube be removed from the set while the current
is on, or if a tube filament should fail or a fila-
ment prong become open-circuited, the voltage
at the A plus and A minus posts will at once
increase considerably. Excess current will con-
sequently be forced on the remaining tubes in
the set. If that shock blows a second tube, an-
other rise occurs and the remaining tubes are
sure to go. With the handy little 42-volt battery
across the set, one tube may be taken out of
a 4-tube set and the voltage on the remaining
three will rise only about fV of a volt (varying
a bit with the internal resistance of the battery),
instead of jumping about rj volts as would be
the case without the dry battery shunt. Conse-
quently a tube filament may fail and the other
tubes in the set will be perfectly safe. On the
same principle this battery also absorbs voltage
surges.

Were the dry battery to be left in this circuit
after the house current was turned off it would
drain itself in a few moments in a vain endeavor
to supply current to the set. To make disconnec-
tion of the battery automatic, a relay comes in
handy wired as indicated in Fig. 2. Any of the
types used for cutting out chargers and cutting
in B power units will serve. In such a relay the
two contacts on the plug by which connection is
usually made to the light socket may be con-
nected to A plus and to A minus, and the leads
from the B power unit socket on the relay con-
nected to the C battery. The actuating coil of the
relay goes in series with the A plus lead. It is
essential that this coil lie between the point at
which + C is connected and the line, and not
between the C battery connection and the set.
In the latter position the relay will not function
properly and there would be a considerable drain
on the C battery as the latter would still be feed-
ing the set through the relay. In the correct
position, the relay action is instantaneous. One
C battery lead may be broken, or two, as many
commercial relays are equipped with a double
set of contacts.

WAYS FOR ECONOMIZING ON CURRENT

OPERATING multi-tube sets on no-volt
d.c. for filament supply is not economical
unless the current-limiting resistances (in the
case described, electric light bulbs) are used at
the same time for lighting the room. A six-
tube set using quarter-ampere tubes would
require the use of the equivalent of 180 watts of
such bulbs, which, on an average use of four
hours per day, would add 22 kilowatt-hours to
the electric light bill each month. To cut this
current consumption the kink is to use the 199-
type tubes up to, but not including, the last
audio stage.

If five tubes be so changed, we have .06 x 5
= 0.3 ampere, instead of .25 x 5 = 1.25 ampere
for 2oiA-type tubes, a saving of nearly one
ampere of filament current. The saving is nearly
120 watts, or two thirds of the original con-
sumption. A 15-ohm rheostat in series with the
A plus lead to the loxHype tubes is sufficient

to cut the 4! volts across the A-battery sub-
stitute to 3 volts for the set of five 199*5.

If the reading lamp used in connection with
this A-battery substitute is to be lighted at times
when the set's operation is not desired, a snap
switch, indicated in dotted lines in Fig. 2, may
be connected across the A plus and A minus just
back of the relay. When that switch is open, the
lamp will light and the set operate; when that
switch is closed, the lamp will light but the set
will be off. It is understood, of course, that the
switch on the set itself is always left in the "on"
position, the whole apparatus being controlled
from the light socket at which the current origi-
nates.

A B-POWER UNIT FOR D.C. OPERATION

THE B-power unit side of a 1 10 d.c. outfit is
simple. One 30- to 6o-henry choke, of d.c.
resistance not exceeding 600 ohms (preferably
lower), and two 4-microfarad condensers (good
by-pass condensers will do, as the voltage does
not exceed 1 10) comprise an adequate filter.
Allowing for a voltage drop through the filter,
an output of 90 to 100 volts is obtained. One
variable high resistance gives the 45-volt tap
for detector. Fig. i shows the hook-up.

With a push-pull last stage audio for four
power tubes, as described in RADIO BROADCAST
for May, 1928 (page 18-19), the B plus is taken
directly from the light-socket connection around
the filter, since thereby the full 1 10 to 115 volts
is obtained for the power tube plates and any
hum is balanced out by the push-pull arrange-
ment.

It is customary to equip these power devices
with 3-ampere fuses. This is of course protection
to the house fuses, but not to the radio apparatus.
At 3 amperes, a short-circuit could play consid-
erable havoc with good radio equipment. To ob-
viate this danger, the writer has inserted an old
199 tube in series with the B plus i lo-volt plate
supply lead. This limits the current through this
lead to 60 milliamperes. The drop across the tube
at a drain of 40 mils is only 2 volts, and all tubes
and associated apparatus are well protected. The
199 tube used may be an old one, whose de-
creased filament emission renders it unfit for
radio set use.

The tapped choke output for push-pull stage
is a satisfactory substitute in the use of two
3o-henry chokes, the two outside connections
going to the plates and speaker, and the inside
common connection to both chokes going to the
B plus. This kink permits the use of chokes
which the builder may have on hand.

Switch

-A

-Relay

Jill

4H V C Bat

(Radio Set)
FIG. 2

-A

Complete Hquipment

For Television

Reception

Fig. 1.

Zeb Bouck's

Televisor

Building Receivers For Television

By ZEH BOUCK

TELEVISION signals are transmitted to-day to a rapidly increasing
number of experimenters throughout the world. The interest and
pleasure associated with their reception is found in the novelty
and fascination of the achievement with home-made apparatus
rather than in esthetic considerations associated with the reproduction.

Television signals when properly
transmitted and received portray the
instantaneous characteristics of the
object being scanned at the transmit-
ter. This is accomplished by inter-
preting at the transmitter the visual
aspects of the televised object as elec-
trical variations which, at the receiver,
are reconverted into properly distri-
buted shades and high-lights.

The quality of the received picture
depends upon how good a signal is
transmitted in the first place, and
upon how well it is reproduced at the
receiving end. Here the amplifier used
following the detector plays the import-
ant part. Theoretically the signal to
be received should contain important
frequencies from as low as the number
of pictures per second to those lying far
above the highest audio frequency used
in music.

However, television programs in
the broadcast band cannot contain
frequencies above 5000 cycles since
this is the highest frequency at which
a broadcasting station is permitted to modulate,
and even when the transmissions are on short

waves, the improvement in reception obtained JN THIS article are given descriptions of the
through the use of a special amplifier going up to work °J iw° experimenters well known to

15 or 20 kilocycles isn't worth the expense of
constructing it — at least it doesn't appear to be
worth doing until the quality of the transmissions
are much improved over what they are to-day.
A start can be made with any good amplifier and,
after the best possible results have been obtained
from it, there will be time to construct an am-
plifier designed especially for television reception.
It is the purpose of these notes to describe the
amplifier and scanning combination used by the
writer. This apparatus will reproduce television
images when attached to any receiver capable of
delivering a signal of requisite strength from the
desired transmitting station.

(Continued on page 36)

?/ two experimenters

readers of RADIO BROADCAST — Zeh Bouck
and James Millen — both of whom have set
up apparatus and successfully received tele-
vision programs. The differences in the
apparatus — especially the amplifier equip-
ment— will give the reader a good idea of the
circuits which may he adapted to present day
television reception. As the art develops more
stringent requirements will he set up as to
the characteristic of the audio amplifier hut it
is evident from the data given here that for the
present at least almost any good amplifier may
lie used.

— THE EDITOR.

By JAMES MILLEN

INSTALLING and operating an experimental television receiver
is not difficult for the average radio set builder or amateur experi-
menter. In this connection a description of the equipment used by
the writer should be of considerable interest, especially as the same
apparatus may be employed by experimenters in other parts of the country

for receiving any television station by
merely selecting a scanning disc of the
proper type for the particular signal
being received. For instance, when
using a 24-hole disc the WGY 21-
meter television signal may be received
all along the West coast. Then, there
are the KDKA experimental signals
which require a ob-hole receiving disc.
The majority of stations, however, are
transmitting a 48-line picture.

Any good receiver capable of being
tuned to the wavelength of the desired
television transmitting station may be
used. The short-wave receiver employed
by the writer is the standard kit sold
by the National Company, all of the
parts of which are available in the open
market. It was described in detail on
page 286 in the August, 1928 issue of
RADIO BROADCAST. The 222-type r.f.
tube is followed by a regenerative
detector. This system prevents radia-
tion— a problem which would soon be-
come quite serious if all the short-wave
receivers were of the radiating variety.
In building any type receiver, especially for
short-wave reception, and particularly one for
television work where a motor and scanning disc
are located in the same room, considerable at-
tention must be given to rigidity of construction.
I his applies to the coils and their mountings as
well as the wiring and other parts of the set.
We find from experience that the ordinary
audio amplifier, such as you use at present for
speech and music, is good enough to provide
picture reproduction sufficiently clear to dis-
tinguish forms, such as the outline of a hand or
head, and to follow motion. This is perfectly
O. K. for a starter, and your present audio
amplifier can be used, provided it has at least
the gain of a good two-stage transformer-
coupled unit.

For receiving 3XK in Boston it was found ad-
(Continued on page 37)

RADIO BROADCAST

- 6V. +
FIG. 3. SCHEMATIC DIAGRAM OF TELEVISION AMPLIFIER

+220
to 450

(Continued from page 35)
The essential parts of a television reproducer
are a tuner (any kind will do that is capable of
providing a good loud-speaker signal in ordinary
reception), an audio amplifier, a neon tube and a
scanning disc turned by a motor mechanically
arranged so that the holes in the disc scan the
surface of the glowing plate.

The amplifier used by the writer is shown dia-
grammatically in Fig. 3, while constructional
points are suggested in the pictures. Figs, i and
2. The amplifier can be built from any standard
apparatus; it employs the usual three-stage
resistance-capacity-coupled circuit.

APPARATUS REQUIRED

T~HE following is a complete list of the parts
' required for the construction of the writer's
television amplifier:

RI — 3 Wire-wound resistors, 100,000-

ohm;

Rj. RJ. R4 — 3 Grid-leak resistors, i-meg., 0.5-
meg., and 5o,ooo-ohm, respectively;
Rj — i Rheostat, lo-ohm;
R« — i Potentiometer, 2Oo,ooo-ohm;
Q — i Mica fixed condenser, o.ooi-mfd.;
Ci — 4 Filter condensers, 4OO-volt, 2-mfd.;
C», Ci. C» — 6 Mica condensers, o.oi mfd.;
L — i Pilot light;
Si — i Toggle switch;

4 Vacuum-tube sockets, ux-type;
6 Resistor mountings;
10 Binding posts;
i Front panel, 7 x 10 x A-inch;

1 Sub-panel, 7 x 10 x A-inch;

2 Sub-panel brackets.

The pilot light and toggle switch are, of course,
unessential, but were incorporated in the am-
plifier for convenience. The amplifier eventually
will be mounted in the cabinet, and the pilot
light will give some indication of current condi-
tions behind the panel. The switch (Si) controls
the filaments of the detector and amplifier tubes,
while the other switch, visible in the pictures,
was used to start the motor turning the scan-
ning disc.

The four filter condensers (Q) are connected
in parallel to bypass the amplifier plate voltage,
and reduce the tendency to motorboat. In the
amplifier pictured the coupling condensers are
built up in stack form, and consist of six o.oi-
mfd. mica condensers.

In operation it will be convenient to be able to
switch easily from the loud speaker to the neon
tube, an operation that is facilitated by the
double-pole, double-throw switching arrange-
ment suggested in Fig. 3. By means of the two
battery taps the voltage applied to the tube is

practically the same with either the loud speaker
or neon tube, thus prolonging the life of the \~i.\
tube and making unnecessary any variation in
the C-bias potentiometer, R6. When the amplifier
is operating the loud speaker about 200 volts
are employed, while with the neon tube the full
450 volts is applied. These voltages can be sup-

Cross Bar •>

f «, — r

Motor Shaf

@ f Disk-*

Looking down or up
FIG. 4. MECHANICAL SPEED CONTROL

plied either by a B battery or a power-supply
unit. A Receptrad Powerizer was used in the
author's laboratory as shown in the picture of the
entire set-up, Fig. i.

SCANNING DISC ASSEMBLY

'TpHE televising apparatus consists of the
* scanning disc driven by an adequate motor,
some form of motor speed control, and the neon
tube, combined in a convenient and efficient
mechanical arrangement. A box frame, such as is
illustrated in the pictures, Figs, i and 5, provides
a simple and satisfactory unit. The motor shelf is
so positioned that the driving shaft can be
centered exactly. The shelf is clamped between
cushions of soft rubber which reduce the
vibration.

The neon tube is mounted with its plates
parallel to the scanning disc, so that the holes
in the latter pass over its entire surface. The neon
tube is a Raytheon Kino-Lamp and it should be
placed in a horizontal position on the upper shelf
as close to the scanning disc as possible. The
Baldor type-MV2 variable-speed motor may be
used for turning the disc. For observing the
picture square hole, ij x 1} inches, is cut in the
face of the front panel exactly in front of the
plate of the neon tube.

The inside dimensions of the entire box. as
pictured, measure 25 x 25 inches. It is built of
half-inch wood (heavier material is desirable) and
the front panel is 7 x 26 inches. Two ten-ohm
rheostats, connected in series, are mounted on

NOVEMBER, 1928

the panel, and these provide a very accurate
motor control. The rheostats are in series with
some additional resistors in accordance with the
directions accompanying the motor.

By means of the rheostats, in conjunction
with a simple mechanical brake described in the
paragraph on operating directions, it is possible
to maintain the speed of the motor sufficiently
close to synchronism with the transmitting disc.

THE CIRCUIT ARRANGEMENT

T~HE detector tube has been incorporated in
*• the amplifier constructed by the writer.
When used with the average receiver, posts one
and three in Fig. 3 will be bridged, and post
two will be plugged into the grid prong on the
detector socket of the receiver. In the case of a
regenerative receiver, the plate of the exterior
detector tube will be wired to the plate terminal
on the detector socket while post three will be
led to the plus detector terminal on the set. In
other words, the tickler or regenerative coil is
connected in between the plate of the detector
tube and the coupling resistor. If it is desired to
use the detector socket in the receiver (and this
may be the more simple procedure in many
cases) post three is led to the plate terminal of
the detector tube.

The neon tube is connected in place of the
loud speaker or output device. This is a simple
series connection and is quite effective. While
there are other systems of inputting to the neon
tube, concomitant complications hardly rec-
ommend them for an initial attempt.

The apparatus described was designed pri-
marily for the reception of the television signals
broadcast from WRNY, New York City, em-
ploying a 48-hole disc at a speed of 450 revolu-
tions per minute. A National Company disc is
used in the illustrated apparatus.

OPERATING DIRECTIONS

THE signal is first tuned in on the loud speaker
in the usual way, a loud clear signal being the
desired result. The motor should be started and
the disc brought up rapidly to approximately
the speed desired. This can be accomplished by
means of a switch short-circuiting the speed-
governing resistors.

I he output of the amplifier is now switched
over to the neon tube, which, when the television
signal is received, should show a definite pattern
when viewed through the disc. As the disc ap-
proaches synchronism with the transmitting
disc, the pattern will resolve itself into more
definite lines slanting away from the perpendic-
ular. 1 he lines become more and more perpen-
(Contimud on page )j)

FIG. 5. REAR VIEW OF TELEVISOR

NOVEMBER, 1928

JAMES MILLEN'S TELEVISION RECEIVER

(Continued from page )$)

visable to use a three-stage amplifier using
National transformers — see Fig. 3. Then
again, the pictures being transmitted by JXK at
present are merely silhouettes, which do not
require an amplifier with as wide a frequency
range as if half-tones were being transmitted.
As a rule, with a three-stage a.f. unit, the
amplifier noise will not be very great. Vibration
from the receiving disc or its motor, which are
transmitted to the amplifier or especially the
detector tube, however, will introduce a periodic
noise that will cause a black streak across the
field of the picture. Any periodic interference,
such as a Go-cycle hum, that may get into the
signal will also cause streaks across the picture,
but these will not remain stationary, but will
move upward or downward across the field of the
picture.

THE OUTPUT CIRCUIT

THE output circuit of the amplifier is ar-
ranged so that the neon or Kino-Lamp is
always illuminated, and, when a station is re-
ceived, the brilliancy of illumination merely
varies in accordance with the signal. A good
background will be obtained if the d.c. current
through the neon tube is limited to 10 or 20
milliamperes. More current will cause the lamp
to glow brighter and brighter but there is no
advantage in this so far as the picture is con-
cerned and it only serves to shorten the life of the
lamp. Accordingly, care should be taken to ad-
just the current to the minimum satisfactory
value.

A Clarostat has been found excellent for such
use, and it may be mounted conveniently on the
front of the frame supporting the scanning
apparatus, as shown in Figs, i and 2. For
illuminating the Kino-Lamp either a standard
high-grade B socket-power unit or heavy-duty
B batteries may be used.

Several different concerns are manufacturing
scanning discs suitable for use in receiving the
signals now on the air. The better grade discs
are well made mechanically, so as to run true
and require little power. The holes in such discs
are also punched to the degree of accuracy
necessary if the received image is to be free from
black lines and streaks. The National disc uses
radially-shaped holes, rather than round holes,
for with this design the "lines" across the image
are much less obvious.

In driving the scanning disc successful results
have been obtained with a number of different
types of small motors. However, the motor
which the writer is using at present is the g-horse-
power type-YIV variable-speed condenser-type
Baldor which is intended for operation on 110-
volt, single-phase, 6o-cycle a.c. line. This is a ball-
bearing motor that operates very smoothly and

quietly. The swish of the
disc through the air con-
stitutes the major por-
tion of the noise, and this
is quite insignificant.
Special rubber vibration
absorbers are supplied
with the motor for mount-
ing purposes.

MOTOR SPEED CONTROL

THE diagram (Fig. 3)
shows the method for
speed control. For the
variable resistor R2, a 75-
watt. 4- to ioo-ohm wire-
wound resistor with a
sliding contact is used.
The other resistor may
be a lo-ohm lo-watt re-
sistance. This is labeled '
and is shunted by the
control leads.

The resistance R-_ is so adjusted that with the
push button released, the motor runs at slightly
below the proper synchronous speed. Then, when
the push button is depressed, the disc tends to
speed up.

Do not mount the television receiver in the
same cabinet with the disc. Vibrations of the
motor will introduce a synchronous noise that
will result in a series of horizontal lines being
drawn across the picture. Therefore, it is im-

NEON Til"' WIVII'W

1 _

FIG. I. GENERAL VIEW OF TELEVISION LAYOUT

RI" in the diagram
push-button speed-

FIG. 2. REAR VIEW OF SCANNING DISC

portant to keep the receiver and amplifier on a
support separate from that for the disc.

The experimenter will find that the following
convention has been adopted by the Raytheon
Company in regard to neon tube mountings. The
tube is fitted with a standard ux base. The plates

inside the tube are placed in a plane at right
angles to the axis of the "Pin" of the base. If
the pin, therefore, is pointed toward the disc
when inserted in the socket, the plates inside
the tube will then be parallel to the disc. The
tube should be mounted at the proper height to
cover the i^-inch square scanned by the revolv-
ing disc. The plates are connected to the "plate"
and "filament" prongs of the tube base.

I Zeh Bouck's Televisor

(Continued from page j6)

dicular as the correct motor speed is approached,
finally forming the image of the televised object.
Final tuning should now be effected on the re-
ceiver. Also, the bias to the last tube should be
varied, by means of the high-range potentiometer
Re, for best results.

MECHANICAL BRAKE NEEDED

IT IS desirable to use some simple form of
' mechanical brake in conjunction with the
rheostats to control the motor. The device
shown in Fig. 4 was designed by the writer for
this purpose.

An iron cross piece was fastened to the box
housing the revolving apparatus so that it crossed
in front of the motor shaft. A hole was drilled to
the exact center of the shaft, and the nut from a
J-inch iron bolt was soldered to the cross arm. A
brass strip was bent as shown in the illustration,
and bolted to the cross piece. The head of the
1-inch bolt was sawed off and a knob mounted
on the end. By screwing in the bolt, the brass
strip is pressed against the end of the shaft,
giving a very delicate braking action. The brass
strip should be taped where it touches the
shaft.

National Disc
Neon
Tube'

180 Volts j' 3Ohenry
•om Separate 25-mA

B-PowerUnit Choke Coil
or Batteries

+ 67 »135 +450

110V.A.C.

RECEIVER AMPLIFIER TELEVISOR

FIG. 3. COMPLETE SCHEMATIC DIAGRAM OF JAMES MILLEN's TELEVISION RECEIVER

A Modulator for the
Wave Trans

A WRITER on the Hartford Times tells
me that all radio men write stories
badly; that is, they always leave the
thrill until the last, instead of putting it into the
headline. In this three-part tale of improved
short-wave transmission, I seem to be guilty of
this offense, for I am waiting
until the third installment to
speak of the uses of such trans-
mitters, and, of course, that is
where the thrill comes in. For the
present we shall give additional
data on the constant-frequency
transmitter described last month,
and explain those devices which
make it useful, which is to say
the key or microphone.

A transmitter without modula-
tion is useless, since it can send
out only a "carrier wave." To
place variations on that carrier
which a radio receiving set can
"unscramble" into code or voice
is the business of the apparatus
described in this article. It was
explained in the October issue
that voice, or radio-phone trans-
mission, has the advantage of
speed while, with the same
power, radio-telegraph transmis-
sion has the advantage of greater
range. Very evidently these two
systems may be used with the
same transmitter to excellent advantage, and
it is my suggestion that provision be made
for both.

CONCERNING INTERFERENCE

7*O MANY amateurs a recommendation of
radiophone is as a rag to a bull; it causes
them to paw up dirt and bellow about the awful
interference caused by the microphone-operated
sets. This opinion is a bit out of date. Two years
ago amateur oscillators were so unstable that
the addition of modulating equipment did have a
distressing way of causing the signal to smear
over a wider band than one cares to think of.
Also, the transmitters did exactly the same thing
when keyed, but, in that case, the interference
consisted of mumbles, thumps, and "yips"
which were harder to identify. Since that time
our preaching for better tuned circuits has had a
good effect, and the coming of crystal-control
has set examples. As a result the number of good
amateur radiophones has increased so that we
must withdraw the accusation against the phone.
To sum up — with an oscillator-controlled bat-
tery set we may modulate with key or voice as
we please with no fear of "wobbulation" being
added to "modulation." The circuit of the
oscillator-amplifier set-up is shown in Fig. 3.

THE CIRCUIT ARRANGEMENT

THE circuit is the almost standard Heising
constant-current arrangement. In Fig. 2A
we review the general principle. Current from the
battery flows through the high-inductance choke,
L, to the point B, then divides and returns in two
parts through the two resistances RI and Rj.
If we suddenly change the setting of the variable

By ROBERT S. KRUSE

resistence Rj we find (by watching the meter I)
that the current through RI changes for only a
moment when this is done. The explanation is
this; the choke L is (after the fashion of induc-
tances) an electrical "stand-patter," i.e., always
opposing a sudden change. Thus, if we suddenly

FIG. ;. TOP VIEW OF MODULATOR

reduce Rz we do not change the total current
through the choke immediately but simply
change its division between the two paths,
therefore, the current through A drops sharply
until L finally agrees to allow more current to
pass. On the other hand, if we suddenly raise the
resistance of R2 we force additional current

A TRANSMITTER, according to Mr.
Kruse, without a means of modulation
is as bad as a ship without a sail. Inasmuch
as the transmitter itself was described in
October, a transmitter that is designed to stay
on its assigned frequency, the modulation
equipment necessarily had to follow. Here it
is. — THE EDITOR

through RI until L permits the total current to
die down. Thus, the effect of any siuldtn changes
in K2 causes corresponding sudden changes in
the current passing through RI.

In Fig. 2i> we show how this principle is used
in our set. The oscillator tube feeds radio-fre-
quency power to the grid of the r.f. amplifier
tube where it is amplified and passed to the cir-
cuit U C3. The current is then transferred to the
antenna, but this does not concern us just now.
The plate power for the amplifier tube is supplied
through a choke coil L, of at least 6 henries, just
as the current in Fig. 2A was supplied. The radio-
frequency choke (r.f.c.) is to prevent the r.f.

wandering into L and the tube "Mod" where it
would be wasted. The vacuum tube labeled
"Mod," which is used as a voice-operated re-
sistance, is substituted in the circuit in place of
the resistor R2. A microphone and amplifier feed
voice-currents to the grid of this tube, and, as
they cause the voltage of this
grid to change, the current to the
amplifier tube varies, causing cor-
responding changes in the r.f.
current in the L; Cj circuit from
which the antenna power is
taken. It is only fair to say that
this method of operation does
not permit the highest amplifica-
tion to take place in the ampli-
fier and that a "straight" r.f.
amplifier should be added if the
full power-rating of that size of
tube is desired. However, this
added complexity does not seem
warranted in a small set since the
range increase is not large.

The circuit arrangement in
complete form is shown in Fig. 3.
The picture in Fig. I shows how
simply the whole works goes to-
gether. In listing the particular
makes of apparatus which are
given below, the writer has no
particular desire to favor any
manufacturer but rather his mo-
tive is to remove uncertainty by
listing those parts which have been used with
satisfaction in short-wave transmitters. In
several cases it was found that other equipment,
which seemingly had every right to be as good,
was thoroughly unfitted for this somewhat
special purpose. If changes in the assembly are
considered necessary they should, therefore, be
made one at a time and the effect noted. The
parts used in my set-up are as follows;

LIST OF APPARATUS

Mike — Western Electric type 348BW or Federal
type 2<5oW.

R! — Used to reduce microphone current to prop-
er value. Not necessary in most cases.

Rj — Gain control. Frost 100,000- or 200, ooo-ohm
type.

Rj — Shunt resistance, exchangeable in clip to
suit tube and transformer used. I megohm is
usually satisfactory.

R( — Filament rheostat, 6-ohm.

L — General Radio choke, type 485-8.

TR| — General Radio type 48j-M (for single
button mike)

TR — General Radio type 485-0

S\\ -Switch to cut off d.c. filaments and micro-
phone. No a.c. circuits should go through or
near this switch.

CONCERNING TUBE EQUIPMENT

COR the sake of simplicity, and to minimize
A.C. hum, it is recommended strongly that
for voice operation 2oiA-type tubes be used in
the oscillator (see p. 344 of October RADIO
BROADCAST) and in the first socket of the modu-
lator, and that their filaments be operated from

NOVEMBER, 1928 A MODULATOR FOR THE 1929 SHORT-WAVE TRANSMITTER

a 6-volt storage battery, which is necessary for
the microphone in any case. The r.f. amplifier
tube in the October article and the modulator
tube in the present set-up may be a 1 12-, 171-,
or 2io-type tube, or a corresponding a.c. tube.
•The UX25O may be used but is not recom-
mended. It is possible to use a.c. tubes in place
of the 210 tubes recommended, but make sure
that they are of the "heater" type, such as the
Arcturus tubes or the UY227, and not of the
"thick-filament" type represented by the 11x226.
Care must be taken to keep the filament leads
clear of the grid circuits and even with these
precautions the hum problem is apt to be bother-
some. With batteries on the tubes mentioned
above this difficulty is removed. Still another
combination is possible, namely to operate 201 A-
type tubes in the first sockets and 2 lo-type tubes
in the Amp. and Mod. sockets with a.c. on all
the filaments. This is done by connecting the
2oiA filaments in parallel and running them
through two equal fixed resistors to posts D and
F of the October set-up. If the October set alone
is used for c.w. these resistors must produce a
drop of 1.25 volts each at a current of \ amp.,
therefore, they must have a resistance of 2.5 to
3 ohms. If the complete set-up, i.e., oscillator.

Cfps.

R.F.C4

L\J!JULM/"<>+ Amplifier

high
voltage

FIG. 3. THE SCHEMATIC DIAGRAM OF OSCILLATOR-AMPLIFIER

FIG. 2

Diagram A (left) : schematic
showing constant-current
modulation principle. Dia-
gram B (above): circuit used
in set. A by-pass condenser
between B- and the plate side of
R.F. C. would improve results.

amplifier, and modulator is being used the cur-
rent will be \ amp. and the resistors may have a
resistance 1.25 to 1.5 ohms each. A pair of \- or
3-ohm filament-ballasts resistors will do very
nicely. It is advisable to twist all a.c. leads into
pairs and to inspect the various center-tapped
resistors to make sure they are in good condition.
An open resistor will cause considerable noise;
an off-rating one will produce less noise that is
still quite noticeable.

denser Cn must of course be readjusted when
such a change is made.

Since the 2io-type tube is used with the
thought of obtaining an increased output it is
operated at high voltage and with a plate current
of about 40 milliamperes per tube — making 80
for the modulator and amplifier. This current
would destroy the windings of the National type
90 choke in the plate circuit of these two tubes,
and, therefore, it must be replaced
by a more substantial choke which
will operate over all the wavebands
we are interested in. I can find
none of the transmitting-type
chokes which will do this, and,
therefore, suggest a combination
consisting of a single-layer choke
in series with a General Radio
tyPe 379 T, Aero Products type
248 (transmitter type) or a Na-
tional Type 90 re-wound with
No. 32 or 34 d.c.c. wire. The single-
layer choke, which is substituted
in the clip for the former choke,
consists of a f" rod of insulating
material wound for ij" with a
single layer of No. 34 or 36 d.s.c.
or s.c.c. wire. The other choke
may be mounted on the back of
the panel near the amplifier tube

THE UX-2IO TUBE

I F ON E uses 1 1 2- or 171 -type tubes
* amplifier and modulator sockets it is
possible to cut down the plate voltage
of the oscillator, thus reducing the
drain from the batteries. This should
not be carried to the point where un-
steadiness results. In my particular
set 90 volts at the oscillator handles a
I7i-type amplifier running at 300
volts with very fine steadiness.

When using the ux-2io tube or
equivalent in the amplifier and modu-
lator sockets the oscillator tube should
have a plate potential of approxi-
mately 180 volts. It must, of course,
never show plate-heating to a visible
degree. If desired, the feed condenser
Cf may be changed in size, provided
it is not made large enough to cause
difficulty in reaching down to the
lo-meter band. The neutralizing con-

in the r.f.

and antenna ammeter. Its business begins at 20
meters where the little choke stops.

I n Fig. SA ot her changes are suggested that may
be necessary if the 2io-type tubes are operated
at voltages above 250 — as they usually are since
their rating is 350. The condenser Cs should be
replaced by a looo-volt Sangamo unit of the larg-
est capacity available. If this cannot be done
conveniently a change to the shunt method of
feeding (shown in Fig. SB) is advised. This may
be necessary in any case if voltages above 400
are used. Here Cs replaces Cs.

The 250-micro-microfarad plug-in condenser

FIG. 4. FRONT VIEW OF MODULATOR UNIT

(C6) used to load the amplifier plate circuit will
not survive with the 210, therefore one of two
things may be done. Either the coil inductance
must be increased or else the 25O-mmfd. tuning
condenser C3 should be replaced by a joo-mmfd.
Equitune. The dial degrees of the two tuned
circuits, i.e., the oscillator and the amplifier, will
then not run together as nicely as was the case
before. Of course, if one is really fussy about
this point it is possible to use two Equitunes, of
the 250 size, set end to end and connected in
parallel, one being fixed at maximum and the
other variable as usual. Personally, I prefer in-
creasing the inductance.

Finally — if someone wishes to use a 2io-type
tube as oscillator, that too can be done by con-
necting posts ABC in Fig. 3 to DEF in Fig. 5.
The plate potential of the oscillator should not
be increased above 180 volts. This combination
isn't bad at the upper wavebands but offers
some slight difficulty at 10 meters. At present
I cannot give the exact dimensions of the some-
what smaller coil that will be required. The
diameter will be ? bout i" smaller than shown in
the October article. The other wavelengths will
shift somewhat but not badly, because of the
large condenser which is used.

KEYING AND OPERATING WITH VOICE

NO MATTER which sort of operating is to be
taken up the first job is to secure a steady
output. Some suggestions were given in the
October article. To these can be added the fact
that it is of comparatively little importance what
plate voltage is used as long as the plates remain
at a sane temperature. For the oscillator this
means no visible color, for the amplifier it means
a red that is not too violent. The type of tuba
used and the recommendations of the maker
should be considered. It is helpful to listen to the
un-modulated output with a little "breadboard"
receiver using the circuit of Fig. 6. With a 199
tube and a 22-volt battery the whole thing can
be put on a 7" x 10" base, including a
4j-volt C battery for the filament
supply. Shielding is neither necessary
nor desirable for such a device; it is a
nuisance in fact. When the note seems
O.K., and free from oo-cycle ripple,
one may key slowly and then proceed
with voice-modulation. For this the
pick-up receiver is stopped from oscil-
lating.

"PERFECT MODULATION"

THE voice-input system shown will
do good work if given a chance.
There are several ways of making the
adjustments. One is to have an assis-
tant speak into the mike — preferably
reading steadily from a book — while
various things are changed and the
results noted as the signal is heard

RADIO BROADCAST

NOVEMBER, 1928

•-»»». T» C

,• f I FTTT? \

- c *

-4510-9

ToAtCol
OscrAmp.

FIG. 5. SCHEMATIC DIAGRAM OF MODULATOR

in the phones of the pick-up receiver. The
assistant must enunciate decently as other-
wise he is worse than useless. He also must
hold the mike in the proper position and at
the correct distance. With these conditions as a
good start one may now adjust the gain control,
microphone voltage and bias of the modulator
tube. The bias may be set at 10 per cent, of the
modulator plate voltage at the beginning, and
varied from this point. In general a large bias
has an advantage in keeping the tube cool. If
one has meters available it will be found good
practice to adjust the currents to modulator and
amplifier tubes so that they are of nearly equal
value. Having once found a good setting one
may watch the antenna meter thereafter, judging
from its movements the degree to which things
remain the same. A better way of doing this is
to put a d.c. milliammeter in the modulator
plate lead — and leave it there.

If an assistant is not available one may place
the "mike" before a good loud speaker running
at a moderate level on some decent input — not
a jazz band. The listening is then done as before.
It is scarcely necessary to say that all adjust-
ments of this sort should be made with the an-
tenna cut off.

The beginner will find himself confused when
trying to determine the difference between good
and bad speech from his own set. He is able to
find some help from the fact that a bad phone
makes no difference between the letters F and S,
and very little between P, B, D and T. In addi-
tion to this it very probably will "blast" on some
notes and on some of the vowel letters, especially
O. Repeating alphabet and the groups of letters
just mentioned, together with reading and
counting are all good tests. One entirely useless
test is to get on the air and work someone. The
truth does not lie in that quarter — or perhaps
I lack faith through being neither a "brass-
pounder" nor very much of a "ragchewer," but
mainly an occasional transmitting experimenter.

One very important point to remember is that
the best of phones will not compensate for

HMr

C,(Phi|inl

RFC.

JIOM, — °'

) -HigMVolUf*

FIG. 8 A.

This circuit is utilised when a 3io-type r.f. tube

is used with a plate potential in the order of

400 volts

sloppy handling of the
"mike." One must keep
at a fixed distance
and speak in an even
tone of voice. Looking
around the room does
not help, nor does a
cigarette or cigar be-
tween the lips. Con-
sider the good care
taken in broadcast an-
nouncements as com-
pared to the ignorant
use of the same equip-
ment by a new speaker
on his first broadcast

from a not-too-good station.

AND AS FOR THE KEY —

\A7 1TH the key one may do many things in-

* V correctly. The best rule is to send little

and listen much until one learns the manner in

which not to do things. This is easily done for

A.KEYS-B

connections are ex-

FIG. 6. PICK-UP RECEIVER USING A
199 TUBE

the average performance is not perfect and the
air is still cluttered up with tireless "CQ" callers
who make the most imperfect phone seem holy
and pure. When one does call — let it be at a
speed where the send-
ing will be readable for
"It isn't the words per
minute but the messages
per hour that count "-
and again — "What prof-
iteth speed when but used
to repeat what was sent
badly?"

Of the set itself little
need be said when oper-
ating for radio-telegraph
transmission only. The
plained in the diagrams; the standard prac-
tices are too lengthy to be put down here.
The Radio Manual by Sterling at this mo-
ment seems alone to contain the new regula-
tions.

One comment with regard to the set can be
made. If for any reason it is desired to use C bias
on the r.f. amplifier tube in place of the un-
orthodox resistance bias shown in the October
paper this may be done by feeding the C battery
to the clips of the cartridge-resistance-mounting.
The oscillator-amplifier set-up has been so laid
out that the C battery can be placed behind
it and leads run in without difficulty. If, as
in my case, the intention is to use the set-
up portably the clips themsevles should not
be disturbed.

Perhaps I have been wrong in the assumption
that tuning the oscillator-amplifier is self-evident.
The procedure is to set the oscillator with the
aid of the wavemeter, then to place the little
lamp-loop near the amplifier plate-coil (Aero
Coil U) and tune that circuit for greatest bright-
ness, finally to revolve the antenna condenser
until the greatest antenna meter reading is ob-

tained. Warning — the antenna meter is easily
burned out if kept off-scale long. If it runs off —
detune or pull the switch instantly. Then shunt
the meter with a length of wire— 6 inches at a
guess — and try again. If it still runs off shorten
the shunt until it does not. The process takes
some practice and should be done for all the
hands after which we will be ready to — but that's
next month's story.

\M\TEUK WAVELENGTHS

THAT there be no confusion regarding who
may transmit, and what frequencies are avail-
able for amateur operations, the following quota-
tion from "Revised Amateur Regulations"
dated March 6, 1928, and signed by W. D. Ter-
rell, Chief. Radio Division of the Department
of Commerce, gives all the necessary infor-
mation.

"An amateur station is a station operated by
a person interested in radio technique solely with
a personal aim and without a pecuniary interest.
Amateur licenses will not be issued to stations
of other classes.

"Amateur radio stations are authorized for
communication only with similarly licensed
stations, except as indicated below, and on wave-
lengths or frequencies within the following bands:

Kilocycles

Meiers

401,000 to 400,000
64,000 to 56,000
30,000 to
16,000 to

8,000 to

4,000 to

2,000 to

28,000

14,000

7,000

3,500

1,500

0.7477 to

4.69 to

9.99 to

18.7 to

37.5 to

75.0 to

150.0

0.7496

5.35
10.71
21.4
42.8
85.7

to 200.O

and at all times unless interference is caused with
other radio services, in which event a silent pe-
riod must be observed between the hours of 8:00
p. m. and 10:10 p. m., local time, and on Sundays
during local church services.

FIG.

B..KEYS R.Fxct. C KEYS BOTH -B 1 R f

7. CONTROL CIRCUITS

Amateur radio telephone operation will be
permitted only in the following bands:

Kilocycles

64,000 to 56,000
3,550 to 3,500
2.00010 1,715

Meters

4.69 to 5.35
84.5 to 85.7
150.0 to 175.O

vQOOj—ij)0(t>— I

R.F.C. R.FC.

FIG

This arrangement must be employed if the conden-
sers will not stand the high amplifier plate voltages

NOVEMBER, 1928

RADIO BROADCAST

No. 11.

RADIO BROADCAST'S Service Data Sheets on Manufactured Receivers

Freshman Model "G" Radio Receivers

November, 1928.

The Model " G " Freshman receiver is six-
tube tuned radio-frequency set having three
stages of tuned radio-frequency amplifica-
tion, a detector and two stages of audio-
frequency amplification. The receiver is
operated directly from 1 10- to 120-volt
(SO-cycle alternating-current house-lighting
mains and employs the G-60-S power-supply
unit to convert the current into the form
necessary for the operation of the receiver.
In normal operation the receiver has all its
grid returns grounded to the frame and
its filaments floating at pos tive potentials
above the frame to furnish the necessary
grid-bias voltages. The power-supply unit
contains a transformer for heating the vari-
ous filaments and a rectifier-filter system
for furnishing the plate current.

1. The Tuning System.

This receiver has four tuned circuits,
including a tuned antenna stage. The pri-
mary of the antenna r.f. transformer con-
nects to A and A?, the first terminal to be
used with a long antenna and Ai to be used
with a short antenna. Across the first tuning
condenser, Ci is connected a midget con-
denser Ci to permit adjusting the first tuned
circuit to exact resonance.

The method of preventing oscillations in
the r.f. amplifier is unusual. "Equaphase"
is the name which has been given to this part
of the circuit, as indicated in the circuit dia-
gram. The circuit, consisting of Ri, C« and the
effective primary inductance, of the r.f. transformer,
when properly adjusted acts like a pure resistance
at all frequencies and the tube cannot oscillate.

2. Detector and Audio System.

A leak -condenser system is used in the grid-
circuit of the detector in this receiver. This is
followed by a two-stage transformer -coupled a.f.
amplifier. A small fixed condenser in the plate
circuit of the detector bypasses the r/. current
to ground and provides the low-impedance path
for the r.f. current, essential if the detector is
to operate efficiently. No output device is included
in the set but it generally is advisable that one

be used unless the loud speaker is equipped with a
coupling transformer that can carry safely about 20
milliamperes, the plate current of a 17lA-type tube.

3. Volume Control.

The volume control consists of a high-resistance
potentiometer, R2, connected across the second-
ary of the first-stage audio-frequency trans-
former. The low-potential end of the secondary of
this transformer is grounded as is the movable
arm of the potentiometer.

4. Filament Circuits.

The filaments of the three radio- frequency and
first audio-frequency tubes are connected in parallel

227

and supplied with current from a 1.5-volt
transformer winding (Sa) in the power sup-
ply. The midpoint of this circuit is obtained
from the center-tapped resistor (R») con-
nected across the filament circuit. The
heater leads from the 227-type detector
tube are fed from the secondary (&)
which delivers 2.5 volts. The winding is
arranged with a center tap which is grounded
to prevent hum. The secondary Si supplies
5 volts for the 1 71 A-type power tube, and this
winding is also used to light the dial lamp.

5. Plate Circuits.

The plates of 226-type tubes in the r.f.
and first a.f. stages are supplied with a
plate potential of about 130 volts from the
power unit. The plate of the detector re-
ceives about 50 volts. The plate of the
power tube receives about 180 volts, which
is the maximum permissible voltage for a
171 A-type tube.

6. Grid Circuits.

At a plate potential of 130 volts, 226-
type tubes require a grid bias of about 9
volts which is supplied, in this receiver, by
the resistor R< connected between the ground
and the center-tapped resistor, R». This
resistor is bypassed by condenser Cs. The
grid leak of the detector is returned directly
to the cathode of the 227-type tube. A grid
bias of about 40 volts for the 171 A-type
tube is obtained by connecting a 2000-ohm resis-
tor (Ri) as shown.

7. Power Supply.

The B-power unit employs the power transformer
(T) with two secondary windings, 84 and Ss. The
280-type rectifier tube is supplied with filament
current from 84 and plate voltage from Ss. The filter
system consists of Li, La and Cio, Cn, and Cn, and
the voltage-dividing resistor Re with the necessary
by-pass condensers Cia and Cn connected across the
output terminals of the filter. Transformer T2 sup-
plies filament energy for the various tubes in the set.
The receiver is designed for operation on a 110- to
120-volt 60-cycles a.c. supply.

226

171-A

THE RECEIVER CIRCUIT

RADIO BROADCAST

NOVEMBER, 1928

No. 12.

RADIO BROADCAST'S Service Data Sheets on Manufactured Receivers

Freed 'Eisemann NR-80 Receivers

November, 1928.

NR-80 is one of the latest re-
** ceivers developed by the Freed-
Eisemann Company. The set is
mounted in a reinforced steel cabinet
and is designed for use on 1 10- to 120-
volt, 60-cycle alternating-current
supply.

TECHNICAL DISCUSSION

1. The Tuning System.

There are four tuning condensers,
Ci, C», Ca and Ci. In the antenna cir-
cuit is employed a special choke which
is designed to give somewhat greater
gain at the low-frequency end of the
broadcast band, thereby off-setting,
to some extent, the opposite charac-
teristic of a tuned r.f. amplifier. The
antenna circuit does not require any
tuning condenser. To compensate for
the slight variations in the tuning coils
of the individual stages small "vanes"
are used. These may be moved nearer
to, or away from, the main inductance
of the tuning coils, and are adjusted
at the factory to the correct position. Each r. f.
transformer is shielded as indicated by the dotted
lines. The various tubes are neutralized by connect-
ing neutralizing condensers, marked N.C. in the
diagram, from the grid of the tube to the secondary
of the following r.f. transformer.

2. Detector and Audio Systems.

The grid leak and condenser type detector used
in this receiver utilizes a 0.00025-mfd. grid con-
denser and a 2-megohm grid leak. The detector
tube is a type 227. The plate circuit feeds into a
two-stage transformer-coupled amplifier using 3:1-
ratio transformers. The 17lA-type power tube feeds
into an output transformer with a turns-ratio of 1 :1 ;
the purpose of this transformer is to keep the d.c.
plate current of the power tube out of the loud-
speaker circuit. The plate circuit of the detector
tube contains a condenser Cs, with a value of 0.001
mfd. to bypass the r.f. currents to the ground.

3. Volume Control.

The volume control of the receiver is located in
the r.f. amplifier. It consists of a variable resistor,
Ri, with a value of 2000ohms. It is connected directly
across the primary of the last r.f. transformer. As

N.C.

THE NR-8o RECEIVER

the arm of the unit is rotated in the direction which
reduces its resistance, it gradually shunts the prim-
ary winding and decreases the amount of signal
voltage which is fed into the detector tube.

4. Filament Circuits.

Four filament windings which supply current to
the tubes in the receiver are placed on the power
transformer. A 1.5-volt winding (83) supplies all the
226-type r.f. tubes, a 2.5-volt winding (So supplies
the 227- type detector tube, another 1.5-volt winding
(Si) supplies the first audio tube, and a 5-volt wind-
ing (Se) supplies the power tube. Two 1.5-volt wind-
ings, one for the r.f. tubes and one for the first audio
tube, are used so that a better hum balance may
be obtained. These windings are shunted by poten-
tiometers R2 and Rs, each of twenty ohms, which
are adjusted at the factory to the point of min-
imum hum in the loud speaker.

5. Plate Circuits.

Three different values of plate voltage are supplied
to the receiver by the power unit. The same value
of voltage is supplied to the plates of the r.f. tubes
and also to the plate of the first a.f. tube; this poten-
tial is approximately 100 volts. The detector re-

ceives about 45 volts, and 157 volts
is delivered to the plate of the
power tube. Individual 0.5-mfd. by-
pass condensers Cs, Ci and Cs bypass
the plate circuits of the various r.f.
tubes so that there will be no r.f.
current flowing through the power
unit where they might cause coup-
ling which would make the r.f. ampli-
fier oscillate. These condensers also
serve to bypass the audio- frequency
currents in the plate circuit of the first
a.f. stage.

6. Grid Circuits.

The grid circuits of the r.f. tubes
obtain a bias from the power unit and
somewhat greater bias is supplied
from the power unit to the first audio
tube. To obtain bias on the power
tube a separate resistor is used, R? in
the diagram, with a value of 1650
ohms. These various resistors which
supply bias are bypassed by var-
ious condensers in the condenser
block. If these resistors were not bypassed, an audio-
frequency voltage would be impressed back on the
grid circuits of the various tubes and either an in-
crease or decrease of amplification at certain fre-
quencies would result.

7. The Power Supply.

The power supply is a conventional one using a
280-type tube as the rectifier in a full-wave cir-
cuit. The filament of the rectifier is supplied with
current by a secondary winding (Si) on the power
transformer T. Plate voltage for the rectifier is
supplied by secondary Ss. The output of the rectifier
feeds into a filter system consisting of C», CM and
Cn located in the condenser block and Li and L^,
which are filter choke coils.

The output of this filter system in turn feeds into
the potential divider, R.i consisting of a number of
fixed resistors connected in series and having the
values indicated on the diagram. At the junctions
between these resistors wires are connected for ob-
taining the various voltages required for the correct
operation of the different tubes in the receiver.
Power to the receiver is controlled entirely by the
switch connected in series with the primary winding
of the power transformer.

171-A

*Det R.F.Fil. +Amp. DetHTR. lA.F.Fil. +Pvm 2A.F.FII.

DIAGRAM OF RECEIVER AND POWER SUPPLY

The Improved Knapp A-Power Unit

DETAINING a satisfactory source
of power for heating the filaments
of tubes in a radio receiver has
been one of the chief problems confront-
ing engineers since the early days of
broadcasting. Of course, from the electri-
cal viewpoint the storage battery is ideal
for the purpose, but broadcast listeners
and radio experimenters demand a device
which is capable of providing equally sat-
isfactory results and which does not re-
quire constant attention. When a receiver
is operated with a storage battery it is
necessary to recharge the battery and
add distilled water at quite frequent
intervals.

Filament-supply units consisting of a
power transformer, rectifier and filter
system have proved to be one of the best
solutions to the A-power problem, and
this article describes one of the most re-
cently developed devices of this type. It is
known as the Knapp A power, and was
developed in the engineering laboratories
of Knapp Electric, Inc., of Port Chester,
N. Y. The unit is available as a kit, i.e.,
in knocked-down form, and the infor-
mation contained in this article relative to
the electrical characteristics, assembly and
operation of the device was supplied by the
manufacturer.

The Knapp A Power converts current sup-
plied by a standard i lo-volt a.c. line into a
filtered 6-voIt d.c. supply, which is satisfactory
for the operation of standard radio tubes. The
rectifier and filter condensers of the unit are of
the dry-electrolytic type, thus avoiding entirely
the usual maintenance operation of adding dis-
tilled water to the cells. Also, because of the
enormous capacities which it is possible to ob-
tain with dry-electrolytic condensers, the a.c.
hum is eliminated almost entirely in the filter
circuit and cannot be detected with the usual
receiver.

From a practical viewpoint this A-power unit
has other advantages. When operated from a
i lo-volt a.c. supply it may be employed to sup-
ply filament current to any standard home-made
or factory-constructed receiver using up to 8
tubes of the 201 A type, or a combination of tubes
which requires a filament current of not greater
than 2 amperes. The unit may be connected
directly with the A binding posts of the receiver
without making any changes in wiring, and it is
connected with the house current by inserting a
plug in a wall receptacle. The efficiency of the
device is comparatively high; that is, it should
not increase the electric bill more than 25 or 50
cents per month.

SIMPLE ELECTRIFICATION SYSTEM

A NOTHER thing which should be pointed
*• out in connection with this power unit is
that it provides the nucleus for converting any
d.c. receiver into a completely-electrified set.
Many excellent devices are available which pro-
vide B and C potentials for the operation of all
types of receivers, thus the use of batteries may
be avoided entirely. Provision is also made in the
unit for controlling the power to the B-supply
unit with the switch employed for turning the A
power on and off; for this purpose a standard 1 10-
volt receptacle is included in the unit.

Power units of the type described in this article
are not a new development; in fact, they have
been in use generally for almost a year. In the

THE POWER UNIT WITH METAL CASE IN PLACE

March, 1928, issue of RADIO BROADCAST an arti-
cle entitled, "A New A-Power Unit" by Ralph
Barclay described an early model of the device
now under consideration. This power unit has
provided many readers with excellent results,
but the new model is improved considerably
and is capable of giving better performance,
especially with modern receivers which employ
amplifiers with better bass-note reproduction
characteristics. Also, in the July, 1928, issue of
RADIO BROADCAST the early model of this power
unit was the basis of the power unit described in
an article entitled, "An Interesting A-B-C-
Power Unit and One-Stage Amplifier" by J.
George Uzmann. This article served to illus-
trate how any home-made or factory-constructed
set could be electrified easily and inex-
pensively.

A glance at the pictures and diagram on these
pages shows how the new Knapp A power differs
from last year's model. In the first place, it is
completely enclosed within a metal case, more
pleasing in appearance and of more sturdy de-
sign. The electrical circuit has been improved
but is fundamentally the same as before. It was
found that the general use of high-quality
amplifier systems in 1929 receivers demanded
that the background of a.c. hum be reduced
still further than it was in last year's model,
and this improvement was effected by a change

VIEW OF POWER UNIT
WITH METAL CASE REMOVED

in the choke construction and through
the use of an extra high-capacity, dry-
electrolytic condenser. Another improve-
ment is found in the rotary switch which
replaces the old-style, plug-type voltage
adjustment. The result of these changes
is that the unit is not only more efficient
electrically but it requires less space in
the cabinet.

POWER UNIT VS. A. C. TUBES

D EFORE continuing further with this
*-* article it might be wise to clear up
one point which probably has entered the
minds of many readers; namely, why
should a power unit be employed for
supplying filament current for d.c. tubes
when it is possible to electrify a receiver
with a power transformer, wiring harness,
and set of a.c. tubes? Of course, both
systems possess merit, but it is safe to
say that the A-power unit provides the
simpler and more "sure-fire" method of
the two, as in order to employ a.c. tubes
it usually is necessary to make a number
of changes in the circuit as well as several
adjustments to reduce the hum. Also, the
arrangement of apparatus in a set may be such
that the constructor will not be able to eliminate
the hum without considerable difficulty.

When comparing the cost of equipping and
operating receivers with these two systems of
electrification several points must be taken into
consideration. With the power-unit method the
cost of electrifying the set is limited to the cost
of the power unit, but when a.c. tubes are
installed it is necessary to buy a power trans-
former, wiring harness, volume control, voltage
regulator, complete set of tubes, etc. With both
systems the current consumed from the i lo-volt
line is about the same, but the cost of replacing
tubes is two or three times as great when a.c.
tubes are used. In both cost and simplicity it
would seem, therefore, that the method of ob-
taining light-socket operation by the use of a
B-power unit in conjunction with an A-power
unit, is to be preferred to rewiring the receiver
for the use of a.c. tubes. A set will operate from
an A-power unit just as satisfactorily as from a
storage battery — and the power unit requires
no attention.

In selecting an A-power unit the amount of
current and voltage which it will deliver at the
output binding posts is another important factor;
that is, it is necessary to make sure that an ample
supply is available for heating the tubes of the
receiver, notwithstanding normal line voltage
fluctuations. In this connection two graphs
showing the voltage-regulation characteristics
of the Knapp A-Power are given in Figs. 2 and 3.
Fig. 2 shows the output voltages obtainable with
the switch, SW2, set at each of the eight contacts
when the input potential is 1 16 volts. Fig. 3
indicates the output of
the power unit with the
switch, SW2, on the
fifth contact when input
potentials of 95, 100,
105 and no volts are
applied.

The graphs described
above show that the
power unit is capable of
supplying about two
amperes of rectified cur-

rent at a potential of six volts, or in other words
it will deliver ample power for heating the fila-
ments of eight tubes of the 2oi/\, i I2A or lyi.A
types. Secondly, it is shown that by means of
the rotary switch, SW2, the voltage may be ad-
justed to the proper value for receiver whether
the line voltage is above or below normal.

CONSTRUCTION OF UNIT

THIS article has now reached the point where
the construction of the A power unit may be
considered. The reader interested in building the
power unit will find a complete list of the ap-
paratus used in this new model at
the end of this article. Also, there
are two pictures which show the
appearance of the unit, and a com-
plete schematic wiring diagram is
given in Fig. i.

The picture of the completed
power unit shows clearly the out-
ward appearance of the device. All
of the apparatus is housed within
the metal case and the voltage-
control switch is regulated by the
knob on the right of the panel. The
output binding posts are located in
the center of the panel near the top, and
at the left of the panel is a receptacle for
plugging-in the B socket-power unit. The metal
box mounted on the front panel conceals
the rectifier unit, the cord on the left is for
connection with the light socket and the cord
on the right is equipped with a switch for turn-
ing the unit on and off.

Fig. i shows the complete wiring of the unit.
T is a power transformer having a secondary with
eight taps for voltage control, and R is a full-wave
dry-electrolytic rectifier. The two heavy-duty
choke coils, which are of identical construction,
are located at Li and L2> and the three dry-
electrolytic condenser units, also of similar de-
sign, are connected at Q, Cj and Ca. P is the
receptacle for the power lead from B-power unit,
S\\'] is the off and on switch, and SW2 is the
voltage control.

The actual construction of the power unit is
very simple. The assembly divides itself into
three major steps; viz, mounting parts on base
plate, mounting parts on front panel and as-
sembly of mounting contacts for the rectifier
unit. After the parts have been mounted the
wiring may be accomplished quickly and
easily.

The base plate for the power unit is die cast
and has been drilled with all holes necessary for
mounting the chokes, transformer, condenser

RADIO BROADCAST

brackets, front panel and steel box body. First,
the two choke coils, LI and L2, are mounted in
place in the positions indicated in the picture.
Next, the transformer is fastened on the right
side of the base opposite the choke U so that the
taps are on the front edge. However, before
mounting the transformer the primary leads
should be arranged so that they pass through
the top and they should be scraped free from
insulation. To complete the assembly of parts
on the base the condensers are fastened in
place with their brackets, as indicated in the
picture.
The front panel of the power unit is supplied

FIG. I. SCHEMATIC DIAGRAM OF THE POWER UNIT

with all necessary holes drilled and tapped, and
the mounting of parts requires only a few mo-
ments' time. The receptacle for the B socket-
power unit is first mounted in the large hole
provided for it in the upper left corner,
the eight-contact switch is mounted in a
similar position on the right side of the panel,
and the output binding posts are located in the
two holes in the center of the panel near
the upper edge.

The assembly of the rectifier mounting con-
tacts is very simple. Five holes are drilled in the
front panel for the contacts which are fastened
in place with nuts and washers.

The wiring of the unit is shown clearly in the
diagram and an explanation of the connections
is hardly necessary. In wiring the complete
unit only eight feet of rubber-covered hook-up
wire is needed and, of course, the connec-
tions should be soldered if best results are
desired.

OPERATION

AFTER the construction of the Knapp A
Power has been completed the wiring
should be checked carefully, and then the unit
may be placed in operation. However, before
the unit is connected to a radio receiver it should
be operated for at least an hour without load.

NOVEMBER, 1928

This precaution is necessary to insure the elimi-
nation of all moisture which may have collected
in the condensers.

Through experience it has been found that best
results are obtained from the power unit when it
is used every day. On the other hand, if the
receiver is not used for a week or two the power
unit should be disconnected from the set and
operated for. an hour without load. Also, the
operation of the power unit may be improved
materially by operating it without the receiver
connected two or three hours each month.

After long service it will be found that the out-
put voltage of the power unit will begin to de-
crease and it will be necessary to
correct the control, Sw2, to a tap
giving greater voltage. The rectifier
unit will finally require replacement.
It is a very simple task to put in a
= new rectifier and these units are

1 ° available at all radio stores.

operating the power unit there
only one knob, Sw2, which may
be adjusted and this controls the
output voltage of the unit. In
adjusting this control the proper
setting may be determined by plac-
ing the switch at the lowest tap
which provides satisfactory performance, but a
much more satisfactory arrangement would be
to connect a o-io volt d.c. voltmeter across the
output binding posts of the power unit. The
meter is particularly valuable in districts
where there are frequent variations in the
line voltage.

LIST OF PARTS

HE following is a complete list of the ap-
paratus included in the Knapp A Power Kit

T — One Power Transformer

LI, Lj — 2 Heavy-duty a.f. choke coils

Ci, Cj. Cs — 3 Dry-electrolytic condensers

R — One Elkon dry-electrolytic rectifier unit

SWi — One Pendant switch and cord

SW2 — One Special 8-point switch, knob and plate

P — i Receptacle for B unit |

Attachment cord and plug

Celeron front panel

Base plate

Set of condenser brackets and clamp angles

Metal box and cover

Rectifier cover

Celeron connector strip

Rubber bushing
2 Output binding posts
i Roll of hook-up wire
Mounting screws, nuts, bushings, etc.

KNAPP A POWER

D.C.Oulput Power v«

A C Line Voltage Curvw

Switch Tap E

1 15

LOAD IN AM PERES-DC
4 6

NO OF'. AMPTUBESU01 A.112A.I71 Aj

FIG. 2.

1 I b

idADINAMPEFECDC

4 b

NO Ori,AMPTUBCSir01 A,11!A.171 A)

FIG. 3.

THESE GRAPHS SHOW THE OPERATING CHARACTERISTICS OF THE KNAPP A-POWER UNDER VARIOUS CONDITIONS

By R. F. GOODWIN

IN THE October number of this magazine the
writer introduced the "Vivetone 29" re-
ceiver. Readers of the article will recall that
in the design of the tuner quality and efficiency
were the main objectives. These same qualities
are outstanding in the power supply and ampli-
fier unit which has been designed to work
in conjunction with the tuner, and which ^^
is described in this article.

Power amplification is, of course, an
admitted necessity in the modern receiver.
Radio entertainment is no longer a nov-
elty or plaything, and radio builders and
owners now demand the quality of repro-
duction that comes only with power am-
plification and the use of an up-to-date
loud speaker, such as the dynamic cone.
This quality is assured in the Vivetone
amplifier by the use of a 310 tube in the
output and a scientifically designed
coupling device. It has enough reserve
power to eliminate the danger of overload
distortion, and it will be found that full
volume is seldom necessary for satisfactory
reproduction in the home.

The power supply incorporated in the unit
furnishes all the operating voltages for the am-
plifier and the tuner, and serves to make the en-
semble completely light-socket operated. One of
the most important features of the power supply
is the incorporation in it of an adequate means
of voltage control. This feature appears to have
been neglected in most of the designs of our
present all-electric receivers.

Practical control of voltage is accomplished in
the Vivetone power unit with two low-resistance
power rheostats, one being a jo-ohm voltage-
control rheostat, PR-O5O, which is connected in
series with the primaries of both the power trans-
formers, while the other is a o.2-ohm rheostat,
PR-2io, which is connected in series with the
ij-volt secondary winding of the low-voltage
transformer, T-2445. Fig ' shows the location and
wiring of these parts.

Many may wonder
why a rheostat was not
also connected in series
with the 2j-volt wind-
ing. The reason is, first,
that the 2s-volt wind-
Vigs of these trans-
formers generally have
the correct voltage,
whereas the ij-volt
winding will be found
to deliver slightly
more than its rating;
and second, the volt-
age-control rheostat
will take care of any
change in line voltage
that would increase or
decrease the voltage of
the 25-volt secondary
or any of the other
voltages after they
have been once ad-
justed. Therefore, only
one secondary rheostat
is required, which is
in the ij-volt circuit.

This method of voltage control is not auto-
matic; it need not be, because line voltage varia-
tions seldom occur more than once or twice in
24 hours, with the exception of dark stormy days
when the power station is heavily taxed. Under
such conditions the voltage control, PR-O5O, is

THE amplifier and power-supply unit described in this
article was designed especially for use in connection
with the "Vivetone 29" receiver which was presented in the
October issue of RADIO BROADCAST. However, equally suc-
cessful results may be obtained when the unit is connected
with other standard a.c. receivers. The amplifier of the unit
employs two transformer-coupled stages with a jio-type
tube in the output circuit. The power-supply circuit pro-
vides normal values of B potential for the receiver and am-
plifier, as well as the necessary a.c. potentials for beating
the filaments of the tubes. — THE EDITOR.

decreased until the meter on the receiver panel
reads one or two tenths below the proper specified
reading.

Although it was designed especially for the
"Vivetone 29" receiver, the amplifier power unit
can be used with other a.c. tuners as an audio
channel and power supply. It will operate ex-
cellently as a phonograph amplifier by connect-
ing a good electrical pick-up unit across P and B
plus of the first-stage transformer.

CONSTRUCTION

THE entire amplifier and power unit is con-
structed on a baseboard measuring 8" in
width and 10" in length, being J" thick. The audio
portion, as shown in Fig. 2 and 3, is mounted
at the left-hand end of the baseboard and the
supply portion on the right. A Micarta 8" by 20"
panel is used to mount the necessary resistances,
binding posts, a.c. outlet and speaker jack.

FIG. 2
Rear view showing layout of parts on baseboard

The audio portion incorporates two stages of
transformer-coupled audio amplification utilizing
the new Thordarson R-joo transformers. For the
output a choke and condenser device is used to
prevent the high voltage from damaging the
speaker windings. For the first audio stage a
type cx-327 tube is used, and in the last
j—^ stage a cx-3io power tube is required.

The power supply portion utilizes a
half-wave rectifier system using a cx-38i
tube. The voltage required to operate this
tube is procured from a Thordarson R-2io
power pack. It supplies the high voltage
for the plate of the rectifier and has two
filament windings, one for the rectifier
tube and one for the 310 power tube.
There are also the two necessary filter
chokes incorporated in the unit.

The low-voltage transformer, T-2445.
which supplies the voltage for the 326
and 327 tubes and the pilot lamp, is also
rather compact in design. It will be
noticed that this unit is mounted along-
side of the power pack, R-2io. Next to it is
the filter condenser block, PL-575, a 12-mfd.
unit properly divided with terminals placed so
as to simplify wiring.

Since the entire layout of the parts is so clearly
shown in the top view and the picture wiring
diagram, Figs. 2 and 3, it will be unnecessary
to give further details concerning their positions

OPERATION

A STUDY of Fig. i and the figures in the
*» article in the October number of RADIO
BROADCAST (page 367-368) illustrating the con-
struction of the Vivetone 29 receiver, will show
the constructor exactly how to connect the two
units for operation. The only operating adjust-
ment that is necessary on the power unit is the
adjustment of the voltage-regulating rheostats,
which is a very simple matter.

To obtain smooth voltage regulation from the
power unit is a simple
matter. With all the
specified tubes in their
respective sockets, all
the leads of the re-
ceiver correctly con-
nected to their proper
posts on the power
unit, and an a.c. volt-
meter temporarily
connected across the
heater terminals of the
detector tube socket
(type 27 tube) the
house power is turned
on, the secondary
rheostat, PR-2IO.
turned entirely to the
left (full resistance),
and the voltage con-
trol rheostat, PR-ojo,
turned entirely to the
right. Allow the detec-
tor tube about one
minute to heat up and
then adjust the setting
of the voltage control
rheostat, PR-O5O, until

RADIO BROADCAST

O Input

NOVEMBER, 1928

cally and electrically equivalent parts can be
made by the experienced set builder. — THE
EDITOR.]

Cost of Parts — Not mer $95.00

I I hordarson filament-supply transformer

type T-2245

fhordarson power pack, type R-2io
Thordarson choke coil, type R-ig6
Thordarson audio transformers, type R-3OO
Dubilier condenser block, type PL-575
Dubilier condenser, i.o mfd., No. 907
Dubilier condenser, 2.0 mfd., No. 903
Centralab heavy-duty potentiometer 10,000

ohms, type HP-oio
Centralab fourth-terminal potentiometer,

6000 ohms, type PF-6ooo
Centralab power rheostat, 50 ohms, type

PR-050
Centralab power rheostat, 0.2 ohm type

PR-2IO

Ward Leonard resistor, 10,000 ohms. No.

FIG.

507-11
Ward Leonard

507-14
Ward Leonard

resistor, 1000 ohms, No.
resistor, 225 ohms, No.

Schematic diagram oj Yivetone Power Unit

the meter registers approximately 2^, volts
(but not over 2j volts).

The meter is then disconnected from the de-
tector socket and temporarily connected to the
filament terminals of one of the r.f. sockets.
Then the position of the secondary rheostat,
PR-2IO, is corrected until a reading of approx-
imately i£ volts is obtained. With this done the
meter is connected permanently to the heater
terminals of the detector socket. The setting of
the secondary rheostat is then permanent and is
not to be disturbed.

To complete the regulation process, the proper
B voltages are to be determined. To accomplish
this a high-resistance voltmeter would greatly
simplify matters but for the benefit of those who
have no meters the position of the arm of HP-oio
on the power unit should be approximately half-
way between both end terminals, whereas the
arm of HP-6ooo should be approximately three
quarters of the way to the right (farthest from the
negative end). The fourth terminal of PP-6ooo
should be approximately \ of an inch from the
negative end terminal. This completes the regu-
lation of voltages.

Now if a line variation occurs it will be indi-
cated by the a.c. voltmeter, and only the voltage
control rheostat, PR-O5O, need be manipulated
to compensate for the line variation. In other
words, by increasing or decreasing the resistance
of this control all the voltages, a.c. and d.c., will
increase or decrease respectively. By keeping the
a.c. meter at approximately 2iV volts all these
voltages will be permanently correct.

In tuning the receiver there is really nothing
difficult. After the current has been turned on,
the tubes should be given at least a minute to
heat up; then signals should come in with great
volume and without hum. Should there be any
noticeable hum it can be eliminated by correcting
the arm position of the potentiometer, PP-OI5,
on the receiver chassis. 1 1 should be remembered
that the arm of the resistor, PP-2OOO, on the set
chassis should be approximately one third of the
way from front end terminal when the condenser
plates are completely meshed.

Should the reader desire complete construc-
tional blueprints or detail information concern-
ing the function of the receiver they may be
obtained by addressing the writer in care of this
magazine. The blueprints are priced at $ i.ooand
are sold at their actual cost, including postage.
They consist of complete full-size wired layout
drawings, schematics, panel layouts, etc. of both

Vivetone receiver and power unit number one.
Blueprints describing another power unit have
also been prepared. This unit consists of a push-
pull 310 power amplifier utilizing a 327 in the
first stage, as in power unit number one, and a
full-wave rectifying system using two 381 recti-
fier tubes. This unit is suggested for those who
require tremendous volume and wish to use
more than one speaker.

LIST OF PARTS

[The list below gives the parts used in the unit
described by the writer. Since all the parts are
of standard design, the substitution of mechani-

507-20

Benjamin five-prong, green top socket, No.
9036

Benjamin four-prong, black top socket,
No. 9040

Benjamin four-prong, red top socket, No.
9040

Yaxley junior jack. No. 701

roll flexible " Braidite" wire

roll Solid "Braidite" wire

Eby Binding posts

Westinghouse Micarta panel, i|" x n"

Westinghouse Micarta panel, i" x 2"

Westinghouse Micarta panel, 20" x S"

Nuts, screws, etc.
For operating the unit in conjunction with an
r. f. tuner, the following accessories are required:
I Cunningham cx-38i rectifier tube
I Cunningham cx-3io power tube
I Cunningham cx-327 tube

FIG. 3

Complete picture wiring diagram

"Our Readers Suggest

9 9

OUR Readers Suggest" is a clearing house
for short radio articles. There are many
interesting ideas germane to the science of radio
transmission and reception that can be made clear
in a concise exposition, and it is to these abbrevi-
ated notes that this department is dedicated. While
some of these contributions are from the pens of
professional writers and engineers, we particularly
solicit short manuscripts from the average reader
describing the -carious "kinks," radio short cuts,
and economics that he necessarily runs across from
time to time. A glance over this "Our Readers Sug-
gest" will indicate the material that is acceptable.

Photographs are especially desirable and will be
paid for. Material accepted will be paid for on pub-
lication at our usual rates with extra consideration
for particularly meritorious ideas.

— THE EDITOR.

Short-Wave Plug-in Coils

THE idea of using "dud" tube bases as a
mounting for short-wave coils is certainly
a good one, but some fans seem to have
difficulty in making a really rigid job. The
method illustrated in Fig. i has been in use in
the writer's set for some time, and although the
coils are subject to a lot of rough usage, they
are just as strong and rigid as when first made.
The'solution to the difficulties associated with
these coils is found in winding the coils them-
selves on separate forms rather than on the tube
base. This also provides greater latitude in wind-
ing specifications.

The coils are wound on a piece of thin bakelite
tubing somewhat larger in diameter than a tube
base. Holes 35a inch in diameter are drilled 120°
apart around the bottom edge of the tubing to
admit a a'j machine screw. Holes are also drilled
and tapped 120° apart about J inch from top
edge of the tube base. The tubing is supported
from the base by small collars made of brass
tubing, and cut just long enough to fill the space.
The whole is held together with & round-head
machine screws, as illustrated in Fig. i.

After the coils are wound, they are given a
coat of cement made of celluloid dissolved in
amyl acetate which insures the wire remaining
tight. The number of turns to use is not specified,
as this information has appeared several times
in RADIO BROADCAST and of course varies
with the tuning condenser
and also with the diameter
of the coil. The reader is re-
ferred particularly to the de-
scription of the Cornet receiver
of Lieutenant Wenstrom, which
appeared in September RADIO
BROADCAST.

Some readers may find diffi-
culty in removing the vacuum
tubes from their bases. The
easiest method seems to be to
pour a little wood alcohol
through a small hole drilled in
the base. After standing awhile
the cement will be softened
enough to pull the tube and

base apart, when the solder is melted from the
prongs.

C. S. TAYLOR, Ft. William, Canada.

Receiving Without an Aerial

HTHE writer has discovered that his particular
* Atwater-Kent receiver functions quite satis-
factorily when operated without an aerial, the
ground wire being connected to the antenna
post, and the ground post left unconnected.
Operation is more selective and good volume is
had on fairly distant stations. Occasionally a
variable condenser, in series with the ground
lead, is effective.

GEORGE N. COOK, Allenwood, Pa.

Bakelite Tube,

Tube Base

Supporting.--'
Collars

FIG. I. COIL KINK

Tube-base plug-in coils of very rugged construction

may be made by mounting a bakelite tube on an old

tube base.

Polarity Indicators

IT IS often necessary to find the polarity of a
' line or pair of wires from a battery. In the
absence of a meter the following methods may be
used to determine the polarity of a source of
direct current.

A small section of blueprint paper, such as
is used in reproducing mechanical drawings,
should be moistened and the two leads brought
into contact with the surface. For voltages up
to one hundred, the leads should be separated
about one-half inch; for higher potentials this
distance should be increased correspondingly.
When the two leads are removed there will be a
small bleached section where the negative lead
touched the paper.

Another simple test for polarity can be made
with a potato. The potato is
cut in half. Stick the two wires
to be tested into the freshly
cut surface, about one inch
apart. In a short time you will
find that the potato has turned
green around the negative
wire.
J. B. BAYLEY, Jersey City, N. J.

To"A Unit I
FIG. 2

When a.c. tubes are used this
simple circuit will often effect
a marked reduction in hum.

STAFF COMMENT

The blueprint paper should
be partially exposed before
being used for the test. The
potato idea is an old one but
worth reprinting for the benefit
of the new fan. The salt-water

test is also simple and definite. Place two leads
from the doubtful d.c. source into a glass of
salt water. Bubbles will rise to the surface about
both leads, but more bubbles will surround the
positive wire.

Reducing Hum in the Detector

\A7HEN a combination of A and B
" socket-power units is used with d.c. tubes,
a slight amount of a.c. hum generally is present
in the loud speaker output. This hum can be
reduced materially, if not eliminated, by con-
necting a 3O-ohm potentiometer across the fila-
ment terminals of the detector tube. The grid
return is then connected to the slider contact of
the potentiometer, instead of to one side of the
filament. When the potentiometer is adjusted
to its mid-position, or approximately so, practi-
cally all trace of a.c. hum should disappear. A 1.5-
volt flashlight cell, connected as in the diagram,
Fig. 2, will provide The positive or negative grid
bias required for the operation of 201 A- or 2OOA-
type tubes respectively.

CHARLES. D. SAVAGE, Portland, Ore.

Reducing Hum in A.C. Sets

A GREAT deal of the hum in an a.c. home-
** constructed set comes from parallel leads
and leads placed too close to the power trans-
former. A simple way to overcome this difficulty
is to use the conventional automobile "BX"
cable for all filament wiring and then ground the
metal casing on these leads as well as the trans-
former shell. This trick has cured more than one
baulky set.

J. B. BAYLEY, Jersey City, N. J.

STAFF COMMENT

The remedy suggested by Mr. Bayley will be
effective in many cases. Shielding of the type he
suggests should always be used, as a precaution,
in wiring a.c. amplifiers for television reception.
Small size BX cable can be obtained at any auto-
mobile-supply store or large garage.

Improving Capacity Feed-Back
Regenerative Circuits

1HAVE just built the adapter described in your
July issue and, while I think it provides the
last word in satisfactory performance, I observe
the presence of the familiar trouble of jumping
suddenly into oscillation. The setting of the plate
condenser is too critical for voice. It follows the
tuning condenser too closely, to say nothing
of the fact that the dial setting of the tuning
condenser is affected by changes in the coupling
condenser. This is an objection I have noted in
all receivers employing this system of oscillation
control.

However, as soon as a universal-range Claro-
stat was connected across the tickler, these ob-
jections disappeared. The variable resistor be

RADIO BROADCAST

NOVEMBER, 1928

FIG. 3. SIMPLE OUTPUT CIRCUIT

With this arrangement it is possible to change from
phones to loud speaker without disturbing the tun-
ing adjustments.

comes the main control and should be mounted
on the panel and the midget condenser, now an
auxiliary, is relegated to the sub-panel. The
over-all flexibility, ease of tuning, and reduction
of body-capacity effects show a marked im-
provement.

E. W. MATTHEWS, Augusta, Ga.

STAFF COMMENT

The control of regeneration by means of a
variable resistor connected across the feed-back
coil is not new, but it seldom is employed in
capacity-controlled circuits. However, as a
matter of general principle, the use of a con-
tinuously variable resistor, so connected in any
regenerative circuit, will provide the smoothest
possible regeneration. The resistor, as suggested,
is connected directly across the terminals of the
tickler coil.

Tuning-in Wiib a Distant
Loud Speaker

DEGENERATIVE detectors, followed by
^ two audio stages, are found in many mod-
ern and ancient receivers. When the speaker is
moved to some remote point, the operator is
confronted with the problem of tuning the set
by the phones. This is easy enough to accom-
plish in itself, simply by plugging into the detector
jack, but it will be found, as a rule, that when
the phones are removed, the constants of the
plate circuit are so changed that the point of
optimum regeneration is passed, and the set
may even break into oscillation. If we plug
into the loud-speaker jack, we get unbearably
loud signals which may injure the phones, to say
nothing of the ears!

There are many what might be called "ortho-
dox" ways of overcoming this difficulty, probably
the simplest of which is shown in Fig. 3. The
value of the resistance R depends upon local
conditions. Between 5000 and 10,000 ohms will
fit most cases, but the best way, if you have a
Clarostat or Royalty or other make of variable
high resistance handy, is to connect it at R, set
it to a_ comfortable volume of an average signal,
measure the resistance and buy a cheap power-
unit resistor near the value found, which is wired
into place permanently.

W. BRUCE Ross, Westmount, P. Q.

STAFF COMMENT

A perhaps more universal and equally simple
arrangement for tuning with the telephone re-
ceiver, is to shunt a Centralab modulator plug
(or any variable resistor having a range between
one hundred and two thousand ohms) across the
telephone receivers and plug the phones into the
loud speaker jack. The volume can be adjusted,

to a comfortable degree, by means of the resistor,
and the set always is tuned to approximately
the same volume.

Home-Made I.F. Transformers

IN THESE days of cheap retail radio prices,
there are few pieces of apparatus that the fan
will find it worth while to make. However, the
intermediate r.f. transformers for a super-
heterodyne are an exception. Efficient trans-
formers may be made easily at a considerable
saving by the super-heterodyne enthusiast.
The following notes describe an intermediate
transformer, designed for 201 A or similar tubes,
having a natural wavelength in the circuit of
about 2000 meters — giving a wide separation
between repeat tuning points on the oscillator
dial.

Two discs of wood J inch thick by i J inches in
diameter and a wooden core J inch thick by J
inch in diameter are required for each transform-
er. These pieces are assembled as in Fig. 4.

Small wire brads may be used to fasten the
discs to the core. Before winding, two holes
should be drilled in each disc, near the outside
for connecting terminals.

The windings consist of 200 turns for the pri-
mary and 800 for the secondary of number 32 s.c.c
or enameled wire. The wire should be wound
haphazard fashion. First wind the primary over
which place a layer of thin paper and then wind
on the secondary in the same direction as the

I A-

±_

FIG. 4. DETAIL OF SPINDLE

The coils of an intermediate-frequency transformer
may be wound on this simple spindle. Heavy card-
board may be used for the end pieces if desired.

primary. Boil the completed unit in paraffine
(make sure that the paraffine is not hot enough to
smoke) for about a half hour. Terminal posts
may now be inserted in the four holes drilled
in the discs and marked PRIMARY, Plate and B
plus, and SECONDARY, Grid and Filament. The
start of the primary is connected to the B plus
and the finish to the plate. The start of the
secondary goes to the filament and the finish to
the grid.

No matching of the transformers is necessary
as they were found to tune close enough.

No filter circuit is required as the combination
of two or three of these transformers provides
a band-pass of about the correct width.

R. W. TANNER, Springfield, Ohio.

Prolonging the Life of the ijiA

HpHE Laboratory Information sheet number
* 204, appearing in July issue of RADIO BROAD-
CAST contained information concerning the
short life of the lyi-type tube, used in a.c. sets.
As a radio service man with considerable ex-
perience in servicing electric sets, I can name
two causes for the trouble, other than fluctuating
line voltage

I believe that the greatest source of difficulty
arises from using lyiA-and }yiA- type tubes in
sets which were designed for the 171 or 371 types.

As these newer tubes draw only one-quarter
ampere filament current, naturally the filament-
supply winding on the transformer, which has
been designed for a one-half ampere load, will
provide an increase in voltage, and soon ruin a
quarter-ampere filament. This same effect has
also been found in battery sets where the audio
tubes are controlled by a fixed resistor.

The remedy is extremely simple. Shunt a 20-
ohm resistor across the filament terminals of the
tube sockets, thereby adding another quarter-
ampere load to the circuit, and the tube will
then have a normal life.

Another trouble which 1 have run across is-
due to a low value of grid voltage on the tube.
This is sometimes caused by a variation in the
resistor which provides the voltage drop for the
negative bias. When this grid voltage is low,
the tube draws an excessive plate current, which
soon lowers its electron-emitting characteristic.

This can be prevented by adding sufficient re-
sistance to the C-bias resistor, to bring the
grid voltage up to the proper value to balance
the plate voltage, which figure may be deter-
mined from the tube chart, supplied by the man-
ufacturer of the tube. Probably an additional
loo-ohm unit would be good as a trial resistance.

I cannot recall a single instance of power tube
failure, where the tube itself has been at fault.
A. H. GOUD, S. Portland, Me.

STAFF COMMENT

If it is possible to locate conveniently the
resistor through which the filament current to
the power tube is fed (in the case of d.c. sets
designed for the 171 tube), this can be replaced
by a 4-ohm resistor, or any quarter-ampere
ballast resistor, with the same result.

Special Soldering Irons for
Difficult Jobs

CET-BUILDERS and repairmen are running
'-' continually into odd soldering jobs which
require a great deal of valuable time and are very
trying on the patience. A set of irons similar to
those illustrated in Fig. 5 will make soldering
less troublesome.

The general type, shown in diagram A, is
followed by the manufacturers of both electri-
cally and externally-heated irons.

Mounting the heads at right angles to the
shafts as shown in drawings B and C greatly
facilitates working in out-of-the-way places.
Drawing D shows a third modification with the
head at a forty-five-degree angle to the shaft
making an arrangement that should do the trick
when all others fail.

A copper bar, one-inch square, was used for
all the irons mentioned and found to be quite
satisfactory.

GEORGE VV. LINN, New York City.

(C;

FIG. 5. SPECIAL SOLDERING IRONS

Difficult soldering jobs are greatly facilitated
by using home-made irons of special design.

THIS PICTURE SHOWS THE ARRANGEMENT OF APPARATUS ON THE CHASSIS OF THE RECEIVER

By BERT E. SMITH

Aero Product^ Inc.

rHEN the course of research, which was
undertaken for the purpose of deter-
mining the best type of circuit arrange-
ment, as well as coil construction for the screen-
grid tube, left no doubt that the final method
evolved gave the optimum results not only with
this tube but also with standard a.c. and d.c.
tubes, it was decided to proceed with the con-
struction of a receiver utilizing this arrangement
which, from its characteristics of adjusted phase
relations, was called the "Chronophase."

Inasmuch as the "Chronophase" receiver was
being planned to secure "DX" reception, through
strong local interference, it was decided that a
signal strength of one per cent., fifty kilocycles
on each side of the resonant frequency, was the
maximum allowable. With an experimental set-
up entirely unshielded and with the coils
separated by eight inches, it was found that such
selectivity was secured by the use of two stages
of radio frequency utilizing a zoo-foot antenna
approximately 50 feet high.

Next came the question of audio-frequency
amplification. The most powerful audio equip-
ment which can be built into a receiver, in most
instances, will employ a I7i-type tube which
has an amplification factor of about three. With
this tube in the last stage 650 milliwatts of un-
distorted output is obtained under proper load
conditions when an alternating potential having
a value of 28j volts r.m.s. is impressed on the
grid. A transformer recently has been made
available of a type with a much flatter curve
over the audible range than is ordinarily obtain-
able and continuing the flat portion of this curve
out beyond the audible range so that there will
be no loss of overtones and harmonics. These
transformers give a voltage amplification of 3, so
if we use a 20 1 A-type tube in the first stage of the
audio amplifier, we have an amplification factor
between the detector output and the input to the
grid of the last audio tube of 3 x 8 x 3 or 72.
Since our tube required a maximum voltage of
28.5 r.m.s. to secure its greatest possible output,
the corresponding detector output would have to
be about 400 millivolts which is readily obtain-
able following two stages of r.f. amplification

with a voltage amplification of about eleven per
stage, which is provided by the "Chronophase"
stages.

MECHANICAL DESIGN

LJAVING both the audio-frequency and

* radio-frequency amplifiers determined, it
was now only necessary to design the receiver in
such a way that no serious losses would result in
the layout and construction.

Always presuming that the coils and con-
densers of a radio-frequency amplifier are of the
best quality obtainable, one of the most prolific
sources of difficulty has been found to be in long
leads, particularly in the grid circuits which are
prone to pick up unwanted signals. The plate
leads come next in sensitivity to external in-
fluences. In order to keep both of these as short
as possible, the arrangement of parts indicated in
the picture was employed. Stators of the conden-
sers, which perforce must be in the grid circuit
anyway, are used as grid leads, allowing the
placing of each tube very close to the subsequent
radio-frequency transformer, and permitting
extremely short leads. A triple condenser with
trimmers for compensating any slight differences
in capacity is used to tune the amplifier. It was
deemed advisable because of antenna variations
to put the midget condenser, used as a vernier for
the first stage, on the panel. A variation of this
was required on both sides of the normal ca-
pacity of the condenser.

The illustration shows the complete layout
finally adopted for the receiver. The Central
Radio Laboratories manufacture a special step-
less variable resistance for the stabilizing control
which can be adjusted to hair-line accuracy.
The Allen-Bradley grid leaks used are built from
a solid block of non-hygroscopic carbon this unit
is unaffected by weather changes, making it
possible to solder the grid leak permanently into
place without using the ordinary grid -leak clips
which are a fertile source of trouble, due to
oxidized and poor connections and consequent
noisy operation.

The fixed condensers were also selected with
great care, since they are in such a position in

the "Chronophase" system that too great a
phase-angle difference in the dielectric would
seriously affect the operation of the circuit.

An inspection of the picture will show that
there are practically no connections on the top of
the subpanel other than those running from the
stators of the variable condensers to the No. 6
terminals on the coils. The coils are mounted
with three machine screws, and the o.ooi-mfd.
condensers between the plates of the tubes and
the taps on coils are connected to the mounting
screw between terminals 3 and 4 on the under-
side of the panel. A piece of flexible wire is at-
tached to the top of this screw, the other end of
which can then be connected to whichever ter-
minal of the coil gives the desired results as will
be explained later.

WIRING THE RECEIVER

AFTER making these connections, wire up
the filament circuit, twisting the leads into
a cable which more or less follows the outside
lines of the sides and back of the panel. All fila-
ment, B-plus, and C-minus wires can be twisted
into the cable wherever convenient, but the plate
and grid leads should always be kept free, with
the exception of the audio output lead which
can be cabled with everything else.

When the wiring job is completed the set can
be tested out before being put into the cabinet
by placing an old dial on the condenser shaft
and attaching the oscillation control and midget
condenser with flexible leads. The circuits should
be balanced, as will be explained in another
paragraph, after it is mounted in the cabinet by
attaching the mounting pillars to the screws in
the subpanel in such position that they will be
exactly over the bottom holes in the cabinet.
Lower the set into the cabinet with the front end
down and start the condenser shaft into the dial,
and then dropping it back in until it fits into
place. Now connect the flexible leads which were
left for the midget condenser and resistance,
tighten up the screws in the holes in the bottom
of the cabinet and the set screw binding the dial
to the shaft, and the set will be ready for
operation.

The matter of balancing up the set is not at all
difficult but will require quite a bit of time. In
most cases, regardless of the type of tube used,
the set will be found to operate to the best ad-
vantage with the antenna on tap No. 2, the first
radio-frequency coil connected to tap No. 3,
and the second radio-frequency coil at tap No. I
or tap No. 2. Fora comparatively short antenna,
it may be desirable to put the tap on the antenna
coil on No. 3 or No. 4, and for maximum selec-
tivity with a long antenna, tap No. I should be
used.

BALANCING PROCEDURE

A SUGGESTED method of procedure for
*» balancing is as follows:

After connecting the taps as suggested, tune-
in a station which will give a fairly weak signal
when the set is not oscillating. Turn up the stabi-
lizer until the set oscillates. Then, with the set in
oscillation, adjust the trimmer units on the two
rear sections of the multiple condenser until
maximum volume is obtained, retuning to a
weaker station if necessary, so that while adjust-
ing, the volume is always kept at a comparatively
low value. When perfectly balanced, the set
should go into oscillation with the stabilizer
about one third of the distance from minimum
resistance, and should go in and out of oscilla-
tion at the same point ; that is, it should be un-
necessary after the set breaks into oscillation to
retard the control beyond the point where it
went into oscillation to clear it up again. If
the set does not oscillate easily enough, raise the
tap on the detector input coil. If it oscillates too
easily, reduce this to tap No. i. If insufficient
selectivity is obtainable even with the antenna
coil at tap No. i, reduce the tapped portion of
the middle coil. With everything properly ad-
justed, the removal of the cap from either of the
screen grid tubes should practically stop the set
from operating, although if it is tuned to a
strong local station, the removal of the cap from
the second tube may leave a trace of signal.
Touching the first section of the tuning condenser
should completely stop operation. Touching
the second section should almost stop operation
and touching the third stage, should reduce the
volume considerably.

If the set is lacking in volume, or if a continu-
ous high-pitched whistle or growling note is
heard, a i.o- or 2.o-mfd. condenser should be
connected across the B minus and B plus 180-
volt taps.

RADIO BROADCAST

Using the particular receiver described in the
article, on a zoo-foot antenna, sufficient selec-
tivity was obtained to bring in woe while WEBH
and WBBM were both in full operation, although
WEBH is located within a mile of the point where
the set was tested. With the same adjustments
exactly, and during the same evening, the set
brought in stations over a thousand miles distant
with full loud-speaker volume. Over a short
period of testing, stations on both coasts were
received.

The need for careful adjustment of both taps
and trimmers on the multiple condenser cannot
be too strongly emphasized. If either adjustment
is not correct, there will be a pronounced lack of
selectivity and the receiver will probably bring
in nothing but local stations.

OUTPUT DEVICE NEEDED

IT PROBABLY will be noticed that no provi-
' sion has been made on this receiver for an
output device. Many of the modern types of
dynamic speakers contain a transformer and,
accordingly, it is unnecessary or desirable to
have an additional transformer built into the
receiver. Furthermore, some builders prefer a
choke and condenser output coupling, while
others are strongly in favor of an output trans-
former. So with the idea of leaving the receiver
as versatile as possible, the matter of output
device was left to the builder.

Under no circumstances, however, should the set
be used without some kind of satisfactory device
to keep the direct current from the loud speaker
windings.

[NOTE: The voltages suggested by the author
for use on the screen-grid tubes are higher than
those recommended by the manufacturers of
these tubes. Normal screen-grid, control-grid,
and plate potentials are 45, 1.5 and 135 volts,
respectively. The simplest way to get such volt-
ages to the proper places within the set is to con-
nect the screen-grid leads to the blue 4j-volt
lead which at present is connected only to
the primary of the first audio transformer. To
get 135 volts on the plates of these tubes it will
be necessary to run a separate lead to a 135-volt
source, or to operate the power tube at this
voltage when no change in the wiring will be
necessary. The lo-ohm resistor Ri should be
connected in the plus filament lead to the two
screen-grid tubes instead of in the negative
lead.

NOVEMBER, 1928

In the Laboratory the difference between
operating the receiver at normal voltages and
those recommended by the Aero Products
Company was hardly noticeable, although this
is evidently not true in all cases, since the Aero
Company advises that they obtained much
better results with higher voltages. The disad-
vantage of using higher than the rated voltage
is that shorter tube life results. — THE EDITOR.]

LIST OF APPARATUS

'"p H E following is the list of parts recommended
1 by the author of this article. Other parts of
similar characteristics may be used if the con-
structor desires. The coils are 2" in diameter,
wound with 90 turns of No. 22 B & S d.c.c. wire
air spaced .005 inches and tapped as shown on the
circuit diagram. The list price of the parts in
kit form is $74.50. — THE EDITOR.

Ci, C2, C» — i Aero gang condenser, .ooo35-mfd.,

Type AE-2IJ5

Cj — i Aero midget condenser, No. 940
Q, C7, CH> — 3 Aerovox mica condensers, .001-

mfd.

Cg — i Aerovox mica condenser, .oo3-mfd.
C« — i Aerovox mica condenser, .ooo25-mfd.
LI, Lz, U — i Aero coil kit, No. U-2O3
L», Ls — 2 Aero NoSkip chokes, No. C-6o
RI — i Aero variable resistor, Type AE2JO
R« — i Bradley Grid leak, 3-megohms
R» — i Daven ballast, No. 5
R» — i Yaxley resistance, No. 810
TI, Tj — 2 Aero audio-frequency transformers,

Type A £770

Y — i Yaxley cable connecter. No. 669
A — Aero split-brass bushings, Type S-i

1 National dial, Type E

3 Kurz-Kasch walnut knobs, No. 98

2 Carter screen-grid connecters, No. 342
i Eby junior binding post

i Aero cabinet. Type 400

i No. 20 drilled Formica base-panel with all

sockets, grid-leak clips, etc. mounted
9 Kellogg solder connecters, No. 2
50 ft. Rubber-covered stranded hook-up wire

4 ft. Rosin-core solder

5 Aero 84 mounting posts

3 Aero 83 bakelite bushings

i Aero No. 3 screw assortment

Brass machine screws R. H., N. P., 5!" x &"

WAM.

0.001 mfd.
» Ti ,,CS

0000257
0.001 mfd mfd. / UT

VT,

VT5

I, .1 ,-l , - 90 turns No. 22 D.C.C.
on 2*formT<|g it 5,10,18.30 turns

FIG. I. COMPLETE SCHEMATIC DIAGRAM OF THE "CHRONOPHASE" RECEIVER

Who Are

tlie Institute?

BY CARL DREHER

A FEW years ago, before industry deprived
me altogether of leisure, I was returning
by train from a walking trip in the
country which had lasted several days and left
me with a coat of tan and a considerable growth
of beard. The train boy, happening to speak to
me when I purchased some of his wares, began a
sentence with the words, "All right, sir," but,
glancing up at me as his lips formed the sibilant
of the respectful title, and beholding my sweater
and beard, he changed it hurriedly to " Feller."
He was a class-conscious train boy. I reflected
that the word Fellow has numerous meanings,
and 1 could not tell whether he intended the
one numbered in Webster 2. A companion;
comrade; associate; contemporary or 3, An
equal in power, rank, character, etc.; or perhaps
even 6, A man of low breeding or of little worth;
but I bought the lemon drops anyway.

Little did that snobbish youth know that the
term which he used disparagingly also has
learned associations, and, in scientific circles,
entitles one to all the honors of a bishop among
the men of God. Nevertheless, it is so, as he
would see were he to consult the 1928 Year Book
of the Institute of Radio Engineers. Of a total
of 4200 members on December i, 1927 (it is
probably thirty or fifty per cent, greater by this
time), 3543 were resident in the continental
United States, the Canal Zone, Philippine
Islands, and Alaska, and of all these radio en-
gineers and aspirants under the flag of the
United States only 74, I ascertained by a statis-
tical investigation, are Fellows. It is easy, if
one is of normally good character, to become
an Associate Member. Full Members, although
not born, may be made. But a Fellow! He is one
of the 2 per cent., and a rare zoological speci-
men even at a meeting of the Institute. Contrary
to the advertisements celebrating the prevalence
of a certain affliction, four out of five cannot even
hope to get it, and most of the remainder are
turned down by the Committee on Admissions,
of which Dr. Frank Conrad was Chairman in
1927, while Mr. R. A. Heising holds the helm
this year. In 1927 10 Members had the temerity
to seek admission to the Fellow grade; six were
admitted, while four applications were denied.
One Fellow was elected directly.

The Constitution of the Institute provides
that a Fellow shall not be less than thirty years
of age. He may be elected under one of several
provisions. If he is a radio engineer by profes-
sion, "he shall be qualified to design and take
responsible charge of important radio work; he
shall have been in the active practice of his pro-
fession for at least seven years, and shall have
had responsible charge of important radio work
for at least three years." Asa professor of physical
science or of electrical engineering he is subject
to equally severe requirements, if he seeks the
honor in that section. By " notable original work
in radio science," inventions, or contributions
to literature giving an applicant ' a recognized
standing at least equivalent" to the above he
may also hope to become a Fellow. Under these
provisions noted radio executives, like Mr.
Sarnoff and Mr. Young, are admitted to the
august company of their own chief engineers,
and.'like them, pay annual dues of $i 5, but they
don't have to read the Proceedings unless they
want to.

Moved by a perhaps unseemly curiosity, I re-

cently read through the list of members of all
grades published in the 1928 Year Book to see
what the 74 indigenous Fellows did during their
business hours. Thirteen, I found, are engineers
employed by Radio Corporation, General Elec-
tric, and Westinghouse, and two are non-engi-

CHIEF OF THE INSTITUTE
Dr. Alfred N. Goldsmith, president,
the Institute of Radio Engineers

neering employees of this group of companies.
Ten are engineers for the American Telephone
and Telegraph Company. Seven are engineers
for miscellaneous radio concerns. Six are con-
sulting engineers. Eleven are executives. One is
in the sound-movie business. Two are definitely
outside of radio in their business connections.
Professors of engineering or allied subjects at
universities number 6. There are 3 army or navy
officers, and 5 in civilian government positions.
In 6 cases no vocational information is given.

My object in printing this is to render some
of the impassioned opponents of the big cor-
porations privy to the hitherto unrevealed fact
that over a third of the leading radio engineers
of the country are already in the hands of these
octopi, and all of them may be swallowed unless
Congress does something about it.

Other interesting facts may be gleaned from
the Year Book. As they are published for all to
see, I may be permitted to comment on them
without scandal. Note, that Major E. H. Arm-
strong, indubitably one of the genuinely great
men of radio, did not become a Fellow until
1927. He could have been one any time after
he wrote his classical paper on the vacuum
tube, in 1915, or as soon thereafter as he became
30 years old. Why didn't he bid for the honor
sooner? Maybe he just didn't give a damn;
that would be like Armstrong. W. R. G. Baker,
Managing Engineer of the Radio Department
of General Electric, is still down as an Associate;

he has since come up for the grade of Fellow,
however. More than one of the luminaries skip
the Member grade, apparently. Stuart Ballan-
tine became a Fellow in 1928, also, rising from
Associate; he must be one of the youngest among
the Fellows, but his recondite and brilliant re-
searches deserve the honor. Harold H. Beverage
remains a Member in the 1928 Year Book. Mr.
H. O. Boehme is also down as a Member ('15).
If you don't remember him, you don't remem-
ber the Atlantic Communication Company
either, and if you don't remember the A. C. C.
you are no radio man. But where is the name
of George H. Clark? He doesn't seem to be a
member of the Institute at all, these days.
John M. Clayton, the Secretary of the Institute,
is a Member. Powell Crosley, Jr. is a Member.
H. P. Davis, the Vice-President of the West-
inghouse Company, who is probably responsible
in greater measure for the existence of broadcast-
ing than any other man and who is an engineer
to boot, is not listed. Dr. J. H. Dellinger was
elected a Fellow in 1923 and became President
of the Institute in 1925. Carl R. Englund of the
Bell Telephone Laboratories, one of the most
brilliant controversialists in the Institute, was
only an Associate in 1927; he has since been
elevated to the Fellow grade. C. L. Farrand seem-
ingly has no desire to rise above the Member
degree. A. H. Grebe, although a Manager for
three years, remains an Associate. Similarly
David Grimes. And ditto Charles A. Hoxie. So
does Edward J. Nally, the first President of
the Radio Corporation of America, modestly
remain an Associate. Hudson R. Searing, an
electrical engineer who was one of the pioneers
among the New York amateurs, and Harry W.
Secor, who was writing radio articles during the
years of radio antiquity, are both associates,
Mr. Secor since 1912, when the Institute was
founded. Hendrik J. Van Der Bijl, the author of
the well-known text on vacuum tubes, is a
Member in South Africa. Irving Vermilya is a
Member also; ten years before there were broad-
cast listeners to be annoyed, he was a master of
spark sets from one end of Long Island Sound
to the other.

On the same page of the Year Book one's eye
strikes the name of Manfred von Ardenne, the
German baron who, as a radio engineer and
mathematician, was able to hold his own with
some of the Fellows of the Institute during his
visit a year ago, but who is so young that he can
only qualify as a Junior. Finally, among the
Z's, just before the name of Prof. Jonathan
Zenneck, the Fellow who came from Germany
this year to receive the Institute's Medal of
Honor, there is Harold R. Zeamans, who ranks
among the Associates, for he is a lawyer and not
an engineer, but for all that he is one of the old-
est members of the organization and has been
presenting it with legal counsel since 1912.

Thus there are great and prominent men
on the lower ranks of the hierarchy, as well as
among the Fellows. Prominent or not, they can
vote, speak their minds at the meetings, and
preserve the Proceedings in their libraries. There
are, however, privilege* which they do not en-
joy. Only the Fellows have the right to wear
badges with blue lettering on a gold background,
only Fellows may aspire to election to the offices
of President and Vice-President, and only
Fellows are ipso facto famous.

New Apparatus

PRODUCTS of radio manufacturers whether
new or old are always interesting to our
readers. These pages, a feature of RADIO BROAD-
CAST, explain and illustrate products which have
been selected for publication because of their special
interest to our readers. This information is pre-
pared by the Technical Staff and is in a form which
we believe will be most useful. We have, wherever
possible, suggested special uses for the device men-
tioned. It is of course not possible to include all
the information about each device which is avail-
able. Each description bears a serial number
and if you desire additional information direct
from the manufacturer concerned, please address a
letter to the Service Department, RADIO BROAD-
CAST, Garden City, New York, referring to the
serial numbers of the devices which interest you,
and we shall see that your request is promptly
handled. — THE EDITOR.

Two-Stage Power Amplifier for
A.C. or D.C. Operation

Xyi

Device: NATIONAL PUSH-PULL AMPLIFIER. This
is a complete two-stage transformer-coupled
power amplifier. It does not contain any power
supply, but it may be light-socket operated by
the use of a 227-type tube in the first audio stage
and two 210- or 25o-type tubes in the output
stage, which is push-pull. An extra socket is
provided so that those who desire may use a

Connect Color to Color

FIG. I.

Diagram shows method of connecting Baldor
motor with condenser block and a.c. line

i i2A-type tube in the first stage and operate
the amplifier from a storage battery. Resistances
are contained within the device so that the proper
C-bias potential is supplied automatically to
the tubes. The unit is wired completely.
Manufacturer: National Company, Inc. Price:
$40.00 completely wired but without tubes.
Application: The amplifier may be used readily
in conjunction with radio tuners or it may be
used with a phonograph pick-up for the electrical
reproduction of phonograph records. It may also
be used to replace poor audio systems in old
sets, thereby improving the quality of the
reproduction.

Improved Motor for Television

X72

Device: BALDOR BALL-BEARING MOTOR. These
motors are intended for use in television receivers
for driving the scanning disc. Two types are
available.

Type YIV is a i-HP, no-Volt, 6o-cycle,
single-phase, i8oo-RPM motor designed for
receiving sets using a 24-inch scanning disc. It is
designed to maintain a constant speed between
750 and 1700 RPM with a 5- to 45-ohm variable
resistance. Price: $30.00.

Type M2V is a A-HP, i lo-Volt, 6o-cycle,

THE NATIONAL TWO-STAGE
POWER AMPLIFIER

single-phase, i8oo-RPM motor for receiving
sets employing 9- to i8-inch aluminum scanning
discs. The motor revolves 1750 RPM at full
load and a variable speed range of 750 to 1700
RPM is obtained with a 6o-ohm rheostat. Price:
$23.00.

Machined flange for scanning disc for all mo-
tors. Price: $3.00.

Rubber-cushioned base. Price: $2.50.
Manufacturer: Interstate Electric Company.
Application: This motor is of the induction type,
not relying upon a commutator either for start-
ing or running. Because of the fact that the motor
is placed in close proximity to the television
amplifier, tiny sparks, such as are produced by
brushes sliding over the surface of a commutator
while the motor is running, would
create disturbing electrical waves,
which if picked up by the amplifier
may cause distortion or fogging to
such a degree that the sought for
picture would be a failure. This
motor is silent in operation and
free from hum, either of which
affects the amplifier because of dis-
tortion and lack of clearness. The
circuit is given in Fig. i .

The following excerpt from one
of the engineering Test Depart-
ment Reports of the Interstate
Electric Co. is of interest: "The scanning discs as
used to-day for television reception are anywhere
from 9 to 24 inches in diameter, and T*« or J inch
thick, usually of aluminum. These discs, may be
driven by an M2V, A HP; Baldor motor, which
has ample power for this purpose. As a matter of

NEW PORTABLE METER MADE
BY WESTINGHOUSE

fact, with a ij-inch disc the motor may run un-
to within 50 to 75 revolutions of synchronous
speed which is 1800 RPM. In order to reduce
this speed to what is required it is only necessary
to place a series resistance in the circuit with a
means of short circuiting about i 5% of the total
resistance in the form of a key. For example, for
a disc 15 inches in diameter, j inch thick with a
required speed of 1080 RPM there is required a
fixed resistance of 160 ohms with a key shunting
around 25 to 30 ohms.

"There is a means of obtaining somewhat
more stable operation, which is to load the motor
in some way (a flat disc is practically a friction-
less load), as for example, by a fan. If we place
six small blades, i by 2 inches on the side of the
scanning disc, we will have accomplished the
result we are looking for; namely, a slight load
on the motor, enabling the operator to hold the
speed of the disc more nearly constant. The mo-
tor now requires only a 3o-ohm fixed resistance
with approximately 5 ohms short circuited with
a key, assuming a 1 5-inch diameter disc is used
and a speed of 1080 RPM is desired."

High Grade A.C. Meters

X73

Device: PORTABLE ALTERNATING-CURRENT IN-
STRUMENTS FOR LABORATORY USE. The Westing-
house Company have recently developed a new
line of portable alternating-current instruments
for laboratory use. These instruments are oper-
ated on the electric-dynamometer principle, con-
taining two stationary coils and a moving coil.
The unit shown on this page is a single-phase
wattmeter. The unit illustrated has a case
made of wood. However, they are now being
manufactured with cases of built-up sheet mi-
carta. This construction of case is novel and
gives the units a great many advantages which
wooden cases do not possess. The finish on the
micarta case is that of burl walnut and is very
hard, not easily scratched or marred like the
finish on the wooden case instruments. The dials
of the instruments are metal under which is a
mirror so as to prevent parallax reading of the
knife-edge pointer. The scale is 55" long with
quite uniform markings throughout. Although
the design of these instruments is primarily for
alternating current use they can be used on di-
rect current and are just as accurate as on alter-
nating current when the average of the direct
and reverse readings are taken. The instrument
element has an iron shield around it so as to
make it immune from external stray magnetic
fields. The accuracy of the instrument is £ of i
per cent., of full scale deflection. The terminals
supplied on the instruments are non-removable,
engraved with the scale value and have large
contact surface.

Ammeters are made in double-scale ranges
from 5 to 30 amperes in capacities multiple of 2.
The voltmeters are also made in double ranges of
multiples of 2 capacities, from 3 volts to 600
volts. The double range 0-3 and 0-6 volts or 0-75
and o-i 5 volts are particularly desirable for mea-
suring the filament voltages of the radio tubes in
laboratory testing. The instruments are also pro-
vided in single-phase wattmeters with voltages
as low as 30 volts up to 600 volts and currents
from 5 ampere up to 30 amperes.
Manufacturer: Westinghouse Electric and Manu-
facturing Company.

Application: The usefulness of such instruments
in any electrical laboratory is obvious. The

NOVEMBER, 1928

NEW APPARATUS

ranges being manufactured cover all values of
current, voltage, and wattage ordinarily dealt
with in radio work.

An Adjustable Resistor Network for
B-Power Units

X74

Device: TRUVOLT DIVIDER. A compact unit con-
sisting of a network of resistances mounted in a
small nicely finished container and designed
for use in conjunction with all types of B-power
units delivering voltages not in excess of 220. It
permits one to obtain readily output voltages of
various values between o and 180 and to also
obtain two values of C voltage. It can be used
with any type of rectifier or filter circuit.
Manufacturer: Electrad, Inc. Price: $12.50.
Application: The problem of obtaining from the
output of a B-power unit the correct voltage for
application to any given receiver is a serious one.
Fixed resistances across the output of a B-power
unit have the disadvantage that the voltage
which will be delivered from the various taps
varies considerably according to the amount of
current drawn from these taps. The use of varia-
ble resistances for each tap has the disadvantage
that the range of voltage which can be delivered
from each resistance is very wide and therefore
rather difficult to adjust accurately. A compro-
mise between these two is probably the best.
We therefore make use in the Truvolt Divider of
the combination of fixed and variable resistances
so that the voltage from each tap is approxi-
mately fixed but can be varied over a range suffi-
ciently wide to adapt the unit to practically any
receiver. This might be called a universal divider
since it can be used interchangeably with any
receiver and any power unit and can be adjusted
to supply the correct voltages to the set. The
unit will find application not only in the con-
struction of new power units but in conjunction
with old power units which perhaps deliver in-
correct voltages or have not sufficient voltage
taps. Also by using the Truvolt Divider we can
obtain from a power unit that ordinarily de-
livers only B voltages two values of C voltages as
well as the B voltages.

The Truvolt Divider is designed to supply four
B voltages and two C voltages. The voltage ob-
tained from the Bi tap is the maximum voltage
from the B-power unit. Tap Bz supplies an
average voltage of 135 but this voltage can be
adjusted to any value between 1 10 and 160 volts.
Tap 63 supplies an average of 90 volts which can
be adjusted between the limits of 65 and 1 10

volts. Tap 64 is the 45-volt tap
capable of supplying a range of
voltages from 20 to 55. Tap 65 is
the negative B. Tap C6 is the low
voltage C terminal and it will
supply a grid bias from minus I to
minus 20 volts. For higher C
biases tap Cy is used. It supplies
voltages from minus 20 to minus
40. The connection of this device
to a typical B power unit is in-
dicated in Fig. 2.

Because the resistances used in
this divider are of a type which
can be accurately calibrated it is
possible by the use of either tables
or curves to adjust the various
controls to give the proper volt-
ages without the use of a volt-
meter. If a voltmeter is available,
it of course affords a simple, rapid,
and certain method of adjusting
the unit. The voltmeter is shunted
between that particular tap which
is being adjusted and the negative
B and the control varied until the
correct voltage is supplied. This
test is, of course, made with the
receiver connected to the unit.

Many experimenters desiring
to use this device will prob-
ably not have available a high-resistance
voltmeter; for them it would be helpful to
have the booklet supplied with the divider,
which describes in detail how to adjust the re-
sistors for any given receiver. This booklet,
which is called the Truvolt Divider Manual, is
not only very helpful in adjusting this device,
but also contains a great deal of excellent in-
formation in connection with the output circuits
of a B-power unit.

Useful Tester for Radio Sets

X75

FIG. 2.

Method of connecting Truvolt divider in a B-power unit

Device: SET TESTER. This tester is designed to
aid in servicing all types of a.c.- and d.c. -operated
receivers. The following paragraphs indicate
some of the tests which may be made.

In balancing the various tuning condensers in
a single-control receiver the tester is very useful.
This test is made by placing a tube in one of the
sockets of the tester and inserting a plug into
the power-tube socket of the receiver, thus con-
verting the tester into an oscillator. When these
connections are made, and the set is turned on,
it will be found that
the milliammeter in the
tester will jump as the
set is tuned to resonance
with the oscillations
produced by the tester.
Each individual tuning
condenser is then ad-
justed to resonance, as
indicated by the move-
ment of the meter.

The tester will mea-
sure the normal emis-
sion, and the emission
when oscillating, of all
types of tubes. In addi-
tion it will indicate the
plate voltage, grid volt-
age, and filament volt-
age of each tube of a
set, both under load con-
dition and with no load.
Another important
application is the detec-
tion of open, short,

RADIO TEST SET MADE BY SUPREME
INSTRUMENT CORP.

and high-resistance joints in any parts of the
circuit.

The instrument contains a complete set of
tools with space for spare tubes. The supplies
include a ratchet screw driver, soldering iron,
hook-up wire, various adapters, test leads, etc.

The instruments in the tester are: a double-
range milliammeter, 0-50 and o-ioo; a triple-
range voltmeter, o-io, 0-50, and 0-250, and a
triple-range a.c. voltmeter, 0-3, 0-18 and 0-150.

The photograph illustrates the Model IOOA
instrument priced at $108.50.
Manufacturer: Supreme Instrument Corporation.
Application: Indicated above. It should prove
invaluable to service men.

-40V.

Hook-up Wire for Various Uses

Device: CELATSITE HOOK-UP WIRE. The follow-
ing kinds can be obtained.

1. Celatsite Flexible Wire. A flexible hook-up
wire consisting of tinned stranded copper wire
covered with a non-inflammable insulation.
Available in the following colors and wire sizes.

No. 22 — $ .70 per 25-ft.
No. 20 — .75 "
No. 18— .80 "
No. 16 — i.oo "
No. 14 — i.oo "

Colors: Black, Yellow, Red, Green, Brown,
Slate, Blue, White, Maroon.

2. Celatsite twisted wire for a.c. filament
circuits. One strand of Red and one strand of
Black No. 1 8 Flexible Celatsite twisted together.
The two colors are used so that, if desired, the
same sides of all filaments can be maintained at
the same relative potential. Packed in 25 ft.
coils. List price: $1.75 per carton.

3. Solid Celatsite wire. Bus-bar made of
tinned No. 14 copper wire and covered with a
non-inflammable insulation. Strips easily for
soldering. Available in same colors as the flexible
wire. Price; $.09 per 3O-inch length.
Manufacturer: Acme Wire Company.
Application: The above material has been used
in the Laboratory for some time in constructing
receivers and power units and has been quite
satisfactory.

Manufacturers' Booklets

A Varied List of Books Pertaining to Radio and Allied
Subjects Obtainable Free With the Accompanying Coupon

E» EADERS may obtain any of tbt booklets lifted bebwbyus-
**• tttg the coupon printed on this page . Order by number only.

i. FILAMENT CONTROL — Problems of filament supply,
voltage regulation, and effect on various circuits. 1928 re-
vised booklet, with circuit diagrams of popular kits. RADIALL
COMPANY.

5. CARBORUNDUM IN RADIO — A book giving pertinent
data on the crystal as used for detection, with hook-ups, and
a section giving information on the use of resistors. THE
CARBORUNDUM COMPANY.

12. DISTORTION AND WHAT CAUSES IT — Hook-ups of
resistance-coupled amplifiers with standard circuits. ALLEN-
BRADLEY COMPANY.

1 5. B-ELIMINATOR AND POWER AMPLIFIER — Instruc-
tions for assembly and operation using Raytheon tube.
GENERAL RADIO COMPANY.

i$a. B-ELIMINATOR AND POWER AMPLIFIER — Instruc-
tions for assembly and operation using an R. C. A. rectifier.
GENERAL RADIO COMPANY.

17. BAKELITE — A description of various uses of bakelite
in radio, its manufacture, and its properties. BAKELITE
CORPORATION

22. A PRIMER OF ELECTRICITY — Fundamentals of
electricity with special reference to the application of dry
cells to radio and other uses. Constructional data on buzzers,
automatic switches, alarms, etc. NATIONAL CARBON COM-
PANY.

23. AUTOMATIC RELAY CONNECTIONS — A data sheet
showing how a relay may be used to control A and B cir-
cuits. YAXLEY MANUFACTURING COMPANY.

30. TUBE CHARACTERISTICS — A data sheet giving con-
stants of tubes. C. E. MANUFACTURING COMPANY.

32. METERS FOR RADIO — A catalogue of meters used in
radio, with diagrams. BURTON-ROGERS COMPANY.

33. SWITCHBOARD AND PORTABLE METERS — A booklet
giving dimensions, specifications, and shunts used with
various meters. BURTON-ROGERS COMPANY.

37. WHY RADIO is BETTER WITH BATTERY POWER— Ad-
vice on what dry cell battery to use; their application to
radio, with wiring diagrams. NATIONAL CARBON COMPANY.

46. AUDIO-FREQUENCY CHOKES — A pamphlet showing
positions in the circuit where audio-frequency chokes may
be used. SAMSON ELECTRIC COMPANY.

47. RADIO-FREQUENCY CHOKES — Circuit diagrams il-
lustrating the use of chokes to keep out radio-frequency
currents from definite points. SAMSON ELECTRIC COMPANY.

48. TRANSFORMER AND IMPEDANCE DATA — Tables giv-
ing the mechanical and electrical characteristics of trans-
formers and impedances, together with a short description of
their use in the circuit. SAMSON ELECTRIC COMPANY.

53. TUBE REACTIVATOR — Information on the care of
vacuum tubes, with notes on how and when they should be
reactivated. THE STERLING MANUFACTURING COMPANY.

56. VARIABLE CONDENSERS — A bulletin giving an
analysis of various condensers together with their character-
istics. GENERAL RADIO COMPANY.

57. FILTER DATA — Facts about the filtering of direct
current supplied by means of motor-generator outfits used
with transmitters. ELECTRIC SPECIALTY COMPANY.

58. How TO SELECT A RECEIVER — A common-sense
booklet describing what a radio set is, and what you should
expect from it, in language that anyone can understand.
DAY-FAN ELECTRIC COMPANY.

67. WEATHER FOR RADIO — A very interesting booklet
on the relationship between weather and radio reception,
with maps and data on forecasting the probable results.
TAYLOR INSTRUMENT COMPANIES.

69. VACUUM TUBES — A booklet giving the characteris-
tics of the various tube types with a short description of
where they may be used in the circuit; list of American and
Canadian broadcast stations. RADIO CORPORATION OF
AMERICA.

72. PLATE SUPPLY SYSTEMS. Technical information on
audio and power systems. Bulletins dealing with two-stage
transformer amplifier systems, two-stage push-pull, three-
stage push-pull, parallel push-pull, and other audio ampli-
fier, plate, and filament supply systems. AMERICAN TRANS-
FORMER COMPANY.

73. RADIO SIMPLIFIED — A non-technical booklet giving
pertinent data on various radio subjects. Of especial in-
terest to the beginner and set owner. CROSLEY RADIO COR-
PORATION.

76. RAom INSTRUMENTS — A description of various
meters used in radio and electrical circuits together with a
short discussion of their uses. JEWELL ELECTRICAL INSTRU-
MENT COMPANY.

78. ELECTRICAL TROUBLES — A pamphlet describing the
use of electrical testing instruments in automotive work
combined with a description of the cadmium test for stor-
age batteries. Of interest to the owner of storage batteries.
BURTON ROGERS COMPANY.

81. BETTER TUNING — A booklet giving much general in-
formation on the subject of radio reception with specific
illustrations. Primarily for the non-technical home con-
structor. BREMER-TULLY MANUFACTURING COMPANY.

84. FIVE-TUBE EQUAMATIC— Panel layout, circuit dia-
grams, and instructions for building a five-tube receiver, to-
Kcther with data on the operation of tuned radio-frequency
transformers of special design. KARAS ELECTRIC COMPANY.

88. SUPER-HETERODYNE CONSTRUCTION — A booklet giv-
ing full instructions, together with a blue print and necessary
data, for building an eight-tube receiver. THE GEORGE W.
WAI KfcR COMPANY.

80. SHORT-WAVE TRANSMITTING EQUIPMENT. Data and
wiring diagrams on construction of all popular short-wave
transmitters, operating instructions, keying, antennas; in-
formation and wiring diagrams on receiving apparatus; data
on variety of apparatus used in high-frequency work.
RADIO ENGINEERING LABORATORIES.

90. IMPEDANCE AMPLIFICATION — The theory and prac-
tice of a special type of dual-impedance audio amplification
are given. ALDEN MANUFACTURING COMPANY.

92. RESISTORS FOR A. C. OPERATED RECEIVERS — A
booklet giving circuit suggestions for building a.c. operated
receivers, together with a diagram of the circuit used with
the new .400-01 ill iampere rectifier tube, CARTER RADIO COM-
PANY.

Radio Broadcast
Laboratory Information Sheets

(tfw. 1-190)
in

BOUND VOLUMES

Ask any news dealer for "Radio Broadcast Data
Sheets" or write direct to the Circulation Depart-
ment, Doubleday, Doran & Co., Inc. See page 56
for further details. Price $1.00

95. Resistance Data — Successive bulletins regarding
the use of resistors in various parts of the radio circuit.
INTERNATIONAL RESISTANCE COMPANY.

98. COPPER SHIELDING — A booklet giving information
on the use of shielding in radio receivers, with notes and
diagrams showing how it may be applied practically. Of
special interest to the home constructor. THE COPPER AND
BRASS RESEARCH ASSOCIATION.

99. RADIO CONVENIENCE OUTLETS — A folder giving
diagrams and specifications for installing loud speakers in
various locations at some distance from the receiving set,
also antenna ground and battery connections. YAXLEY
MANUFACTURING COMPANY.

101. USING CHOKES — A folder with circuit diagrams of
the more popular circuits showing where choke coils may
be placed to produce better results. SAMSON ELECTRIC
COMPANY.

102. RADIO POWER BULLETINS — Circuit diagrams,
theory constants, and trouble-shooting hints for units em-
ploying the BH or B rectifier tubes. RAYTHEON MANU-
FACTURING COMPANY.

104. OSCILLATION CONTROL WITH THE "PHASATROL" —
Circuit diagrams, details for connection in circuit, and
specific operating suggestions for using the "Phasatrol"
as a balancing device to control oscillation. ELECTRAD,
INCORPORATED.

105. RECEIVING AND TRANSMITTING CIRCUITS. Con-
struction booklet with data on 25 receivers and transmitters
together with discussion of low losses in receiv r tuning cir-
cuits. AERO PRODUCTS COMPANY.

In tending the coupon below, make sure that your name
and address are included and are plainly written. Also
make sure that the listing of booklets from which you
choose is that of the latest issue of the magazine, as Radio
Broadcast cannot guarantee the delivery of booklets not
listed in its current issue.

USE THIS BOOKLET COUPON

RADIO BROADCAST SERVICE DEPARTMENT
RADIO BROADCAST, Garden City, N. Y.
Please send me (at no expense) the following book-
lets indicated by numbers in the published list above:

\umher) (Street)

(City) (State)

ORDER BY NUMBER ONLY
This coupon must accompany every request. R B

11-28

108. VACUUM TUBES — Operating characteristics of an
a.c. tube with curves and circuit diagram for connection
in converting various receivers to a.c. operation with a
four-prong a.c. tube. ARCTURUS RADIO COMPANY.

1 12. HEAVY-DUTY RESISTORS — Circuit calculations and
data on receiving and transmitting resistances for a variety
of uses, diagrams for popular power supply circuits, d.c. re-
sistors for battery charging use. WARD LEONARD ELECTRIC
COMPANY.

1 13. CONE LOUD SPEAKERS — Technical and practical in-
formation on electro-dynamic and permanent magnet type
cone loud speakers. THE MAGNAVOX COMPANY.

114. TUBE ADAPTERS — Concise information concerning
simplified methods of including various power tubes in
existing receivers. ALDEN MANUFACTURING COMPANY.

115. WHAT SET SHALL I BUILD? — Descriptive matter,
with illustrations, of fourteen popular receivers for the home
constructor. HERBERT H. FROST, INCORPORATED.

118. RADIO INSTRUMENTS. CIRCULAR "J" — A descrip-
tive manual on the use of measuring instruments for every
radio circuit requirement. A complete listing of models for
transmitters, receivers, set servicing, and power unit con-
trol. WESTON ELECTRICAL INSTRUMENT CORPORATION.

120. THE RESEARCH WORKER — A monthly bulletin of
interest to the engineer and home builder. Each issue con-
tains special articles on radio design and construction with
special emphasis on resistors and condensers. AEROVOX
WIRELESS CORPORATION.

121. FILTER CONDENSERS — Some practical points on the
manufacture and use of filter condensers. The difference be-
tween inductive and non-inductive condensers. POLYMET
MFG. CORP.

123. B SUPPLY DEVICES — Circuit diagrams, characteris-
tics, and list of parts for nationally known power supply
units. ELECTRAD, INC.

124. POWER AMPLIFIER AND B SUPPLY — A booklet
giving several circuit arrangements and constructional
information and a combined B supply and push-pull audio
amplifier, the latter using 210 type tubes. THORDARSON
ELECTRIC MFG. Co.

125. A. C. TUBE OPERATION — A small but complete
booklet describing a method of filament supply for a.c. tubes.
THORDARSON ELECTRIC MFG. Co.

1 26. M ICROMETRIC RESISTANCE — How to use resistances
for: Sensitivity control; oscillation control; volume control;
regeneration control; tone control; detector plate voltage
control; resistance and impedance coupling: loud speaker
control, etc. CLAROSTAT MFG. Co.

129. TONE— Some model audio hook-ups, with an ex-

§lanation of the proper use of transformers and chokes.
ANGAMO ELECTRIC Co.

130. SCREEN-GRID AUDIO AMPLIFICATION — Diagrams
and constructional details for remodeling old audio ampli-
fiers for operation with screen-grid tubes. THORDARSON
ELECTRIC MFG. Co.

131. THEMERSHON CONDENSER — An illustrated book-
let giving the theory and uses of the electrolytic condenser.
AMRAD CORPORATION.

132. THE NATIONAL SCREEN-GRID SHORT-WAVE RE-
CEIVER— Constructional and operating data, with diagrams
and photographs. JAMES MILLEN.

133. THE NATIONAL SHIELD-GRID FIVE — A circuit dia-
gram with constructional and operating notes on this-re-
ceiver. JAMES MILLEN.

134. REMLER SERVICE BULLETINS — A regular service for
the professional set builder, giving constructional data,
and hints on marketing. GRAY & DANIELSON MFG. Co.

135. THE RADIOBUILDER— A periodic bulletin giving ad-
vance information, constructional and operating data on
S-M products. SILVER-MARSHALL, INC.

136. SILVER MARSHALL DATA SHEETS — These data
sheets cover all problems of construction and operation on
Silver-Marshall products. SILVER-MARSHALL, Inc.

139. POWER UNIT DESIGN — Periodical data sheets on
power unit problems, design, and construction. RAYTHEON
MFG. Co.

140. POWER UNIT PROBLEMS— Resistance problems in
power units, with informative tables and circuit diagrams.
ELECTRAD, INC.

141. AUDIO AND POWER UNITS — Illustrated descriptions
of power amplifiers and power supplies, with circuit dia-
grams. THORDARSON ELECTRIC MFG. Co.

142. USE OF VOLUME AND VOLTAGE CONTROLS. A com-
plete booklet with data on useful apparatus and circuits for
application in receiving, power, amateur transmitter, and
phonograph pick-up circuits. CENTRAL RADIO LABORATORIES.

143. RADIO THEORY. Simplified explanation of radic
phenomena with especial reference to the vacuum tube,
with data on various tubes. DE FOREST RADIO COMPANY.

144. A.C. DETECTOR TUBE. Data on characteristics and
operation of 2.^-volt a.c, detector tubes. ARCTURUS RADIC
COMPANY.

145. AUDIO UNITS. Circuits and data on transformer
and impedances for use in audio-amplifier circuits, plate and
output impedances and special apparatus for use with dy-
namic speakers. SANGAMO ELECTRIC COMPANY.

146. RECEIVER CIRCUIT DATA. Circuits for using re-
sistances in receivers, and in power units with descriptions ol
other apparatus. H. H. FROST, INC.

147. SUPER-HETERODYNE CONSTRUCTION. Constructor
and operation of a nine-tube screen-grid super-heterodyne
SET BUILDERS' SUPPLY COMPANY.

148. SHORT-WAVB RECEIVER. Constructional and opera-
tion data on a four-tube short-wave receiver. KARAS ELEC-
TRIC COMPANY.

149. FIVE-TUBE SCREEN-GRID RECEIVER. Blueprint
with full constructional details for building a broadcast re
ceiver using two screen-grid tubes. KARAS ELECTRIC COM
PANY.

iso. FIVE-TUBE A.C. RECEIVER. Blueprint for con
structing a five-tube a.c. receiver employing the "equamalic
system." KARAS ELECTRIC COMPANY.

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Name.

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The Radio Broadcast

SHEETS

Since they began, in June, 1926, the popularity of the Information Sheets has increased
so greatly that it has been decided to reprint the first one hundred and ninety of them
(June, 1926-May, 1928) in a single substantially bound volume. This volume, "Radio
Broadcast's Data Sheets" may now be bought on the newsstands, or from the Circulation
Department, Doubleday, Doran & Company, Inc., Garden City, New York, for $1.00.
Inside each volume is a credit coupon which is worth $1.00 toward the subscription price
of this magazine. In other words, a year's subscription to RADIO BROADCAST, accompanied
by this? i. oo credit coupon, gives you RADIO BROADCAST for one year for $3.00, instead of
the usual subscription price of $4.00.

— THE EDITOR.

No. 233

RADIO BROADCAST Laboratory Information Sheet November, 1928

Balancing Radio Receivers

AN EASY METHOD

'"THE change in the plate current of the detector
A tube, when a signal is being received, may be
utilized to balance the various tuned circuits in a
single-control receiver. If the several tuned circuits
in a multi-stage r.f. amplifier are not properly
ganged, the set will be in-
sensitive and the selectivity
will be poor. It is essential,
therefore, that the various
stages be accurately aligned.
How this can be done is
the subject of this Lab-
oratory Sheet. The method
used is simple and is based
on the action of a deteC'
tor when a signal is being
received.

If a milliammeter with a
range of about 2 milliam-
peres is connected in series
with the B-plus lead to the
detector, as indicated in the
diagram, it will be found to
read about 1 mA. if the de-
tector is of the grid leak and
condenser type and about
0.2 mA. if a C-battery type
detector is used. If a station
is tuned-in, the plate cur-

IstA.F.

rent of the detector tube will decrease if the former
arrangement is used and increase with a C-battery
detector circuit, .the amount of the increase or de-
crease being proportional to the strength of the sig-
nal— the stronger the signal the greater the change
in current. Therefore, when the set is accurately
tuned and all of the condensers are perfectly aligned
the deflection of the meter —
and therefore the output of
the set — will be greatest.

Balancing therefore be-
comes a matter of tuning in
some station, preferably one
operating on a short wave-
length, and then adjusting
the various condensers, by
whatever means are pro-
vided by the manufacturer,
so that the greatest change
is indicated on the meter in
the plate circuit of the de-
tector. When circuits have
been adjusted so that the
greatest current change is
obtained, the set is balanced.
It is best to make this ad-
justment with the set tuned
to a short wavelength, for it
is in this part of a receiver
range that the greatest lack
B <• Del of balance is liable to occur.

No. 234

RADIO BROADCAST Laboratory Information Sheet November, 1928

The Audio Transformer

OPERATION AT HIGH AUDIO FREQUENCIES

TN LABORATORY Sheet No. 227, in the October
1 number, we studied the characteristics of audio
transformers and pointed out that the lowest
frequency response depends upon the ratio of the
reactance of the transformer to the plate resistance
of the tube. Here we will consider the high fre-
quencies. For convenience we have reprinted here
the diagram from Sheet No. 227.

At high frequencies the reactance of L is very
large in comparison with C. and it may therefore be
neglected. Essentially, we then have a circuit con-
sisting of Rp, La and C in
series. As L& and C come
into resonance, the impe-
dance of the circuit will
decrease and more current
will flow, thereby tending
to increase the voltage
across C, which is the
voltage applied to the grid
of the next tube. How-
ever, the voltage across C,
for a given current, is in-
versly proportional to the
frequency, and this will

tend to lower the voltage across it at high fre-
quencies. In some transformers, however, there is
a marked peak at about WXX) cycles, corresponding
to the resonant frequency of La and C in series, the
output falling off rapidly beyond the point.

If this upper resonant peak is very pronounced
the gain of the entire amplifier will increase greatly
at this point, tending to make the amplifiers oscif-
late. Good design requires that the peak be kept as
small as possible.

At frequencies higher than that at which La and
C resonate, the reactance of La continues to in-
crease, and the reactance of C to decrease; there-
fore, the voltage across
C rapidly falls. If some
transformer curves are
examined, it will fre-
quently be found that
the curve drops rapidly
beyond the upper reso-
nant point.

The problem of design
is to adjust the leakage
inductance, La, and the
effective capacity, C, so
as to give satisfactory
high-frequency response.

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No. 235

RADIO BROADCAST Laboratory Information Sheet November, 1928

Television

FREQUENCY BAND REQUIRED

TN TELEVISION transmission a problem which
•*• must be given careful consideration is the width
of the band of frequencies which must be transmit-
ted to reproduce at the receiver end, with good
quality, the scene being scanned by the television
transmitter.

Theoretically, a television signal contains com-
ponents of all frequencies from zero to infinity. In
practice the frequency band is much more re-
stricted and depends upon various factors.

The width of the band of frequencies which musi
be transmitted is a func-
tion of the number of ele-
ments scanned per second
at the transmitter. For
example, if the number of
lines into which the pic-
ture is broken — which is
equal to the number of
holes in the scanning disc
— is 50, then the number
of elements into which the
picture will be broken will

No. of
lints

25
50
75

100

3.100

11MKX)
28,01 X)

;»(!.< XX)

be 50 times 50, or 2500. If we transmit 20 pictures
per second, the lolal number of elements trans-
mitted per second will be 50,000. The highest
frequency which must be transmitted, to get good
quality, can be taken as equal to half this figure,
or 25,000 cycles. The table given herewith shows
how the value of the highest frequency which should
be transmitted varies with the number of scanning
lines and the number of pictures per second. For
example, a 50-line picture sent 15 times per second
requires up to 19,000 cycles.

A station transmitting within a broadcast band
is limited to 5000-cycle modulation. Therefore, any
broadcast station trans-
mitting television pro-
grams and using a number
of lines and number of
pictures per second such
l hat requires a frequency
band greater than 5000
cycles must either modu-
late above the legal limit
or suppress in the ampli-
IHTS ! he frequencies ;il«>ve
;>(XX) cycles.

No. of pictures per second

4,700
19.000
42.000
75.000

25. (XX)
5<>.000
HXUXJU

No. 236

RADIO BROADCAST Laboratory Information Sheet November, 1928

Moving-Coil Loud Speakers

THEIR OPERATION

'T^HE important characteristic of the dynamic or.
*• more properly named, moving-coil type loud
speaker is the fact that it has a coil fastened directly
to the cone, which is caused to move back and forth
in an air gap in a magnetic circuit, the movements
being in accordance with the frequencies flowing
through the coil.

The moving coil is mounted at the apex of the
cone, as indicated in the diagram, and connects to
the secondary of the transformer, T, the primary of
which connects to the plate of the power tube. The
moving coil of a well-designed unit has a fairly
constant impedance over the entire range of audio
frequencies and the transformer, T, is designed to
"match" the coil's impedance to the output im-
pedance of the tube. So long as the power tube
works into an impedance about equal to or some-
what greater than twice the tube's plate impedance,
satisfactory power transfer from the tube to the
moving coil will be obtained. The instructions
covering the use of one of these loud speakers should
indicate what tubes or combination of tubes are
recommended for use with the unit.

The term "dynamic loud speaker" is not a very
accurate description of a type of loud speaker whose
distinguishing feature is that it has a moving coil.
The word "dynamic" is defined as "mechanics
treating of the motion of bodies and of the action
of forces in producing or changing their motion."
Since all loud speakers move they can all be called
"dynamic." We have seen descriptions and adver-

tisements of "dynamic" loud speakers which con-
sisted mainly of an ordinary electromagnetic unit
coupled to a cone. This term used to describe such
loud speakers is probably misleading to sonic al-
though technically it is not incorrect.

Leads from -
moving Coil'-.. .-J

No. 237

RADIO BROADCAST Laboratory Information Sheet November, 1928

Power Output

HOW IT DEPENDS UPON IMPEDANCE RATIOS

TT HAS been proved mathematically and experi-
A mentally thai a tube delivers the maximum
amount of undistoited power when it works into a
load resistance equal to twice the plate resistance
of the tube; maximum power output, however,
is obtained when the load resistance equals the
tube's plate resistance. The curve on this sheet in-
dicates relatively in TU how the power in the load
varies with the ratio of the load resistance to the
tube's plate resistance ( sometimes called plate im-
pedance). The X on the curve indicates when- ;t
tube is normally operated, the load resistance at the
point being twice the tube resistance.

We frequently see statements to the effect that
the loud speaker we use must
be matched to the lube to get
the largest amount of undis-
torted power into the loud
speaker. Such is 1 he case, but t he
curve indicates that there can be
considerable mismatching with-
out serious loss of power. For ex-
;imple, even when the load resis-
tance is about five times greater
than the tube's resistance , there
is only a 2 Tt; loss — a loss which
would hardly be noticeable in
the ear.

It is unwise, however, to work
a tube into a load resistance less
than its own resistance, because

under such conditions the tube's characteristic is
curved (see Laboratory Sheet No. 12-1) and this
curved characteristic introduces distortion.

In cases where the element of the loud speaker
has a low impedance, for example, it is necessary
to use a transformer between the lube and tin- luml
sjH-aker to compensate the differences in impedance.
A moving-coil type, i.e., dynamic loud speaker,
might have an impedance of, say, 20 ohms at some
frequencies, and if it is to be used with a 2000-ohm
tulw i!7lA) which requires a load impedance of
4000 ohms to get maximum undistorted power,
then the coupling Iransformer would have an im-
pedance ratio of 4<KX) divided by 20. or 200. cor-
responding to a turns ratio of the square root of
200. or 14.

-3

3 4

LOAD RESISTANCE
PLATE RESISTANCE OF TUBE

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No. 238 RADIO BROADCAST Laboratory Information Sheet November 1928

A Hook'Up for Short- Wave and
Broadcast Receivers

A METHOD FOR SWITCHING OVER

TT IS general practice in constructing short-wave
-1 adapters to arrange them with extension leads so
that they may be plugged into the broadcast set in
the detector socket in place of the regular detector
tube. This practice is all right when one is building
an adapter that perhaps will not be used continually,
but when both the broadcast and the short-wave
tuners are going to be used frequently, it is better to
arrange the circuit as indicated in the diagram on
Sheet No. 239, which permits one to change from
broadcast to short waves by a simpler means than
taking out a tube and plugging in an adapter.

The diagram shows the eetector of the broadcast
receiver and the detector of the short-wave re-
ceiver. They are both wired to the same A and B
voltages, and either set is thrown in or out of oper-
ation by simply turning the proper filament switch, Si
or S?; Si turns on and off the broadcast receiver and
82 similarly controls the short-wave set.

The two plates are permanently wired together,
and for this reason the arrangement we have in-
dicated should only be used when the two sets are
located close to each other (which is usually the
case) so that the plate lead running from one set
to the other is not more than 1 or 2 feet long.

Most of us have available only one antenna to
use with both sets. To use it with both receivers a
single-pole double-throw switch, Sj, can be placed
in the antenna circuit; thrown to one side it con-
nects the antenna to the broadcast set, and thrown
the other way it connects the short-wave receiver.

An easier arrangement, which works well in
practically all cases (how well it works depends upon
the characteristics of the two receivers) is indicated
by dotted lines. The antenna is connected directly
to the broadcast set and through a 50-mmfd. con-
denser, C, to the short-wave set. This small con-
denser will block the broadcast signals from the
short-wave set but permit these latter currents to
pass quite readily.

No. 239 RADIO BROADCAST Laboratory Information Sheet November, 1928

Circuit for Short- Wave and Broadcast Reception

BROADCAST RECEIVER

SHORT-WAVE RECEIVER

No. 240 RADIO BROADCAST Laboratory Information Sheet November, 1928

Television

DATA ON THE BELL TELEPHONK LABORATORIES'
METHOD

""THE demonstrations of television given by the
•*• Bell Telephone Laboratories, associated with
the American Telephone and Telegraph Company,
rank higher, in our opinion, than any of the other
demonstrations so far given, in quality of the results.
In the following paragraphs are summarized some
of the most important elements of the apparatus
used by these Laboratories.

(a) The scanning discs contained 50 holes and
revolved at a speed of 1062.5 revolutions per min-
ute, giving 17.7 pictures per second.

(b) The' output voltage of the photo-electric cells
at the transmitter was about 10 microvolts.

(c) The range of frequencies decided upon as
being essential tor good quality extended from 10 to
20,000 cycles. Overall measurements on the linal
amplifier indicated a frequency characteristic
constant within plus or minus 2 TU over this
range.

(d) The signals from the transmitter were ampli-

fied and delivered to the transmission line at a level
of 10 milliwatts. The amplification from the photo-
electric cell to the line was i;tO TU.

(e) Synchronization was accomplished by the use
of synchronous motors containing 120 poles and
having a synchronous speed of 1062.5 r.p.m. The
angular phase displacement was above 0.07 de-
tin t-s. This magnitude of phase displacement corre-
sponds roughly to the angular twist in a steel
shaft 6 feet long of 1 inch in diameter, operated at
full load.

(f) With regard to the effect of extraneous cur-
rents due to noise, it was found that satisfactory
results were obtained if the average picture currents
were 10 times greater than the average noise cur-
rents.

This corresponds to 20 TU, or a power ratio of
100. In ordinary sound broadcasting the noise in
the telephone lines is kept at a level 60 TU below 10
milliwatts, giving a power ratio of l.OOO.OOO. It is
evident that it is permissible to have the noise
level much higher in television reception than in
sound reception.

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Volume to Fill Any House
*— "the Last \VordL in Realism
"— »f rom Any Phonograph

Listen-In
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NOT with any ordinary radio receiver, of
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your wonders might sound very tame to him.
Perhaps by this time he is only interested in
New Zealand and Japan! For in short-waves al-
most anything is possible; amazing feats of distant
reception are becoming a matter of common
knowledge. (See prices on opposite page).

True Tone Quality

from
New S-M And Jos

S-M dough-system audio trans-
formers are guaranteed uncondition-
ally to give better tone quality than
others, with higher ampli/ication, re-
gardless of size, weight, or price. They
sell in tremendous quantities, by
simply comparing results with others
in the comparison amplifiers used in
S-M demonstrations at recent radio
shows.

The large-size transformers (S-M
225, 1st stage; and 226, 2nd stage) are
$9.00 each; the smaller ones (S-M 255
and 256) are $6.00 each. The above
guarantee applies to both sizes.

Western Radio Mfg. Co.

"The Big Friendly Radio House"
128 W. Lake Street Chicago, Illinois

The new 2-stage S-M 678PD Phonograph Amplifier is
priced so low that, while particularly adapted for dance halls
and small theaters, it is ideal for the home also. Used with
any 110 volt D.C. dynamic speaker, it takes input from any
magnetic phonograph pickup, or from the detector tube of a
broadcast or short-wave receiver, and, by means of its S-M
Clough-system audio transformers, supplies to the speaker
undistorted the full power output of its 250-type tube. All
input power is taken from the 110 volt A.C. house-lighting
mains. Price, wired, $73.00; complete kit, $69.00.

Or you can get 250-tube power right in your present set by
inserting a 250 tube (with an adapter) in the last socket of
the set, and using the S-M 675ABC Power Supply — which
furnishes ABC power for the 250, and B power to the entire
set (or full ABC power to A.C. tube sets). Price, 675ABC kit,
$54.00, or factory-wired, $58.00.

We carry a full line of
S-M Products: each of
the kits, amplifiers, and
power supplies listed on
the opposite page is a
specialty with us, and
can be shipped promptly
from stock. A new edition
of our catalog is just out;
it contains a splendid
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quality kits, partsj and
accessories to be found
anywhere. The coupon
will bring it to you, free.

MAXIMUM

DISCOUNTS TO

DEALERS

WESTERN RADIO MFG. CO.
128 W. Lake St., Dept. SM-12
Chicago, 111.

Please send at once your new FREE catalog, list-
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Address

Town and State.

RADIO BROADCAST

DECEMBER. 1928

KEITH HENNEY

Director of the Laboratory

WILLIS KINGSLEY WING, Editor
EDGAR H. FELIX

Contributing Editor

HOWARD E. RHODES

Technical Editor

Vol. XIV. No. 2

*^>

Cover Design - - - From a Design by Harvey Hopkins Dunn

Frontispiece - - Old Saywlle Station Returns to Limelight 84

Quartz Crystals R. C. Hitchcoc^ 85

Phonograph-Radio Amplifiers Howard E. Rhodes 88

The March of Radio An Editorial Interpretation 91

Radio, the Corner Stone of the Amusement With the Broadcasting Stations

Industries The Progress of Aircraft Radio

WCY Protests Allocation Plan Radio Abroad

The joo-Mile Chain Regulation The Visual Broadcasting Field

The Fight for Short Wavelengths News of the Radio Industry

The "Skyscraper" Screen-Grid Receiver - - Clifford Denton 95
"Strays" from the Laboratory .,.,., Keith Henney 97

Accuracy of "Slide-Back" Voltmeters Data on Underground Aerials

Hum in the "Lab." Circuit Receiver Type in Tubes in First Audio Stage

A Test Set for the Radio Service Man - B. B. Alcorn 99

The Service Man's Corner - - 101

As the Broadcaster Sees It Carl Dreher ioa

Information on Sound Motion Pictures Broadcast Standardization

Some Experiments with Band-Pass Filters - - Kendall dough 104
"Radio Broadcast's" Home Study Sheets 107

No. ii. Resonance in Radio Circuits — Part I No. 11. Resonance in Radio Circuits — Part II

Loud Speakers — A Debate - - 109
Removing Nonsense from Short- Wave Transmission

Robert S. Kruse in

"Our Readers Suggest — 114

An A.C. Screen-Grid Booster Push-Pull with Standard Transformers

Work Table Clamp Home-Made Soldering Lug

The New A.C. Screen-Grid Browning Drake Receiver

Glen H. Browning 115

Book Reviews Howard £. Rhodes 116-125

An A.C. Band-Pass Screen-Grid Receiver— The Master "Hi-Qag"

William E. Barton 117

New High- Voltage Metallic Rectifiers for B-Power Units

/. George Uzmann 120

"Radio Broadcast's" Service Data Sheets - - - 121

No. ij. The A.C.-66 Dayton Receiver No. 14. The Fada Models 50, 70, 71 and 71

Problems in Synchronizing Television Receiving Discs

Boyd Phelps 123

The "Chronophase" for A.C. Tubes - - - Bert E. Smith 126

New Apparatus and Their Applications - 127

Manufacturers' Booklets 128

"Radio Broadcast's" Laboratory Information Sheets - - 130

No. 141. Supplying Power Devices from 110 No. 146. Power Output Characteristics of

Volts A.C. Tubes

No. 141. Resistance-Coupled Amplifiers No. 147. Television

No. 143. Resistance-Coupled Amplifiers No. 148. Frequency Characteristics of Tele-
No. 144. Alternating-Current Ratings vision Amplifiers
No. 14T. Power Output

Letters from Readers 137 j

Short-Wave Stations 141

The contents of this magazine is indexed in The Readers' Guide
to Periodical Literature, which is on file at all public libraries.

AMO7s[G OTHER THINGS. . .

WITH this issue, we start the promised department for
radio service men. The service man is a most important
element in the present radio structure but for some reason or
other he has been inarticulate. We know that a great many
readers of RADIO BROADCAST are doing service work, either on
whole- or part-time and we hope that those who are doing this
work will write us, telling of their problems, how they are being
solved, and of topics they would like to see discussed. Incident-
ally, the head of one of the largest New York organizations
specializing in this work, Mr. John S. Dunham, writes: that
although he believes service articles are of real value that "the
average service man could derive far greater benefit by pains-
taking, thorough study of RADIO BROADCAST'S Data Sheets
from the beginning and the very excellent series of Home Study
Sheets, recently inaugurated. From our own experience, we
believe that service men generally need to increase their basic
knowledge." With this, we agree, but we are certain that the
experiences of service men are of deep interest to others working
in the field. It certainly goes without saying that no service man
can really do his work intelligently unless he has a thorough
background in fundamentals.

THE application of power amplifiers, microphones, moving-
coil loud speakers and similar apparatus for so-called public-
or group-address work seems to be increasing rapidly. This de-
velopment is a natural and fortunate one and in our opinion due
largely to the increasing appreciation of what good radio ap-
paratus can do. The question which most frequently comes up
is not the simple one of how to connect or to operate the gear
but what power output is essential for a given service. RADIO
BROADCAST Laboratory Data Sheets Nos. 245 and 146 in this is-
sue discuss this interesting question.

EARLY in 1926, Mr. Howard E. Rhodes joined the staff
of RADIO BROADCAST and since that time, his excellent
articles have added much to the interest and technical value of
our pages. The popular Laboratory Data Sheets are Mr.
Rhodes' work. We are pleased to announce that effective
November ist, Mr. Rhodes was appointed Technical Editor of
this magazine.

THE current issue contains many articles on subjects of
great interest to many radio folk. Notable among these are
the following: the article on cutting and grinding quartz
crystals, the data on underground antennas in "Strays from the
Laboratory," the references to sources of information on sound
movies, the article on experimental band-pass filters, the dis-
cussion of moving-coil and "magnetic" type loud speakers,
Mr. Kruse's article on amateur experimenting and finally Boyd
Phelps' description of his ingenious work in television synchron-
izing which is found on page 123.

THE January issue will contain the long-promised article
on moving-coil speakers and is worth waiting for. Con-
structional stories on interesting receivers, and power amplifier
units are promised as well as more useful data for the experi-
menter and service man.

— WILLIS KINGSLEY WING.

OOUBLEDAT, OORA^ & COMPACT, IA[C., Garden Qity, Hew

MAGAZINES

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TORONTO: DCH'BLEDAY, DORAN & GUNDY, LTD. L. J. MCNAUGHTON, Assft Treasurer

1928, hi the United Slatei. Newfoundland, Great Britain, Canada, and other countries by Daubleday, Doran & Company, Inc. All rijhtj reserved.

TERMS: $4x90 a year; single copies 39 cent*.

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Old Sayville Station Returns to Limelight

forX »„ * /' / "•<"«'»'«'«» engineer of the Federal Telegraph Company, is stationed at Sayville, Long Island
for the purpose of reconditioning the once-famous radio transmitter which was built there by the Germans in 1912 This

th moZll f"rer ^ ^ U- S'Na?y *"",",* the W"rld War and •***>•<* <•» MS; however, it may soon be one of
Ztrnrnr>. '" the.a""rld- ™* interior «>«' **•«» "Portion of the transmitting equipment. The

Tf"/'fj7cA'Bra^ra'«s"m;«»'^»»'^/«^/>«''^'» the rear, on the left is the adjustable inductor
tuning the counterpoise, and the variometer in the foreground is connected in the antenna circuit.

FIG. i. A NATURAL QUARTZ CRYSTAL CUT LN FOUR PARTS

'OST radio experimenters are now fa-
miliar with the advantages of using a
quartz crystal to control a radio-
frequency transmitter. Although many articles
have been written on the use of a crystal oscil-
lator, the actual procedure of cutting a proper
plate from a crystal of quartz has received rela-
tively little attention.

This article is concerned mainly with the
actual operations and calculations used in cutting
and grinding a quartz oscillator plate from a
quartz crystal. It should be noted that a quartz-
crystal oscillator does its most reliable work when
certain factors are kept constant. These are
plate and filament voltages, plate and "tank"
circuit tuning, and most important of all, the
temperature of the crystal.

Assuming the tuning, lo remain fixed, when a
crystal temperature is maintained constant, the
ordinary plate and filament voltage variations
cause only small frequency changes in a crystal-
controlled circuit. The frequency of such a crystal-
controlled circuit varies about one five-hundredth
as much as that of a similar tuned grid-oscillator
circuit with the same plate and filament voltage
variations.

SELECTING THE RAW MATERIAL

A PERFECT quartz crystal is rare, but
fortunately a perfect crystal is not required
for an oscillator. There are a few simple tests for
determining the suitability of a quartz crystal,
which require only the unaided human eye. A
good quartz crystal may be rough and dirty on
the outside, but generally is clear inside when
viewed by the naked eye, showing no colors or
dark regions. It also must be free from bubbles
and cracks. Quartz is often cracked mechanically
during mining. This of course would make it
unsuitable for oscillator use, as a crack would
lengthen into a complete break.

The optic axis of the quartz crystal, so called
because of the unique optical effects obtained in
this direction, is located parallel to the edges

formed by adjacent hexagonal crystal faces. It
is in the direction of the dimension W in Fig. 3.
For reference in cutting as well as convenience
in clamping, a crystal should have at least one
of its natural hexagonal faces present. An ordin-
ary crystal has all six hexagonal faces, and a pyra-
midal point on one end. The end opposite the
point is generally full of flaws and is broken off
in mining. Fig. I on this page shows a
natural quartz crystal, with the first three cuts
made for crystal oscillators. The left section
contains the point, and is of a quality suitable for
oscillator use. The right-hand section is full of

-}&•>

(~^R YSTAL-CONTROLLED transmitting
^—' stations are rapidly becoming the rule
rather than the exception. The new amateur
bands which become effective in 1929, and
the insistent demand for greater frequency
stabilisation among broadcasting stations,
make it imperative that transmitters use
quarti plates which will bold the frequency
of the station within very close limits. These
plates must be sawed from quarti crystals,
and then ground to the desired thickness.
This article, by R. C. Hitchcock, of the
Ifestinghouse Electric &• Mfg. Co., gives
the details of the modern technique of crystal
grinding, and should appeal to amateurs
and broadcast station engineers alike.

— THE EDITOR.

bubbles and other flaws which make it unsuitable
for an oscillator.

Although the pyramid at the point of a quartz
crystal is bounded by true planes, the hexagonal
sides are seldom planes, their faces more often
showing stria t ions. These striations resemble
steps, the crystal cross-section being gradually
made smaller by these definite steps in the direc-

tion of the point. Striations are sometimes closer
than fV inch and quite deep, and others may be
over an inch apart and so shallow that the
crystal must be examined closely to distinguish
them.

The striations, if they are present, are useful
as reference marks, as the first cuts are to be
made in exactly the direction of the striations,
that is, perpendicular to the hexagonal faces of
the crystal.

The usual specifications for a good quartz
crystal, or rock crystal, for oscillator cutting are
that it should be free from twinning, have na
flaws, and be of optical quality. It is true that
sometimes a crystal plate is a satisfactory oscilla-
tor while all the other properties mentioned above
are lacking. But in general, until one has had
experience and is willing to risk cutting plates
with the certainty that some will not oscillate,
the best recommendation is that the proper
quartz crystal be secured from a reputable dealer
in minerals.

The cost of a quartz crystal depends on its size
and quality. For the quality specified above ihe
usual price is about five dollars a pound. A crystal
2y" across the hexagonal flats, and 3" over all
in length, weighs about a pound. For larger
crystals the price per pound increases. Clear
quartz crystals having dimensions all less than
one inch are quite cheap, but are not large enough
for quartz oscillators. These tiny crystals can
be used in fused quartz work and for ceramic
glazes, and are sold for a few cents a pound.

CUTTING QUARTZ

QUARTZ is very hard and some form of
machine for cutting is advised, although for
grinding quite satisfactory work may be done by
hand. Fig. 2 shows how an inexpensive polishing
head may be arranged for cutting a quartz
crystal. A lathe could also be used for this work,
but special precautions would have to be taken
to prevent the grinding compound from ruining
the bearings and the ways.

RADIO BROADCAST

Quartz- Crystal
^-Cutting Wheel

Pan

plash
Guard

FIG. 2

A copper or brass disc -,1," thick and 6" to 8"
in diameter revolves in a pan nearly filled with
No. 150 carborundum and water. More of this
cutting compound has to be added as the cutting
progresses, as a good deal of the material splashes
out. Shields should be placed to prevent the
spattering of walls and floors if the cutting ap-
paratus is set up at home. A splash guard should
also be provided as shown in Fig. 2, and a ring,
R, put on the shaft to prevent the compound
from working into the bearings. During cutting,
the compound in the pan should be stirred so
that the carborundum is kept in suspension.

The crystal is bolted to a hinged piece as shown
in the figure. A thin wooden block is placed on
top of the crystal so that the crystal will be cut
clear through before the cutting wheel reaches
the hinged piece. The cutting speed should be
250 r. p. m. or slower. If an 1800 r. p. m. motor
is used, the pulley on the polishing head should
be 1800/250 = 7.2 times as large as that on
the motor. A motor of i h. p. is about the right
size when the crystal rests on the cutting wheel
of its own weight, as shown in Fig. 2. About
twenty minutes will be taken to cut through a
two-inch crystal. If a weight is added to the
crystal to make it feed faster, a more powerful
motor will be required.

As shown in the picture in Fig. i, the
first cuts are to be made perpendicular to the
crystal's hexagonal faces, and the section cut will
be a right prism. These cuts should be 1.25" apart
so that the finished size can be made 1.10" or
28 mm. This width dimension is called W in
F'g- 3-

METHODS OF SLICING QUARTZ CRYSTALS

TTWO methods of slicing an oscillator crystal
* from the right prism are shown in Fig. 3.
The original method, given in 1880 by the Curies,
is still used for all low-frequency crystals, and
sometimes for high radio-frequency crystals.
This is shown as Method 2. For frequencies higher
than 600 kc. (less than 500 meters wavelength) a
Method i crystal controls more power, is easier
to make oscillate, and uses less quartz. For a
given frequency a Method I crystal is about
two-thirds as thick as a Method 2 crystal.

Fig. 3 shows clearly how the slices are made;
a Method I plate has its faces parallel with the
crystal hexagonal faces; a Method 2 plate has its
faces at right angles to these faces. The dimen-
sions L and W are not critical but good results
will be obtained if L is about i" or 25 mm, W
being about 1.10 inch, or 28 mm.

The quartz prism is bolted to the hinged piece
and slices made according to Method i or 2 are
cut. The cutting disc wastes material and does
not cut squarely, so a larger slab should be cut
than is needed for the actual crystal size. For
frequencies above 600 kc. a slice \" to §" thick
should be made.

The edges of the L and W dimension are

ground square, then beveled so that the crystal
will not crack along the edges while the flat sur-
faces are being ground.

The distance T is the oscillating dimension
and Fig. 5 gives the value of T for frequencies
between 600 and 4000 kc. The frequency and
wavelength are given in the left column, and the
oscillating dimension in both millimeters and
inches is given in the right column. The short
center column is a constant K, called the "meters
per millimeter," meaning that for each milli-
meter of a quartz oscillator there is a definite
electromagnetic wavelength. The larger T is, the
longer is the wavelength, X, the relation being
X = K T (,)

The "meters per millimeter," or K, is found by

/ \

X. /

"*\J

i J

' L — •

METHOD 1

METHOD i
FIG. 3

dividing both sides of equation (i) by T, so that
1 = meters = K (2)

T millimeter

For Method I, K varies from 140 to 150 meters
per millimeter, while for Method 2, K is 100 to
1 16 meters per millimeter.

To use Fig. 4, line up the frequency at the
left column, with K in the center column, finding
T in the right hand column.

To be on the safe side, it is best to use the
smaller value of K for preliminary work so that
the crystal will be thick enough to allow for a
final adjustment. For precision better than 10
per cent, a wavemeter or some source of standard
frequency should be used. Fig. 5 gives only ap-
proximate values of T. A more exact method of
figuring T and how to find the proper K for a
given piece of quartz will now be given.

CALCULATING K FOR A CRYSTAL

UARTZ crystals have different values of K.
noted above, and the only sure way to
know the frequency is to measure it. However, a
fair determination of K can be made for a given
crystal, if the frequency is measured. Suppose a
Method I crystal for 1800 kc. is wanted, and
the value of 140 meters per millimeter for K is
used. This shows T to be 1.19 mm. When T is
ground to I.IQ mm. the frequency is measured to
be 1760 kc. sec. Using equation (4), to be derived
later, it will be found that the correct K for this
piece of quartz is.

l^_ 300,000 _ 300,000 _ 143 meters
TF 1.19x1760 millimeters

DECEMBER, 1928

Putting this value of K in equation (4), the
correct T is

300,000 _ 300,000

K F 143 x 1800

= 1. 166 millimeters

The thickness measurement is used for con-
venience in grinding, and is not as reliable as a
measurement of the frequency. The final adjust-
ment of a crystal for frequency should be made
with a wavemeter or some known standard of
frequency.

For testing, the oscillator circuit of Fig. 4 is
suggested. A io,ooo-ohm wire-wound resistor
and C battery provide grid bias. The use of a
grid choke coil should be discouraged; a good
crystal does not need it, and a crystal which will
not oscillate without a choke should not be de-
pended on for control. At 1000 kc. over 50 watts
can be obtained from a single crystal-controlled
tube using only a grid resistor, and no grid
choke. For power work, from 20 to 50 watts, the
crystal must be kept at a fairly low working tem-
perature to prevent overheating.

For frequencies below the broadcast band, and
as low as 25 kc. / sec., crystals are cut by Method
2, oscillating along the L dimension ,T being from
2 to 4 mm. K has the same meaning for these
low frequencies as for the regular Method 2
crystals oscillating in the T dimension.

NECESSARY PLANENESS OF CRYSTAL PLATES

'"THE faces of an oscillator plate should be as
1 nearly fiat and parallel as possible. That is,
T should be the same throughout the crystal in
order to have the whole crystal oscillating use-
fully as a unit. Tests have shown that the best
crystals have variations in T such that the cor-
responding natural frequencies are not more
than .5 kc. different. For instance, a looo-kc.
crystal may have thicknesses corresponding to
frequencies ranging from 999.75 to 1000.25 kc.
This makes it necessary to grind a high-frequency
crystal very accurately plane and faces parallel,
the accuracy required increasing as the square
of the frequency.

If X is the wavelength in meters, and F the

frequency in kc. sec., the well-known relation is

x _ 300.000 (3)

Eliminating X between equations (i) and (3).
300,000 (4)

KT =

or, T =

To find the relation between small changes in
T and F, equation (4) is differentiated, giving

For frequencies from 600 to 4000 kc. the
curves of Fig. 6 give the thickness variation, dT,
for a frequency variation dF = .5 kc. To take
a particular case, a Method i crystal of 3000 kc.
should have a thickness variation of only 8.5 ten
ten thousandths of a millimeter, or 3.3 hundred
thousandths of an inch, if the .5-kc. variation is
allowed. From Fig. 6 it will be seen that Method
i crystals must have only two thirds as much
thickness variation as Method 2 crystals of the
same frequency. This offsets to some extent the

FIG. 4

DECEMBER, 1928

advantage of using less quartz
for the Method i crystal.
However, the advantage of
greater power still favors the
Method i plate. '

Accuracy as great as given
above is impossible by ordin-
ary grinding but indicates
the desired goal. Crystals
with thickness va rial ions
twenty times as great as
given by Fig. 6 are often good
enough, even though their
frequencies are 10 kc. differ-
ent at various points. If a
crystal is ground with two
frequencies nearly the same
— for instance, one half the
crystal at 1000.0 kc. and the
other half at 1000.5 kc.— an
audible beat note of .5 kc.
will probably be produced
and the crystal will not be
satisfactory for control. This
will not occur if the two
thicknesses are irregularly
spaced over the whole
crystal.

For very high frequencies,
due to the difficulty in mak-
ing crystals plane enough,
it is recommended that a
tow-frequency crystal be used
in a circuit whose harmonics
are amplified to obtain the
desired high frequency.

GRINDING QUARTZ

THE thickness grinding
1 may be done by hand,
using a micrometer to mea-
sure the thickness. Crystal
plates should be ground with
their faces oriented as nearly
as possible like the plate

QUARTZ CRYSTALS

METERS

80 — »

90-

100-

150-

200-

300-

400-

500-

600-

KC/5EC.

-3,000

•E.OOO

-1,500

-I,OOO

750

METERS

PER
MILLIMETER

6.0-
5.0-

•*.0 —
3tO —

100-

150-

2.0 —

•600
•500

FIG. 5

1.0-
.3-

.a-
.7-

INCHE5

-.3

-.1
-.09

-.07

_— .06

— .05

_ — .04

shown in Fig. 3. However, if a crystal has its
faces quite accurately parallel, a change of a few
degrees in having the crystal
0 2 plate line up with the one

{JJ U shown in Fig. 3 is not serious

\ Jt except as it changes the value

^ of K. As this factor varies for

different crystals, and has to be determined for
each one by a frequency measurement, the im-
portant thing is to have the faces parallel.

Gasoline or benzol should be kept handy to
clean the grinding compound from the crystal
before measuring or testing in a circuit. After
cleaning, a crystal should be handled as little as

600
700

5OO

800

400

900
1000

300
ZOO

9v:

<X*P-

,,c

,0,

2000

x
/w

3000

100

4000

aO
60

4
4

4
i
4

4
4

1
»
4

n
1

(

«-

4
4

INCHES

J
m

(— r i i i i — ' — i — iii' — — .f.
% S 8 8 Q MILLIMETERS g g
o o o o p Q

ALLOWABLE THICKNESS VARIATION CdT) FOR
A FREQUENCY VARIATION (dF)=. 5 KC/SEC-

FIG. 6

possible to prevent getting
grease on it from the fingers.
For the first stages of
grinding an ordinary piece
of glass is flat enough. First
use No. 150 carborundum,
then 302^ and 303! emery.
Pour the powder upon the
glass, adding a few drops of
water, and m\x it with the
powder to make a grinding
paste. Move the crystal plate
around in circles, holding it
with the finger tips. If desired,
the crystal may be fastened
to a block of metal with low
melting wax, and the metal
block held by hand.

During grinding the thick-
ness should be measured
from time to time; be care-
ful not to grind down the
edges too far. If a certain
spot is too high, a small brass
piece charged with the grind-
ing compound should be
rubbed over it.

When approximately the
right thickness by micro-
meter, a crystal should be
tried in its circuit and the fre-
quency measured. If the
frequency is much too low, a
new calculation of T should
be made, as already men-
tioned.

Sometimes a crystal jumps
abruptly to a higher frequency
during grinding, just before
the calculated thickness is
reached. This does not neces-
sarily mean that this crystal
is useless, as often the fre-
quency will jump lower by
grinding either L or W a
little, of the order of 0.020 inch. If a crystal
refuses to oscillate when thin, although it
oscillated well when thicker, this is due to ir-
regularities in the T dimension, and a grinding
off of the high spots will cause the crystal to
oscillate.

When using a micrometer as the sole means
of testing a crystal plate thickness the plate
often deviates from a plane, the reason being
that a hollow on one side and a high spot on
the side corresponding can not be detected by
an ordinary micrometer. When grinding very
thin crystals this is especially likely to happen,
and often prevents the oscillation of an other-
wise satisfactory crystal plate. One remedy is
to use a metal straight edge as a reference
in keeping the crystal plate flat and free from
high or low spots. The crystal is held up to the
light, and the straight edge moved slowly over
the surface. The high spots will be seen to
touch the straight edge, and cracks of light
will indicate low portions.

Polishing with rouge may be done with a
leather surface, but if the thickness is nearly as
good as the 0.5 kc. variation of Fig. 6 gives, no
great increase in power or other advantage will
be gained by polishing the faces of the quartz
crystal.

It has been shown that high-frequency crystals
must be very accurately flat and parallel. The
grinding of the thickness is by far the most im-
portant item of the whole operation of grinding
a crystal. A few hours spent in careful and pains-
taking work in grinding the thickness will be
repaid by the successful operation of the quartz
oscillator crystal plate.

— .03

HOWARD E. RHODES

Technical Editor

THERE is a natural link between the phono-
graph and the radio, for they both consti-
tute means of bringing entertainment into
the home. In a sense neither is quite complete
without the other and both may be combined
advantageously into a single instrument. There
are many commercial examples of this — Victor,
Columbia, etc. — with which the reader is prob-
ably familiar. From a small but carefully se-
lected group of records one can obtain a great
deal of pleasure, and, when the radio program be-
comes tiresome (as it frequently does), it is con-
venient to be able to turn on the phonograph
and listen to one's favorite selection. The pictures
in this article illustrate some apparatus, both
home-constructed and manufactured, that can
be utilized readily in assembling a phonograph-
radio combination.

What apparatus do we need? For the radio set
we require a tuner with which we may select
and detect the radio signal, an audio amplifier,
and finally a loud speaker. For the phonograph
we require an electrical pick-up unit an au-
dio amplifier and a loud speaker. The audio
amplifier and loud speaker may be arranged so
that they may be used interchangeably with
either the radio or the phonograph; these two
sections will differ, therefore, only in the first
part, a tuner being used for radio and an electri-
cal pick-up for the phonograph. This article
is devoted to a description of an amplifier and
loud speaker combination designed to fit into the
lower compartment of a cabinet which also con-
tains space for a radio receiver; located in the top
of the cabinet is a phonograph turntable. All
of the apparatus may, of course, be light-socket
operated. The choice of apparatus should be
limited only to the extent that good parts must
be used. The apparatus can be arranged in any
fashion suiting the desires of the individual con-
structing the set.
The picture, Fig. i, is typical and shows the

installation of an audio
amplifier in the bottom
compartment of a cabi-
net designed to house a
phonograph and radio
The loud speaker is
placed on the baseboard
on which the audio am-
plifier was constructed.
The inside of the cabi-
net has been lined with a
layer of porous material
so as to prevent to some
extent cabinet resonance
which tends to make
some loud speakers
boom at the low fre-
quencies. The blank
space at the upper part
of the cabinet is for the
radio tuner; we have
not shown a receiver in
this position because we
wish to make it quite

evident that any good tuner may be used, be it
tuned r.f., superheterodyne or any other type.
The lid on the cabinet, shown in a slightly
opened position, closes down on the compart-
ment holding the phonograph turntable. There is
ample room in this compartment to permit plac-
ing the electrical pick-up in the correct position
relative to the turntable.

In front of the loud speaker is placed a baffle-
board shown in the picture lying against the left
door. This baffle should preferably be con-
structed of a piece of board about I or ij inches
thick of such dimensions as to fit into the open-
ing in the cabinet. To hold the baffleboard in
place it may be screwed to the front of the base-
board on which the amplifier is constructed.
There is supplied with this cabinet a decorative
screen which fits in front of the baffle and helps

FIG. 2. THE PHONOGRAPH-RADIO AMPLIFIER CONSTRUCTED BY THE WRITER

FIG. I. THE PLATTER PHONOGRAPH-RADIO CABINET WITH AN
AMPLIFIER AND REPRODUCER IN THE LOWER COMPARTMENT

to improve the final appearance of the instru-
ment.

The amplifier, shown in Fig. 2, constructed in
the laboratory to give the reader an idea of the
kind of apparatus which may be utilized (any
good amplifier may, of course, be used), employs
an a.c.-operated two-stage transformer-coupled
circuit. A 22y-type tube is used in the first
stage and a 25o-type power tube in the out-
put stage. The output of this tube — about 4.5
watts — is more than enough for all purposes.

LIST OF PARTS

npHE circuit diagram of this amplifier is
' given in Fig. 3, and those who have had
experience in the home construction of such
units will obtain the information they require
from the circuit diagram and the pictures. Those
who haven't had such experience will do better,
we feel, to buy a complete amplifier or a complete
kit of parts, which can be assembled very easily.
The parts used in the amplifier are listed below.
Other makes of parts electrically equivalent
may, of course, be used.

The following is a list of the apparatus used

in the power amplifier constructed in RADIO

BROADCAST'S Laboratory:

Ci, C2, C3, €4 Four Acme Parvolt by-pass con-
densers, i-mfd., 40o-volts;

Cs, Q Two Acme Parvolt filter condensers, 2-
mfd. iooo-volt;

C? One Acme Parvolt filter condenser, 4-mtil ,
6oo-volts;

Q, C», Cio, Cn Four Acme Parvolt by-pass con-
densers, i-mfd. 400- volt;

L One dial light, 5-volt;

LI, Lj Two Samson filter choke coils, type-312,
3o-henry;

RI. One Ward Leonard fixed resistors, 5000-
ohm;

R3 One Ward Leonard fixed resistor, 2ooo-ohm;

RI One Polymet metallized grid leak, 25,000-
ohm, typeG-ijoj;

R< One Ward Leonard resistor, jooo-ohm;

Ra One Polymet metallized grid leak, 25,000-
ohm, type G-IJOJ;

R« One Polymet wire-wound resistor, I5oo-ohm,
type, W-1702;

DECEMBER, 1928

R7 One Electrad Truvolt resistor, jo-ohm,

type V-5o;
Rs One Electrad Truvolt resistor, lo-ohm,

type V-io;
R9 One Ward Leonard resistance bank,

type 507-6;
RIO One Centralab power potentiometer,

175-ohm, type PF-iyj;
Tj, 1 2, Two Sangamo audio transformers,

type A;

T3 One National filament-lighting trans-
former;

T< One General Radio power transformer
type 565-6;

T6 One General Radio loud-speaker filter, type
587-6;

Three Benjamin sockets, ux-type;

One Benjamin socket, uv-type;

Belden Hook-up wire;

Eight Eby binding posts;

One Platter orthophonic cabinet, type PR-o,i8.

The following tubes are required:

One power tube, 25O-type;

Two rectifier tubes, 281 -type;

One tube, 227-type.

The input to the amplifier is through leads
Nos. i and 2 which form a complete circuit

PHONOGRAPH-RADIO AMPLIFIERS

ANSMISSION UNITS

— / —
— / —

"^ Dotted curveshows ef
freuuencvehararLerisi
using filtering infrid
supply to tubes

«*ton "**»^
c of not ^5
and plat*

\
\

-9
-10
-11

—

-j

500 1000
CYCLES PER SECOND

5000

FIG. 4

RESPONSE CURVE OF THE AMPLIFIER
DESCRIBED IN THIS ARTICLE

for the audio-frequency currents, these currents
being kept out of the B-supply unit by the
resistor R,. Filter systems, consisting of resistors
and by-pass condensers, are used in the grid
and plate circuits of the 227-type tube, and also
in the grid circuit of the 25o-type tube, for the
purpose of keeping all of the audio-frequency
currents out of the B supply. The filter in the
grid circuit of the 227-type tube consists of
R2, C2, the plate-circuit filter is R4, C3 and the
grid-circuit filter of the 25o-type tube is Rs, C4.
The output of the amplifier feeds into the choke-
condenser unit Tj.

The B-supply unit for the amplifier
is conventional, consisting of two 281-
type tubes in a full-wave rectifier sys-
tem. In series with the output of the
rectifier is placed a small 5-volt flash-
light bulb, L. If a short circuit occurs in
the filter system, or at any other point
in the circuit, the current through this
lamp will increase sufficiently to burn it
out, thereby protecting all the apparatus
from damage.

The excellent frequency characteristics
of this amplifier are indicated by the solid
curve in Fig. 4. To give an idea of the impor-
tance of the various filter circuits, mentioned
in a previous paragraph, we have also shown
in dotted lines the frequency-characteristic
curve of this amplifier without the filters. The
importance of such filtering is obvious.

The home experimenter who likes to construct
his own gear may desire to build such an ampli-
fier as we have described, but the professional
set builder who has or hopes to get some orders
for the construction of a phonograph-radio
combination can do the job more quickly by buy-
ing a kit of parts or a completely wired amplifier.

SAMSON PAM-I7

SILVER-MARSHALL

678- PD
WITH CASE IN PLACE

SILVER-MARSHALL

678-PD
WITH CASE REMOVED

SAMSON
PAC-2

NATIONAL PUSH PULL
AMPLIFIER

SILVER-MARSHALL
682-250

FIG. 5. A GROUP OF AMPLIFIERS SUITABLE FOR USE IN PHONOGRAPH RADIO COMBINATIONS

RADIO BROADCAST

DECEMBER, 1928

FIG. 3. SCHEMATIC DIAGRAM OF THE WRITER'S AMPLIFIER

necessary A, and B voltages to both the ampli-
fier and the radio-tuner unit.

A large number of special amplifiers which,
in some cases, may be adapted to phonograph-
radio combinations, are also made by many manu-
facturers, including those mentioned in Table i.
As an example we might consider the installa-
tion of power-amplifier equipment in a hotel.
In such a case one would require one or more
amplifiers arranged in the form of a group, each
amplifier supplying a certain portion of the
power for the loud speakers. The utility of such
an installation is, of course, increased if it is
arranged so that music from either radio sta-
tions or phonograph records can be transmitted
throughout the system. When this arrangement
is employed it is necessary to connect the input
posts of all the amplifiers to the same set of
terminals, and a switching system is needed to
connect either the output of a radio receiver or
the output of a phonograph pick-up to these
terminals. Any readers interested in the details
of such amplifiers will do well to write the
manufacturers mentioned in this article for
complete descriptions of this equipment.

An excellent power amplifier which, unfortu-
nately, is not illustrated in this article, is the
Amertran type 2-AP. This is a complete two-
stage transformer-coupled amplifier, the output
stage being push-pull. Either 171- or 2io-type
tubes may be used in the push-pull stage and
the first audio amplifier tube may be either a
227- or a standard 201 .\-type tube.

Many such amplifiers are made — several are
illustrated here — and a list of a few of the best
units, with their characteristics, is given in
Table i.

COMMERCIAL POWER AMPLIFIERS

IN FIG. 5 are illustrated two Silver-Marshall
' "Unipacs;" both of them satisfactory for use
in a phonograph-radio combination. Data on
these and other S-M. amplifiers is given in
Table I. As indicated, these amplifiers may be
obtained either completely wired or in kit form.
The 678-po amplifier is especially interesting in
connection with this article since the circuit is
arranged so that the field of the dynamic loud
speaker acts as the filter choke and is energized
by the d.c. current flowing through the filter
circuit. This amplifier, with its 25O-type output
tube is capable of supplying up to 4.5 watts of
undistorted audio-frequency power, to the loud
speaker.

In Fig. 5 are illustrated also some of the power
amplifiers made by the Samson Electric Com-
pany, which may be used in constructing a
phonograph-radio combination, or any other
unit from which high-quality reproduction may
be desired. Data on the various models are
given in Table i. It will be noted that the
amplifier PAM-I7 is similar to the PAM-I6 ex-
cept that it supplies field current for a dynamic
loud speaker. The type PAC-2 amplifier should
be used where the unit is also to supply B volt-
ages for the radio tuner. This amplifier, it will be
noted, also supplies a C voltage of minus 45 volts
which may be used to bias the grids of the r.f.
tubes in the receiving set.

The National Company also makes a power
amplifier that may be used. This amplifier is also
illustrated in Fig. 5, and Table I gives complete
data on the various models. Model 8110 is a
complete power amplifier and B-supply unit, but
the Push-Pull amplifier does not contain any
power supply and, therefore, must be used with
a separate power unit designed to supply the

Table I — Data on Power Amplifier Units and Kits

Manufacturer

Type
No.

Prices

Voltages Available for
Receiver

Description of Amplifier

Kit

Wired

B Voltages

A. C.

Voltages

Silver-Marshall

682-250

$96.50

$111.50

45, 90, 135

1.5,2.25

Two-stage transformer-coupled.

226 in first stage and one 210

or 250 in output stage. Recti-

fiers are two 281 tubes. An

874 glow tube is used.

Silver-Marshall

682-210

$102

$117

45, 90, 135

1.5, 2.25

Same as 682-250 except that

output stage is push-pull.

Either 210 or 250 tubes may

be used.

Silver-Marshall

678-PD

$66

$73

None

Two-stage transformer-coupled.

226 in first stage and one

250 tube in output stage.

Rectifier is one 281 tube. This

model supplies current for

field excitation of a dynamic

loud speaker.

Samson

PAM-17

$125

None

Two-stage transformer-coupled.

227 in first stage and 210's in

push pull in output stage.
Rectifier is one 281 tube.

This model supplies current

to the field winding of a

dynamic loud speaker.

Samson

PAM-16

$125

None

Same as PAM-17 except that no-

provision is made for supply-

ing the field of a dynamic

loud speaker.

Samson

PAC-2

$175

45, 90, 135

1.5,2.5

Two-stage transformer-coupled.

227 in first stage and 210's in

push pull in output stage.
Rectifier is one 281 tube. An

874 glow tube is used. This

model also supplies a — 4.5

volt C bias.

National

8110

$85

45, 67

None

Three-stage resistance-coupled.

(adjustable)

Uses one 210 power tuht- in

output stage. Rectifier is one^

281 tube. One 201 A and one

240 are used in the first two

stages.

National

8050

$75

45,67

None

Same as type 8110 except that

(adjustable)

the output tube is a 250 and

the rectifier is a type 280 tube.

National

Push-

$40

None

This unit is a two-stage trans-

Pull

former-coupled push-pull am-

Am-

plifier using type 210 or 250

plifier

tubes in the output. It does.

not contain a power supply

and must therefore be used

with a separate unit such as

the National Type 250.

American Trans
former Co.

2-AP

$60

None

Two-stage transformer-coupled.
Either a 227-or 20lA-type tube

in the first stage and 171 A- or

210-type tubes in the output

stage which is push-pull II doe»

not contain a power supply.

I HK MA

Nt-.W.S AND

uAmo Kvnsrnr

Radio May Become the Cornerstone of the Amusement Industries

IT IS the privilege of all men to consider the
age in which they live as the zenith of human
progress. In our little world of radio, the
last decade has been a kaleidoscope of evolution
and to-day we stand at the brink of a titanic
realignment of the communication and enter-
tainment worlds, with the versatile vacuum
tube as its cornerstone. We cannot escape the
conclusion that this decade will prove the most
significant in the history of the stage, the screen,
the phonograph, and the broadcasting industry.

Radio has grown from a humble sideline of
the electrical industry and a pursuit of the former
amateur experimenter, who refused to abandon
his hobby, to the position of key industry of the
entertainment world. The application of vacuum
cube amplification to practically every phase of
aural and visual entertainment promises to make
broadcast reception only one phase of the many-
sided business which will constitute the radio
industry of the future.

Five years ago, the prostrate phonograph
industry was revitalized by adopting the meth-
ods of the broadcast studio in recording and the
audio system of the radio receiver for reproduc-
tion. More recently, the motion-picture industry,
by an almost identical process, has incorporated
sound entertainment as an integral part of
screen reproduction and is, in consequence, en-
joying an amazing revival.

Slowly but surely, drama, concert, vaudeville,
motion pictures, phonograph, and broadcasting
are being drawn into the vortex to form a huge,
unified entertainment business, destined to
reach staggering proportions in volume of busi-
ness and to achieve undreamed of heights in the
character of entertainment and education which
it brings to the home. By this
process, also, the economic
problems of broadcasting will
be solved definitely and the
spasmodic character of pro-
duction in the industry signifi-
cantly readjusted.

SOME PREVIOUS PREDICTIONS

IN THE January "March of
Radio," we ventured some
predictions as to an ultimate
home-entertainment machine,
comprising broadcast receiver,
phonograph reproducer, radio
picture recorder, film projector
and, some day, television re-
producer. Nebulous as this
conception then appeared, it
seemed to us inevitable because
of the natural technical and
artistic alliance of these once
separated fields. In June, we
were able to chronicle the first
practical step in this direction,
the rumored merger of the
Radio Corporation of America
and the Victor Talking Ma-
chine Company. To-day, all
the important phonograph com-
panies are in the radio business.
Concurrently, came the talk-

ing-movie boom, utilizing many of the inven-
tions developed for radio.^More recent develop-
ments are providing the structure for the actual
manufacture of such a device.

The principal motion-picture producers are
licensed already by the Bell System to use one
or both of their two methods of sound synchron-
ized film systems. Vitaphone uses phonograph
records mechanically synchronized with the
film; Movietone records sound impressions
directly on the edge of the film by means of a
light shutter system. These light impressions are-
converted into sound at the motion-picture
theatre by passing light through the sound track,
upon a photo-electric cell.

The R. C. A. more recently entered the field
by exploiting a system developed by the General
Electric Company, using the oscillograph princi-
ple to make the sound record on the film. Having
entered the field later, there are, as yet, only a
few Photophone licensees, as the R. C. A. system
is termed, but with the prospective alliance with
the Keith-Albee-Orpheum circuits and the Film
Booking Offices of America, a huge number of
theatre installations by Photophone are in pros-
pect. Several other systems are soon to appear.
Acute shortage of equipment exists and there is
a feverish rush to speed theatre installations for
the reproduction of sound pictures.

At the present time, the R. C. A. and the Bell
System are in competition in the sound-picture
field. If the precedent of broadcasting is followed,
a combination of these rival interests will be
effected ultimately. Five years ago, the Bell
System laid the foundations for the National
Broadcasting Company by operating the first
chain of stations with WEAF as the key, while the

Radio Corporation and its associates maintained
wjz as the competing key station to a chain
connected through telegraph lines. Intense
competition proved uneconomic, with the result
that the National Broadcasting Company was
formed as a merger of the two systems.

UNIFICATION INCREASES EFFICIENCY

NEW SUPER-DIRECTIONAL HORN

The huge loud speaker illustrated above was developed by the Victor Talking Machine
Company, Camden, N. J ., and was designed to have marked directional characteristics.
The horn is used to remove the hazards of landing dirigibles by providing a means of
communicating directly with the ship while it is in the air. Successful results have
been obtained in tests with the U. S. Navy Dirigible J -4 flying at 7500 feet over the

Victor building.

the standpoint of efficient and eco-
nomic operation, unification of broadcast
studio management, concert bureau direction,
recording of musical accompaniment for sound
pictures, phonograph recording and vaudeville
management is a natural alliance. The operation
of broadcast input amplifiers, of electrically
operated devices for phonograph recording and
of sound-film recording devices, as well as of
reproducing equipment in theatres and public
address systems, is technically similar. Nothing
could be more natural and logical than the
merger of these activities.

There are, however, some practical obstacles
to the joining of so many forces. Political senti-
ment is against the concentration in a single hand
of so many potent means of influencing public
opinion as are presented by broadcasting and
motion pictures. The leaders in the radio field
have, at no time, been in greater need of unified
public support and of intelligent management of
their public relations. The very fact that all the
prospective mergers are announced as being
only in the negotiation stage is recognition of the
need for public approval in advance of actual
consummation.

The principle of unification and concentration
in industry is founded upon efficiency in public
service. So long as the policies of huge corpo-
rations are directed with im-
partiality, we not only tolerate,
but encourage the unification
of such important agencies of
general welfare as the telephone
service. Likewise, we may look
forward to centralization of
broadcasting, motion pictures,
phonograph recording and ulti-
mately television, provided that
service to every element of the
public, every taste, every strata
of society, and every shade of
religious and political belief is
considered in proportion to
their needs. The actual com-
pletion of such mergings may
have to await additional legal
safeguards but, more likely
than not, the immense detail
of negotiation is the only im-
mediate problem to be met.

The merger of radio, phono-
graph, and theatre interests by
a leading group of the industry
will, undoubtedly, result in
similar alliances on the part of
the other radio manufacturers.
There can be no practical
monopoly of any artistic effort
and, undoubtedly, in the pro-
spectively combined fields, we

RADIO BROADCAST

DECEMBER, 1928

will see competition of a character similar
to that now applying in our more limited
radio spectrum. The Radio Corporation ac-
tivities, it appears to us, are only an example
which will be followed by the entire industry in
time. \Vo also regard it as likely that, although
patents may be an important part in the Radio
Corporation structure, by the use of alternative
methods or by licensing, a competitive field will
be built up.

RESULTS OF UNIFICATION

TO THE radio manufacturer, the combi-
nation of these now separated industries
promises an immensely increased volume of busi-
ness and less seasonal fluctuation in production.
To the home user of radio equipment, it will
offer a more versatile source of entertainment
of both aural and visual character. To the artist,
it means greater opportunity to participate in a
much wider range of activity, instead of restric-
tion to a single field of entertainment, such as
recording, radio, screen, or theatre. The unit of
sale in radio equipment will rise manyfold, and
a billion dollar industry will soon appear. Every
element of the industry will enjoy greater pros-
perity, proportionate to the greater diversity
and service which it renders.

WGY Protests New Allocation Plan

THE most vociferous objections to the
allocation plan have been advanced by
WGY, which has been allotted a "daytime
only" channel, which it may use up to the point
that it interferes with KGO. Under the Com-
mission's definition of sunset, this is three hours
after sunset in the East, limiting WGY to trans-
missions up to about eight p. M. in winter and
nearly eleven p. M. in summer. If KGO stands by
for one hour after sunset, WGY gains most of the
important hours when it serves its more distant
listeners. The Commission points out, in answer
to WGY'S protests, that the First Zone, and
particularly New York State, has more than its
share of powerful stations and that there is one
station of 30,000 watts and one of 50,000 watts
within a hundred and fifty miles of WGY. Cer-
tainly, if the principle of equitable division
among zones and subdivision of zone quotas
according to states by population is to be ob-

served, there must be time sharing of one form or
another on the part of at least one of the power-
ful stations in the First Zone with a station in
another zone, so long as there are but eight
exclusive channels per zone. WGY was selected
for such sharing, no doubt, because it is not the
original source of chain programs but acts
principally as a relay station. Had wjz or WEAF
beeh selected for sharing with a Pacific coast
station, the effect would have been a great hard-
ship upon the largest broadcasting audience in
the world.

WGY HAS NATIONWIDE POPULARITY

\A/GY has pointed out that it is one of the
* * pioneer stations with the most widespread
audience of any station in the United States.
RADIO BROADCAST'S questionnaires certainly
support the contention that WGY has the most
enthusiastic "distance" audience of any station
in the country. The requirements of equitable
distribution are, however, inescapable. The ex-
clusive channels of the First Zone cannot be
assigned exclusively to chain stations or only to
those within service range of New York City.
The only alternative offered the Commission
is to suggest a time-sharing arrangement by WGY
with one of the New York stations, either wjz or
WEAF. While this might be satisfactory to WGY,
it certainly would be a blow of such serious pro-
portions to New York listeners that it accounts
for the fact that the Commission did not con-
sider that course. The original engineers' plan,
calling for fifty exclusive channels, provided
room not only for WGY in the First Zone but better
allocations for additional leading stations in all
zones. Reduction of the cleared band to forty
channels has complicated greatly the problem of
providing adequately for all the good stations
in all the zones.

One way out of the present situation might be
a more liberal definition of "sunset." The Com-
mission has ruled that daytime stations shall
close down at the average time for sunset during
a given month, at the point where the western
station, subject to interference, is located. How-
ever, night broadcasting conditions do not pre-
vail immediately upon the setting of the sun, but
only after quite complete darkness. Therefore,
the same sundown regulation as is used for the
lighting of lights on motor cars may be more

VIEW OF SAN FRANCISCO KI-.CI-. I VIN< , STATION AT DALY CITY, CALIFORNIA, SHOWING
ROTABLE LOOP AERIALS. THIS STATION IS OPERATED BY THE FEDERAL TELE-
GRAPH FOR THE RECEPTION OF MARINE SIGNALS

suitable for broadcasting regulation. The ad-
dition of an extra evening hour at such peak
times as ten and eleven p. M. would greatly
lighten the economic burden now placed upon
stations limited to daytime broadcasting. We
urge that experiments be made to determine the
proper time for establishing an official broadcast-
ing sunset, because we believe this offers a loop-
hole for improving the position of the worthy
stations, now compelled to sign off just at the
hours when they begin to have a fighting chance
to make enough revenue to meet their expenses.
The Chicago stations which protest and ask for
better channel assignments do not receive much
sympathy from the average broadcast listener.
Chicago stations have dominated the dials for
too long a time in the memory of the broadcast
listener to cause anything but glee when it is
announced that the Commission has somewhat
reduced the proportion of ether territory as-
signed to stations in that city. Chicago has had
its way about radio long enough and it will be a
relief to listeners, who like dial twisting, to find
something other than Chicago stations on the
clear places.

Reasons for the joo-Mile
Chain Regulation

THE regulation of the Commission, requir-
ing that the same program shall not be
duplicated in the exclusive channels by
stations separated by less than three-hundred
miles, has, for the time being, been waived, pend-
ing further investigation of the subject. Some
months ago, in considering the problem of the
frequent duplication of chain programs in the
few clear channels, we pointed out that an ideal
solution lay in limiting the nmmber of exclusive
channels assigned to stations of the same chain
to four or five widely separated points, requiring
that the bulk of chain broadcasting be conducted
on regional rather than nationally clear chan-
nels. In practice, however, such regulation leaves
an insufficient number of high-grade, independ-
ent stations, now carrying non-chain programs,
to fill the clear channels thereby freed.

Some form of regulation is necessary, how-
ever, if the real objective of the clear channel is
to be accomplished. The distant listener, beyond
the high-grade service range of any broadcasting
station, depends upon the nationally cleared
channels for his program service. If he finds all
stations within his range on these cleared chan-
nels radiating the same program, the funda-
mental objective of giving the rural listener the
best broadcasting service and the greatest
variety through cleared channels is not achieved.
It was such a consideration which caused the
Commission to pass the 3OO-miIe separation
regulation. The principal reason that the regu-
lation adopted failed is that there is an in-
sufficient number of high-grade independent
stations to fill the cleared channels; not that
there is anything fundamentally wrong with the
regulation itself.

The Fight for
Short-H/ave /^locations

£> ECRETARY of War Dwight S. Davis
^V has requested the Federal Radio Com-
^J mission to set up a new amateur band
between 5000 and 10,000 kc. This proposed band
is to be used for amateur work in cooperation
with Army radio stations. Oldtimers will re-
member that one of the first broadcasting sta-
tions in New York was WVP, operated under ihe
supervision of the Army with the cooperation of
a committee of amateurs. This station did its

WITH THE BROADCASTING STATIONS

share in the early days to introduce broadcasting
to New York.

/"\\E of the more ambitious of applicants for
fcx short waves is the newly formed Universal
Wireless Communication Company which wants
no less than 116 short-wave channels. To begin
its proposed service, it asks for an experimental
license to operate a New York-Chicago circuit,
to be followed, upon its success, by stations in
fifty leading cities. There is no indication of the
competence of the organization involved or its
financial resources in the press dispatches. Cer-
tainly, its sponsors are not overburdened with
modesty because they have the courage to ask
for frequency space worth many millions of
dollars.

A considerableTiumber of non-communication
companies have applied for short-wave channels.
Perhaps one of the most interesting applications
is that of the Montgomery-Ward Company,
which wishes to link nine factory branches, three
hundred existing stores, and ultimately 1500
stores by short-wave radio telegraphy. The means
now used for communication purposes for this
extensive group are the mails because telegraphy
is too expensive for the purpose.

If a rental had to be paid the Government for
the use of a channel, proportionate to its worth,
it is unlikely that many of the private services
now contemplated would be undertaken. The
situation with respect to short waves is exactly
similar to what would occur if we could have
free telegraph lines, the only expense to the user
being to furnish key and operator. Perhaps many
of the Government's problems could be solved by
turning over the channels in each class of service
to the highest bidder. Such a process would be a
shocking one to those w.ho consider radio a demo-
cratic Utopia, but it would eliminate a lot of the
useless fighting now going on; limit the employ-
ment of radio to services in which it is truly
superior and essential; avoid filling short-wave
channels with private services, requiring dis-
crimination against late but deserving applicants
and, in addition, make radio a revenue producer
for the Government.

IN SUPPORT of the Radio Corporation of
* America's application for 67 short wavelengths
its representatives stated before the Commission
that its principal purpose was to distribute
10,000 incoming transatlantic radiograms and
2000 transpacific radiograms daily. 95 per cent,
of these messages are addressed to individuals
and corporations in thirty leading cities, which
it is proposed to link. The R.C.A.'s represent-
ative stated that Western Union, with its 25,000
offices, and Postal with its 2000, are unwilling to
make satisfactory arrangements for handling
these messages. The proposed radiogram for-
warding business is not enormous and it seems
unfortunate that a special system of communi-
cation must be set up in competition with exist-
ing nationwide systems of wire communication
to handle such a reasonable amount of traffic. It
is within the Commission's jurisdiction, in con-
sidering the merits of this application for short
waves, to inquire why the wire facilities of the
country are not available on satisfactory terms
to handle the traffic involved.

Will) the Broadcasting Stations

ANEW departure in political programs was
offered by the Democrats when they put
on Irving Berlin, William Collierand Gene
Buck on a coast-to-coast network, together
with Fred Barrens' Democrat Orchestra. That
political speeches require entertainment support
has always been recognized, but this was the

THE RADIO OPERATOR OF THE COURTNEY FLIGHT AND HIS APPARATUS
Hugh Gilmour, the radio operator who accompanied Captain Courtney on bis attempt to fly the Atlantic, is
shown in his London home with the apparatus which he removed from the Dornier Wai flying boat u-ben he
was rescued by a life boat from the " Minnewaska" after floating in the ocean for fifteen hours.

first political broadcasting accompanied by a
goodwill, musical program.

\\J ALTER DAMROSCH, the dean of Ameri-
• * can orchestral conductors, has always
found the education of children in music his
happiest work. Under the sponsorship of the
Radio Corporation of America, he is now en-
abled to carry this out on a more extensive scale
than he dreamed possible ten years ago. He is
directing forty-eight school concerts in four
series, each series designed for a different group
of school and high-school students. The first
series is for children of the third and fourth
grades; the second for the fifth and sixth grades;
the third for seventh grade and junior high
school; and the fourth for high schools and
colleges. In this series, Mr. Dam'rosch takes up
each instrument and describes its part in the
spectrum of music. He also analyzes various
musical themes in a carefully worked out curri-
culum. Schools are actively taking advantage of
this meritorious educational use of broadcasting.

THE Bureau of Standards is undertaking
the calibration of crystal oscillators for
broadcasting stations on a moderate schedule of
fees. The tremendous volume of work entailed
in preparing for the new allocation requirements
is, no doubt, overwhelming the personnel of the
Bureau. We can count on the customary faithful-
ness of Bureau of Standards employees to do the
job with the utmost speed consistent with the
standards of accuracy which they maintain.

AN IMPORTANT addition to the Columbia
chain is WBBM of Chicago which, for its
power, has extremely good coverage in the cen-
tral west.

VA/LS, the famous Sears-Roebuck station in
' " Chicago, has been sold to the publication,
The Prairie Farmer. E. L. Bill has been retained
as its director. Inquiring persons point out that
the Commission is not required to approve this
sale and that it is not under obligation to grant
a license to the station under the new ownership.

DY INCREASING its power to 10.000 watts,
*-* KWKH becomes the South's most powerful
station.

XA/LW of Cincinnati and WTIC of Hartford,
' ' Conn., officially have been granted 25,000-
watt construction permits and may use an ad-
ditional 25,000 watts experimentally.

THE Federal Radio Commission quickly
yielded to the protest of Iowa, demanding
an exclusive channel, to which it is entitled
under the quota arrangement of the Davis
Amendment. Because of congestion in Chicago,
the Commission, in its original set-up, had bor-
rowed a channel for that city from Iowa. This is
the first, but not the last, example which will
show how useless the borrowing clause will prove
in practice. When there is a universal shortage
of any commodity, it becomes impossible to find
anyone who will loan freely to others. Certainly,
nobody feels inclined to make any sacrifices so
that Chicago may have additional stations.

AN OPINION by Federal Judge James H.
** Wilkerson confirmed the right of the Federal
Radio Commission to regulate wavelengths and
the power of broadcasting stations under the
Radio Act of 1927. This decision was rendered in
Chicago in the case of Stations WCRW and WEDC,
which stated that the Commission's power re-
duction, required under the new allocation plan,
represented confiscation of property without
due process of law. The effect of the decision
is merely to change the scene of argument from
a Chicago court to one in Washington where,
under the Radio Act, such appeals must be
brought.

Progress in the Field
oj Aircraft Radio

THE first of a more powerful type of radio
transmitter installations at an important
aircraft junction point is to be completed
at Cleveland, Ohio, in the near future. It is of
20oo-watt power and promises to give reliable
radio-telephone communication with aircraft in
flight for a distance of a hundred miles and many
times that distance by telegraphy. The standard
aircraft frequency of 335 kc. will he employed.
Eleven aircraft centers, other than Cleveland,
are being installed at the present time, and all
of them will be in operation within the next sic
months.

RADIO BROADCAST

DECEMBER, 1928

I pTLIZING 5OO-watt high-frequency trans-
^ mitters a complete chain of stations has
been established as the communications net-
work for the Chicago-Dallas air route. The
cities at which the transmitters are located are
Fort Worth, Oklahoma City, Wichita, Kansas
City, Unionville, and Moline.

D ELLEFONTE, PA., is the location of another
^ radio aircraft installation. This important
center for air mail service will have a directive
radio beacon, somewhat similar to that installed
at College Park, Md. Announcement is also made
of a new type of vibrating reed which is used as
an indicator on the plane, in which change in
frequency with temperature is practically elimi-
nated.

Radio Abroad

A VITALLY important patent decision was
made by the Comptroller General of the
Patent office in England when a compul-
sory license was granted to Loewe Radio Com-
pany. Under this compulsory license, the Loewe
Company will make three- and two-element radio
valves at license fees of 10 s. and 7 s. 6 d. re-
spectively, instead of £i, 17 s. 6d. and £i, 55.,
heretofore asked by the Marconi Company. In
the decision of the Comptroller General, he
stated that he was "satisfied that a case of
abuse of monopoly rights had been established."
The decision further states: "When we come to
consider the scale on which the applicants hope
to manufacture in this country, the Loewe valve
will be recognized as offering a new advantage
or utility for which it may be well worth sacrific-
ing the superior sensitiveness which the use of
reaction admittedly affords. Broadcasting has
become a feature of our national life and a
commercial policy which excludes large sections
of the public from its full enjoyment is not, we
think, a justifiable policy.

"The Marconi Company have secured a
dominant position by gathering together in their
own hands a large series of patents which, taken
collectively, cover almost all
broadcasting receivers of an effi-
cient character. It is to be remarked
that not one of the five patents
now in question is for an inven-
tion originating with the Marconi
Company. Three of them are
American, one is French and one
is German in origin.

" It is a kind of super-monopoly.
The applicants cannot seek alter-
native terms from competing
monopolists. They are absolutely
in the hands of those who have
gathered together this far-reaching
aggregation of monopolies."

THE British Broadcasting Com-
pany's educational curriculum
includes literary readings in
French, German, and Latin from
biographical sources, and a wide
range of subjects presented in
English on literary, historical,
geographical, and musical subjects.
This educational program is sum-
marized completely in a brochure
on the subject which should in-
terest ambitious American pro-
gram directors.

THE League of Nations an-
nounces that its plan of estab-
lishing a powerful radio-telegraph
station has advanced to the point

that it has now purchased a ;o-kilowatt trans-
mitter which will be operated, in normal times,
by Radio Suisse. The station and its personnel
may be taken over by the League, upon notice,
in emergencies.

THE Government of Australia, under a new
law . has taken over the ownership, equip-
ment, and facilities of broadcasting stations of
the Class "A" type. A system somewhat similar
to that now established in New Zealand is being
adopted for the control of broadcasting.

A COM MISSION has been appointed in
** Canada with a view to laying plans for the
establishment of a national radio system, similar
in character to the British Broadcasting Com-
pany. The commission will study the situation,
not only in Canada but in Great Britain and the
United States. Naturally, the owners of broad-
casting stations are opposing the move.

In the Visual Broadcasting Field

tOMMlSSIONER H. A. LAFOUNT is
reported as advising stations in his zone,

1 interested in visual broadcasting, that.
"In my opinion, the Commission will shortly
issue orders for the regulation of television and
picture transmission, particularly in the broad-
cast band. I, therefore, suggest that you defer the
purchase of any equipment or the making of any
investment until such action has been taken."
If restrictive regulation must be applied to
this experimental art to the discouragement of
those who are risking their time and money in so
problematical a field, it would be a good idea to
get it over with. There is.no known reason for
regulation at this time because picture broad-
casting is causing no trouble of any kind, but
the Commission has so often and so repeatedly
thrown the shadow of threatened regulation
upon it that progress in extending picture broad-
casting has been totally stopped. If the new field
must be stifled by the Commission, the sooner it
is done the better, because all the money, so far

V_>i

courageously spent by experimenters, is practi-
cally a total loss should the Commission pass any
serious restricting regulations. Further waste in
experiments may as well be stopped, sooner
rather than later, and protracting the agony is
not in the least helpful.

A VERY fine publicity story emanated from
** WGY, following the broadcast, on the eve-
ning of September 1 1, of the radio play from their
studio. The television transmitter was working
on the occasion. The story stated that synchron-
ization of speech and vision was perfect, but
there were no comments as to the character of
the images received. Those, technically ac-
quainted with the subject, were inclined to smile
at the statement that perfect synchronization
was accomplished, not because there was any
doubt that it had been done, but because it
would have been wonderful if anyone discovered
a practical means of transmitting speech and
television out of synchrony under the conditions
involved. A statement by Dr. Alexanderson, who
is receiving WGY'S transmissions at Lake George,
a distance of two-hundred miles, reported trouble
from a mirage or delayed image. Television may
prove useful in securing data with reference to
the heaviside layer.

THE Experimenter Publishing Company has
sued a magazine which was to appear under
the title Television. The application for the
injunction was, of course, denied by the New
York Supreme Court. Somebody ought to pub-
lish a list of patented words.

News o] the Radio Industry

IN URGING industry support of the R. M. AJ
patent pooling plan, Le Roy J. Williams of that
Association pointed out that the automobile
industry, which solved its patent difficulties
by means of an identical scheme, did not find a
single patent basic. The R. M. A. plan, as our
readers will remember, provides for the pooling
of non-basic patents at the option of their hold-
ers, and permits patent holders
to exempt so-called basic patents
from the pool. The argument,
however, does not hold good for
the radio industry because there
are many patents of a decidedly
basic character still ,in force. The
pooling of non-basic patents is ol
negligible importance.

A RECENT s
R. M. A. ir

NEW AUTOMATIC SOS RECEIVING APPARATUS
The S. S. "Cedar Bank" recently has been equipped with Marconi apparatus
for automatically receiving SOS signals. The signals are received by the three-
tube regenerative set shown on the left, and below the receiver is the selector re-
lay •which rings bells, in the radio room, on the bridge and in the operators'
• tiitcrooni, whenever an SOS signal is picked up.

statement of thq
innocently rises to
the point of humor when it seeks
to throw aspersions on the .t ecu-
rate and comprehensive statistics
which the National Electricaj
Manufacturers' Association ha<
issued with the cooperation of th^
Department of Commerce. Th«
R. M. A. now proposes to go into
the statistics business also, utiliz-
ing agencies of a character undis-
closed to assist it. "The study of
available, but incomplete statis-
tics, largely estimates, is being
made by the Committee which
will present recommendations to
the R. M. A. Board of Directors
for the development of real stat-
istics which are reliable and may
be of actual service to all branches
of the radio industry." Silly1
publicity of this character cer-
tainly does not reflect glory upon
those who issue it. — E. H. F.

FROA[T VIEW OF "SKYSCRAPER" RECEIVER

The ^Skyscraper** Screen-Grid Receiver

FR a long time the neutrodyne type of
eceiver, consisting of a two-stage neu-
tralized radio-frequency amplifier using
2OiA-type tubes, a non-regenerative detector
and a two-stage audio-frequency amplifier,
justly has been considered an excellent set —
sensitive, selective, and easily controlled. The
receiver described in this article has these plus
some additional good characteristics. This set
uses two stages of r.f., detector and a two-stage
audio amplifier, and to this extent is to be likened
to a neutrodyne — but here the similarity ends.

In this receiver two 222-type screen-grid tubes
are used in the r.f. amplifier and, as
a result, it becomes unnecessary to neu- ^gg
tralize the r.f. circuits and greater am-
plification is obtained. The use of the
screen-grid tube in the r.f. stages makes
it essential that shielding be used and
that all the plate and filament circuits be
bypassed and filtered to prevent common
coupling between the various circuits,
which would result in oscillation. As part
of the shielding, it is recommended that
Remler tube shields be placed over each
of the 222-type tubes.

Starting with the antenna connection
we will review the circuit arrangement
used in this receiver, and in this way we
will be able to bring to the attention of the
reader the various interesting design fea-
tures which the set possesses. The circuit c#s"~
diagram of the receiver is given in Fig. i.

The input circuit to the receiver is arranged so
that either an antenna or a loop may be used.
The loop is connected to the two terminals
marked "Loop" in which case no ground need be
used, although a ground may be connected
to the ground terminal if one desires. The
antenna connects to the first coil LI, through a
small o.oooi-mfd. fixed condenser, Q. The switch
SW2 adapts the set for use on either a short or a
long antenna, and, when the set is put into
operation, reception should be checked on both
positions of the antenna switch. In locations
where there are several near-by broadcasting
stations and a selective receiver is essential, the
antenna switch should be thrown to terminal
No. i. With the antenna switch in this position
the set is very selective; if the switch is thrown
to terminal No. 2 the volume will increase but

By CLIFFORD DENTON

I t-rranti. Inf.

the set will lose some selectivity. In out-of-town
locations, where very sharp tuning is not re-
quired, the set generally will give most satis-
factory operation with the antenna switch on
terminal No. 2.

The output of the tuned circuit LiQ feeds the
grid circuit of the first 222-type tube which in
turn feeds into the r.f. transformer consisting of
L2L3. This transformer is wound on a threaded
hard-rubber tube. The plate coil is wound first
with a small-size wire in the bottom of the
grooves and on top of this primary winding the
secondary is laid, the secondary wire being of a

CT'HE receiver described in this article is a development
•*• of Robert Arnold, in association with the laboratory of
the Ferranti Company in the United States. The set was
examined and tested in operation in New York Citv by a
member of the staff of RADIO BROADCAST Laboratory and
it performed very_ satisfactorily. The set is quite selective
and the fidelity is excellent. RADIO BROADCAST will be
pleased to hear from those who undertake the construction
of the set.

Constructional data is not given in this article for
complete information of this sort may be obtained directly
from the Ferranti Company.

— THE EDITOR.

size such that it lays on top of the groove. The
turns ratio is i to i. The coils are mounted on a
standard four-prong base and plug into sockets
mounted on the sides of the shields, as indicated
in the picture of the receiver.

This secondary, Ls, of this transformer feeds
into the second radio-frequency tube whose out-
put circuit supplies energy to the next trans-
former consisting of L4L6; this transformer is
similar in construction to the preceding one. A
C-bias detector is used; the plate is supplied
with 90 volts through the jo.ooo-ohm resistor,
Ri, and minus 6 volts is used on the grid. The
output of the detector goes to the first audio
transformer, Ti, across the primary of which is
connected a jack, Ji, to which a phonograph
pick-up unit may be connected. The output cir-
cuit of the audio amplifier is push-pull with

type tubes. The output transformer, T3, should
be of a type designed for use with the particular
loud speaker which is to be used with the set.
The designers of the receiver, feeling that many
experimenters might have available an audio
amplifier and, therefore, desire only to construct
the radio-frequency and detector circuits, have
shown the jack ]i in the plate circuit of the de-
tector. If a separate amplifier is to be used it
should be connected to this jack.

As we mentioned in a previous paragraph all
the battery circuits of the receiver have been
filtered carefully in order to prevent common
coupling which is often the cause of oscil-
y-^ lations and motorboating. The screen-grid
circuits of the two r.f. tubes are filtered
by the [o,ooo-ohm resistors, Rj and RZ
and the condensers, C6 and Q. The plate
circuits of these tubes are filtered by resis-
tors RI and Rs, each with a value of 50,000
ohms, and the by-pass condensers, Cy and
C8. The detector circuit is filtered by the
4-mfd. condenser C9 and the resistor RI.
Filtering in the detector circuit is es-
pecially important since even a small
amount of coupling at this point will
affect the characteristics of the audio am-
plifier and either make it distort or hum
badly, and in some cases the coupling may
be sufficient to make the audio amplifier
oscillate. i

^Or* Three filter circuits are located in the
audio amplifier. The grid circuit of the
first audio-frequency amplifying tube is filtered
by a 2-mfd. condenser, Cio, and a 50,000 resistor
Ry. The plate circuit is filtered by a 2o,ooo-ohm
resistor, R8, and the condenser Cn with a capacity
of 2 mfd. The third filter circuit is located in the
grid circuit of the input push-pull transformer,
T2, and it consists of a io,ooo-ohm resistor, Rj,
and the condenser, Ciz, whose capacity is 2 mfd.
| Push-pull amplifiers frequently have a ten-
dency to oscillate especially if the power tubes
have slightly different characteristics. If the
amplifier does oscillate it can be overcome by
placing a 5O,ooo-ohm resistor between the
center-tap of T2 and R9, in the grid circuit
of the input push-pull transformer. This resistor
should not be by-passed. The use of this resist-
ance will not in any manner adversely affect the
quality. Editor.]

RADIO BROADCAST

~222~

1928

C-40*sV.

FIG. I. COMPLETE SCHEMATIC DIAGRAM OF THE "SKYSCRAPER" RECEIVER

The values of resistance used in the filter cir-
cuits are such that both of the r.f. tubes and the
first audio tube may be supplied with 180 volts
t-om the power supply, and the filter resistance
\v ili reduce this voltage to the correct value for
me operation of the particular tube. An ex-
ample will make this clear. Consider the first
audio stage, in which socket is recommended
the use of a H2A-type tube. The d.c. plate
resistance (not the a.c. plate resistance) of this
tube is about 20,000 ohms. This 2o,ooo-ohm
plate-circuit resistance is in series with a 20,000-
ohm filter resistance, R8, across 180 volts. There-
fore, half the voltage will appear across the tube
and the other half across the filter resistance.
Therefore, there will be about 90 volts on the
plate of the ii2A-type tube.

Volume is controlled in this receiver by vary-
ing the potential applied to the screen grids of
the r.f. tubes, this adjustment being accom-
plished by RIO, a 6ooo-ohm potentiometer. In
series with this 6ooo-ohm potentiometer is
placed a fixed resistance, RH, with a value of
20,000 ohms so that not more than the rated
value of 45 volts can be applied to the screen
grids.

On the front panel of the receiver are four
controls, besides the on-off switch. The two small
knobs at the lower-right and left-hand corners
are the filament rheostat and the volume con-
trol, respectively. The drum dial on the left
tunes the antenna condenser, Cj, and the right-
hand dial tunes the other two condensers, C3
and Q, which are ganged together.

For the past several months development
work on the "Skyscraper" receiver has been in
progress in the laboratories of the Ferranti
Company. For this reason the writer feels
certain that those who construct the set will
obtain as satisfactory performance as he has
from the various models which have been under-
going tests. Optimum performance can be as-
sured only by following as closely as possible
the arrangement of apparatus illustrated in
the pictures on these pages and in the wiring
layouts supplied with the construction booklet.
For this reason it is recommended that set
builders, who contemplate building the receiver,
send for this booklet. If, after the construction
has been completed, the set does not perform
in an altogether satisfactory manner the wiring
should be checked carefully and the various
tubes should be tested. It is also a wise plan to
examine the H power-supply device, as poor re-
sults may be caused by incorrect plate voltages.

LIST OF I'ARTS

~THE apparatus used in constructing the
* model of this receiver illustrated in these
pages is given below. The total cost of the
parts listed is $95.00. The builder may substitute
electrically equivalent parts.

The last four items in the list, and also the
tuning coils, are especially designed for use with
this receiver, and may be obtained by writing
directly to the Ferranti Company. Those who
desire to construct this set can also obtain a
booklet giving complete constructional ilut.i from
the Ferranti Company for Ji.oo.
The list of apparatus follows:

Ci One Tiny-Tobe fixed condenser, o.ooot-

mfd;.
Cv, C», Q Three Remler SLAV condensers.

o.ooo5-mfd.;
Cj, Cs, C?, Cg, Cu, CM, Cu, Cie, Ci7, Cn Ten

Tobe by-pass condensers, o.5-mfd., type 300;
C9 One Tobe condenser, 4-mfd., type 240;
Cio, Cu, Cu Three Tobe condensers, 2-mfd . type

202;
LI, L2-I_3, LrLs Three Ferranti tuning coils,

" Skyscraper"-type;
Le One Hammarlund r.f. choke coil, type RFC-

85;

Ri One Tobe fixed resistor, 5O,ooo-ohm, 2-watt;
Ji One Yaxley jack, open-circuit;
J2 One Yaxley jack, closed-circuit;
Rj, RJ, R- Three Tobe resistors, lo.ooo-olim, 2-

watt;

tv-[k

1<4, Ks, Rs. Rn Four Tobe resistors, 2o,ooo-ohm,

2-watt;

Rs, RT Two lobe resistors. in.uoo-ohm, 2-watt;
Rio One Carter midget potentiometer, type

MW-6M;

RI-, One Carter rheostat, 6-ohm, type 506;
Ris, RH, Two Carter resistors, 15-ohm, tv(

J-5-'5;

TI One Ferranti transformer, type AF-5;
\-2 One Ferranti transformer, type AF-5C;
T3 One Ferranti transformer, type OP-8C foi

magnetic speakers, type OP-4C for dynamic

speakers;

Two Remler tube shields, type 56;
Three Hammarlund shaft couplers, type FC;
Three Na-AId sockets, type 424;
Three Na-Ald sockets, type 428, for ptug-ir

coils;

Three Benjamin sockets, type 9040;
One pair of Benjamin brackets, type 8629;
Two National drum dials, type VF with type-2>

illuminators;

Two darter screen-grid connectors; type 337
One Yaxley mounting plate and cable, 7-wire;
Four Eby binding posts, insulated;
Ten Lynch resistor mounts;
Acme flexible wire in colors to match Yaxlej

cable, for wiring;

One Aluminum base, lo-gauge; drilled;
One Bakelite panel, 8" X 24" X ?»", walnut

finish, drilled;
Three Aluminum Co. of America standar<

shields, drilled;
Thirteen rubber insert rings, for feed line

through base.

THIS PICTURE SHOWS THE EXACT ARRANGEMENT OF PARTS ON THE CHASSIS

Accuracy of the

"Slide-back"

Voltmeter

MANY engineers take
the vacuum-tube volt-
meter for granted, i.e.,
they seldom stop to
consider its limitations and inaccuracies.
Many times within the past two years
we have seen described in more or less
technical papers the "slide-back" volt-
meter with a direct or implied statement
that it is an infallible device for measur-
ing voltage.

The circuit diagram for a "slide-back"
voltmeter is given in Fig. i. It consists of
a vacuum tube biased so that the plate
current is quite small and so that a.c. in-
put voltages change this steady C bias,
thereby changing the plate current. The
plate current is then brought back to its
original value by changing the steady C
bias by means of the potentiometer. The
difference between the two values of bias
l^with and without a.c. input) is the peak
value of the input voltage. The negative
halves of these cycles drive the grid more
negative, but, since the tube is already
overbiased, the plate current changes but
little. The positive halves of the cycles,
however, reduce the steady negative C
bias, and the plate current increases. Then
the potentiometer is varied and the steady
C bias is "slid-back" until the same plate-
current reading is obtained.

To test the accuracy of the instru-
ment we used a Weston model 301
milliammeter with a full-scale reading
of i 500 microamperes. With 45 volts on
the plate and a C bias of minus 4 volts,
the plate current was 200 microamperes. This
steady current was balanced out of the microam-
meter by using the A battery voltage or by
means of a "bucking battery" (B) as shown in
dotted lines in Fig. i. Now, when an a.c. voltage
of 2.05 peak was placed on the input, the plate
current increased, and we "slid-back" the po-
tentiometer until the C bias meter read an in-
crease of 0.35 volts. When an input of 4.0 volts
was used a net change of 1.6 volts in the C bias
was required before the current plate was the
same.

Clearly the method fell down. A change of
C bias due to a.c. voltages could not be balanced
out by an equal change of d.c. voltage.

In the next test we put .33 milliamperes
through the meter by means of the battery B,
in Fig. i, and the C bias on the tube was ad-
justed so that no change took place in the de-
flection of the plate-current meter when the
plate voltage was turned on or off — in other
words, we placed sufficient bias on the tube so
that the plate current was zero. This point can-
not be determined exactly, of course, but if

row

the meter has an initial reading changes in this
deflection are noted easily.

Now when an a.c. input voltage was placed on
the tube, the C bias changed, the plate current
changed and a deflection was noted. The steady
bias was then increased so that turning on or
off the plate battery to the tube made no change
in the reading of the plate-current meter.

When an input peak a.c. voltage of 10 was ap-
plied to the tube a change of 9.5 volts steady
bias was necessary to reduce the plate current
to zero; other readings are noted in Table i.

TABLE I

Steady current through 500 microammeter=331 micro-
amperes. Steady bias to cause no deflection on meter=
7., volts when Ep=45 volts

input a.c.
peak volts

4.05

6.1

8.2
10.1
14.5
16.3
18.3

/ d.c. volts
to reduce
IP to zero

3.6

5.8

7.5

9.5
14.3
15.6
17.5

accuracy

89%

FIG. 1. SLIDE-BACK VOLTMETER

Substituting a more sensitive meter, say a
\\estinghouse 500 microampere meter, or a
Weston zero-center galvanometer with a sensi-
tivity of 60 microamperes per division, increased
the accuracy somewhat. But even with the 1500
microampere meter we could balance out an a.c.
voltage with a d.c. voltage with an accuracy of
about 90 per cent. Larger input voltages could
be read more accurately.

When used in this manner the "slide-back"
voltmeter is accurate enough for all ordinary
measurements. The device is inaccurate when
operated, as is often done, so that a fairly large
steady current is obtained in the plate circuit,
and the bias is so adjusted after an input is
applied that the plate current returns to this
value. The nearer one can get to the actual zero
plate current point, the more accurate the in-
strument as a whole becomes.

FOR A long time we
Hum in the "Lab" threatened to throw
Circuit Receiver out our B-SUpply unit

and build a new one.
With the four-tube "Lab." circuit re-
ceiver (August RADIO BROADCAST) con-
siderable hum appeared in the loud
speaker in spite of rather thorough filter-
ing in the B supply itself. We began to
wonder where the noise came from; was
it inductive pick-up from the power line
running near the audio-transformers, or
was it picked up in the first- or second-
audio, detector or r.f. tube?

The audio amplifier of the receiver was
perfectly quiet, as evidenced by shorting
its input through a io,ooo-ohm resistor.
Running the detector from a 45-volt B
battery helped a bit, but the hum was
still too loud. Running the r.f. tube from
a go-volt B battery killed the hum com-
pletely. Larger filter condensers across the
90- volt B-supply lead did no good. What
could be the trouble?"

Let us look at the r.f.-tube circuit in
Fig. 2. Notice that the cp-volt lead from
the power-supply unit — which has some
a.c. in it no matter how well it is filtered
— is connected directly to the detector in-
put coil. This is a very high-impedance
circuit, equivalent to Fig. 3, and any a.c.
current flowing will build up a large volt-
age and subsequently will be amplified
by this detector and audio tubes.

The first experiment was to wind a
primary coil about the detector input
coil, as in Fig. 4. The noise dropped out.
The solution was then simple: isolate the primary
winding of the coil from the secondary, as in Fig.
5, and ground the lower end of the detector coil.
Now the power-frequency noise is effectively
grounded as far as the detector input goes, arid,
therefore, no hum gets on to the tube's grid.

Figure 5, then, gives the circuit diagram of an
r.f. amplifier and detector for the "Lab." Cir-
cuit which will iron out a.c. hum entering the
receiver from the B-supply unit via the r.f.-
amplifier plate circuit.

In the course of the experiment leading to the
elimination of the hum from the circuit, the leads
to the regeneration condenser were reversed.
Considerable difficulty was experienced in
"holding-down" the circuit, and it was impos-
sible to neutralize the amplifier completely with-
out placing a shield between the regeneration
condenser and the detector tuning condenser.
When, however, the regeneration condenser was
connected correctly, that is, stator to the de-
tector plate, all difficulty disappeared, and a
high-gain stable amplifier resulted. Readers who
have trouble with the circuit, evidenced by the de-
tector or amplifier oscillating continuously, might
try reversing the leads to this small condenser.

Det.

FIG. 2. ORIGINAL "LAB" CIRCUIT

RADIO BROADCAST

DECEMBER, 1928

More Data on
Underground
Aerials

SO FAR as we are concerned
the following quotation from
a letter from our good friend
Dr. G. W. Pickard closes the
subject of trick and underground antennas: —

"On page 259 of the September issue of RADIO
BROADCAST I notice an appeal for definite quan-
titative data on the underground antenna.
Probably by this time you have found the vari-
ous references necessary, but in case you have
not, I'll give some of the desired facts.

"As you know, there are underground antennas
and antennas. Some of these consist of plates or
coils of wire, variously insulated, but aside from
the sucker type of radio fan, no one has taken
them seriously, and so far as I am aware, no
quantitative measurements have been made.
But the real, more-or-less-useful type of under-
ground antenna, consisting of a long, straight
insulated wire buried at a slight depth in the
ground, is the subject of a considerable technical
literature, and its reception characteristics are
quite well known.

"First, consider 'Short- Wave Reception and
Transmission on Ground Wires (Subterranean
and Submarine)' by A. Hoyt Taylor, Pro-
ceedings I. R. E. Vol. 7, No. 4, August, 1919.
Taylor points out that the buried wire antenna
is strongly directive, receiving signals best in
the direction of its length, and but feebly from
directions normal to the wire. He also explains
that reception is possible because of a tilt in the
wave-front, which gives a com-
ponent of the electric vector
parallel to the wire.

"Next, take my paper, ' Static
Elimination by Directional Re-
ception,' Proceedings I. R. E.,
Vol. 8, No. 5, October, 1920,
wherein it 'is explained that if
static and signal come from
different directions, properly
oriented directional aerials will
eliminate more static than
signal.

"Finally, consult 'The
Wave-Front Angle in Radio-
telegraphy' by L. W. Austin,
Washington Academy of Science
Journal, pages 101-106, March 4, 1921, wherein
it is shown that waves are slightly tilted forward
in the direction of propagation, this tilt being
small and of the order of o°-3°.

"It is obvious without going further into the
literature of the subject that buried wires often
give better signal-static ratios than does the
conventional open antenna, simply because they
receive directionally.

"Now for a numerical answer to RADIO BROAD-
CAST'S question; what is the relative signal
strength of a fifty-foot wire in the open, and the
same length of wire buried? If the wire in the
open is truly vertical, and the earthed wire is hori-

O.Q00015

0.001

zontal and but lightly
covered with earth,
reception on the buried
wire will be somewhere
between oand 3 percent,
of that on the vertical
wire, depending upon soil
resistance and the con-
sequent tilt of the wave.
If the wire is buried
quitedeeply, both theory
and Taylor agree that
the wave tilt will in-
crease, and hence a some-
what better signal will
be obtained on the more
deeply buried wire. If
you really wish to pur-
sue this matter to the
bitter end, that is, liter-
ally run it into the ground, see Wireless
Telegraphy by Zenneck, McGraw-Hill, 1915,
particularly pages 260-262 and Figs. 310-317
showing examples of tilted waves for different
soil constants.

"You see, there is no mystery, no magic about
this matter. There is no division of the wave
from the transmitter into two distinct parts,
one traveling under and the other over, the
earth's surface. There is no inexplicable filtering
action in a layer of dirt which will strain the
static out and let the signal through. A long
buried wire is merely an in-
efficient but directional an-
tenna, and, if it can be aimed
at the signal and not at the
static, it will give a favorable
signal-static ratio.

"But more power to you in
your attack upon the thousand
and one fake contraptions
which grow like weeds upon our
unfortunate roofs, burrow fool-
ishly in the ground and litter
the tables of the uninformed
fan. I am afraid an appeal to
reason will not reach effec-
tively the class you would
protect, therefore, the best way
would be to make fun of these fakes. With wire
cones, triangles and other Euclidian-looking
objects on the roof, weird tangles of wire in
pits on the front lawn, Geppert and other din-
gusses guarding the radio Deceiver, a poor,
puzzled radio wave must scratch its head and
wonder how it would ever get in."

000025

T "Total number of turns
in PandS

FIG. 5. THE IMPROVED "LAB" CIRCUIT RECEIVER

demand on the part of listeners for low-frequency
tones all out of proportion to their natural
values. Mr. Rhodes's figures (Table II) show that
working a good amplifier out of an impedance
for which it was not designed may ruin its char-
acteristic.

Is a 112 Tube
Needed in the
First stage?

R.F.

0.00025*

FIG. 4

FROM time to time we see
the statement in an article
about high-quality audio am-
plifiers that it is a good plan
to use a 1 12-type tube in the detector socket be-
cause of its low output impedance. The object
is that under these conditions the first audio
transformer works out of a tube whose im-
pedance is, ostensibly, 5000 ohms, instead of
12,000 ohms for the2oi-A. Therefore, the articles
argue that the low-frequency response will be
better. We have often suspected this to be a
piece of nonsense, and recent tests made in the
Laboratory by Howard Rhodes on a two-stage
Sangamo amplifier have proved our contention.
Little or nothing has been said about what
happens to the high-frequency response of an
amplifier when the impedance out of which it
works is changed. It seems to be assumed tacitly
that nothing happens, or if it does, the difference
does not matter. This may be part of the gen-
eral negligence on the part of amplifier designers
to consider the high frequencies as unimportant,
occasioned without a doubt bv the unreasonable

Rp ohms

TABLE II

Frequency cycles

5000
10.000
20,000
30,000

60
-1.7
-1.7
-2.0
-2.3

1000
0
0
-0.5
-1.0

2000
+1.3
+0.6
-1.1
-3.2

4000
+2.8
0
-3.5
-8.0

6000
+2.6
-1.3
-5.2
-8.6

The amplifier consisted of two Sangamo type-A
transformers, a cx-32y first-stage and cx-3;o
second-stage tube. The output device consisted
of a General Radio Type 587-0 which is made
up of a i5-henry choke and two 2-mfd. con-
densers. The current into 4000 ohms was read
while the input voltage was kept constant. The
table gives the necessary data on the measure-
ments. The current at 1000 cycles when the am-
plifier worked out of 10,000 ohms was taken as a
zero level; all other adjustments of frequency
and input impedance are compared to this value.
Thus, at 60 cycles and worked out of 5000 ohms,
the amplifier is down 1.7 TU compared to 1000
cycles when worked out of 10,000 ohms.

The amplifier has the best characteristic when
worked out of 10,000 ohms, and it is assumed it
was designed with this impedance in mind.
Working it out of 5000 ohms causes a rise at
the high frequencies where the secondary ca-
pacity begins to resonate with the leakage re-
actance of the transformer. Perhaps this is be-
cause lower input resistance is equal to lower re-
flected resistance in this resonant circuit so that
the resonance becomes more pronounced. This
theory is borne out by the fact that many am-
plifiers, which are perfectly stable when operated
out of 10,000 ohms, begin to oscillate badly at
5000 or 6000 cycles when worked out of low
impedances.

The Sangamo amplifier begins to fall off when
worked out of 20,000 ohms but is still much bet-
ter than many when worked out of the imped-
ances for which they were designed.

The truth of the matter is that a well-designed
transformer is engineered with some particular
input resistance in mind. When worked out of
this resistance the characteristic will be the
best, if this resistance is something else the char-
acteristic goes bad. If the resistance is 5000
ohms a 112-type detector tube will give better
over-all frequency response; if it is 10,000 ohms
a 2OI-A detector will be better.

The question remains, what is the output
impedance of an average detector circuit? Who
knows? — KUTH HFNMI

A Test Set

CfHlS is the second article on practical
•*• service problems by Mr. Alcorn. In bis
first story, the author discussed what seemed to
him the ten leading service troubles, analyzing
some of them in detail. This one deals with
short circuits and the "case histories" rep-
resent actual and most interesting experi-
ences which should be useful not only to
those who have occasion to service or use the
particular sets referred to, but also to those
doing service work on almost any set. Sur-
prisingly little really helpful data has ap-
peared in print on service problems. It is our
hope that these articles by Mr. Alcorn, and
other similar articles which we may publish,
will in a measure fill this real need.

—THE EDITOR.

IN SERVICING radio receivers repair men
and radio dealers are confronted constantly
with mystifying problems which tax their
ingenuity to the extreme. In some cases several
hours of experimenting may be necessary before
it is possible to locate the trouble and start work
on the repair, whereas other times an unusually
baffling condition may be cleared up acciden-
tally in a few moments. The writer does not wish
to infer that he has found the solution difficult in
all, or even a majority, of the problems in radio
servicing, for this is not the case. On the con-
trary, the owner's description of the receiver's
performance often permits one to diagnose
the trouble, and other times it is possible to
analyze the difficulty by merely turning the
tuning controls for a few moments. However,
there are a sufficient number of unusual ailments
which a receiver is apt to contract to make this
line of work full of interest.

In last month's article of this series the writer
described several unusual experi-
ences which he has had in locating
troubles caused by open circuits. It
is admitted that the difficulties cited
were out of the ordinary, but they
serve to point out the type of work
a service man must be prepared to
tackle. In all probability open cir-
cuits of the types described may
never be experienced by the reader,
but they are typical of the peculiar
problems which present themselves
every day. While on this subject
there is one new case of open-circuit
trouble which may be of interest.

The writer was called upon to re-
pair an Atwater Kent Model 35
receiver, and after testing the various
circuits it was found that there were
a great number of open circuits.
Both audio transformers were burned
out, the grid suppressors were open,
the primary windings of the radio-
frequency transformers were defec-
tive and, in addition, several by-pass
condensers were blown out; in fact
the set was almost a total wreck.
However, after all of these parts had
been repaired or replaced the set still
refused to function, although the
usual tests showed the continuity

By B. B. ALCORN

The second installment
of a series of articles
relating the experiences
of a radio service man

of all parts to be in order, except that the
B potential did not reach the plates of the
tubes. After considerable checking, the trouble
was located in the battery cable, but not in
the place where one would normally expect to
find it. The A+ wire in the battery cable of this
receiver is fitted with a lug for connection with
the storage battery, and the B — wire is connected
with the cable in this lug. In some way the B —
wire had become loose, and an oxide had formed
which insulated the wire from the cable. When
this difficulty was repaired no further trouble
was experienced with the receiver.

GOOD EQUIPMENT NEEDED

IN THE servicing of radio receivers a repair
' man is lost without the proper tools and test-
ing equipment. Both of these items are of
equal importance and their selection deserves
the most serious consideration. With a well-
designed portable test set it is possible to locate
most all causes of trouble in a fraction of the
time that would otherwise be required, and, if a
complete set of tools is available, the receiver
often may be repaired in the customer's home.
However, when equipping himself with tools
and test apparatus the service man should al-
ways consider the equipment from the viewpoint
of portability and convenience.

In addition to its utility value a service man's
equipment accomplishes another very important
result; namely, it creates a favorable impression
in the mind of the client, and this is very im-
portant. Therefore, when building or buying
portable equipment it is wise to consider its ap-
pearance as well as its usefulness.

THE AUTHOR S TEST SET
99

There are a number of excellent test sets
available on the market, and, if the service
business will stand the strain on its pocket-
book, one of these instruments is an excellent
investment. Such apparatus is now being made
by Hoyt, Jewell, Supreme Instrument, Weston,
and other instrument manufacturers. The writer
recently had an opportunity to experiment
with the test set made by the Supreme Instru-
ment Company, and, while it is rather expensive,
it is as complete an outfit as could be desired.
The set is designed especially for use in the field,
but it is also excellent for work in the laboratory.
It consists of an oscillator, a wavemeter, a
calibrated variable condenser, and a power-
supply circuit which operates direct from the
a.c. line. The set is housed in a neat case which
also provides space for carrying tools, tubes, and
enough supplies to meet the usual requirements.
It may be considered a portable laboratory.

Unfortunately most service men cannot con-
sider purchasing elaborate test equipment of
the type referred to in the above paragraph,
because of the high cost of such apparatus.
However, it is possible to build a very satis-
factory set-checking device at a considerable
saving in expense. It is true that such a set tester
may not be quite as versatile as a commercial
product, but it may be constructed so that it is
satisfactory for most purposes. The set checker
designed by the writer will indicate short cir-
cuits, open circuits, and the general condition
of tubes in both a.c. and d.c. receivers, and it is
made up of meters that every service man
should own. Of course, if high-grade meters
are used the cost may be as high as $40, but this
is considerably less than the average commercial
outfit.

PARTS NEEDED

A LIST of the apparatus required for trie
^* construction of the set checker is as follows:
One double-range panel-mounting high-
resistance d.c. voltmeter, 0-8 and
0-200 volts;

One panel-mounting d.c. milliam-
meter, o-ioo milliamperes;

One portable triple-range a.c. volt-
meter, 0-4, 0-8 and 0-150 volts:

Two Benjamin sockets, ux-type;

Nine Carter tip jacks;

Two single-pole, double-throw push-
button switches;

One double - pole double - throw
switch;

One double-pole single-throw switch ;

One wooden case (large enough to
provide space for accessories) ;

One bakelite panel.

The accessories used with the test

set follow:

One set of test prods;

One socket adapter, ux 199 to uvigg;

One socket adapter, uv 199 to ux 199;

One socket adapter, uxigg to uv-
ZOIA;

One Jewell UY socket adapter, type-

521;

One test cord (made by connecting

two uxigg bases by six feet of

four-wire battery cable).

The complete schematic diagram

of the set tester is given in Fig. i.

The portable a.c. voltmeter is not

100

RADIO BROADCAST

DECEMBER, u);8

connected directly in the tester, but it is a
necessary piece of equipment which should
be on hand at all times. One of the sockets
called for in the list of parts is connected in the
position indiacated by the tube in the diagram;
the four wires marked "test plug" are connected
to the, terminals of the second socket. The
notations on the drawing indicate many of the
ways in which the tester may be used, but a few
additional pointers may be of interest.

In checking a battery-operated receiver with
this set tester it is best to start by removing
each tube of the set in turn and inserting it in
the tube sockets of the tester, the test plug socket
of the tester being used for the test cord which is
also plugged into the empty socket of the re-
ceiver. Next, throw the double-pole switch to
one side, but change it quickly to the other side
if the d.c. voltmeter is incorrectly connected, i.e.,
if the needle tends to move in the wrong direc-
tion. With the tester connected in this manner
the milliammeter will indicate the condition of
the circuit under test; if the deflection of the
meter is correct for the tube used, it is necessary
to assume that the circuit and tube are operating
properly, but a large deflection indicates a short
circuit and zero deflection indicates an open
circuit.

After the above check has been completed it
may be necessary to make further tests in
order to locate the exact position of the opened or
shorted circuit. In this connection it is interest-
ing to note that tip jacks have been provided on
the tester so that each of the meters may be
used separately. When testing a.c. sets the
portable a.c. voltmeter is used to measure the
filament or heater potentials, and then the cir-
cuit is checked in the usual manner with the
tester. It should be remembered that the d.c.
voltmeter will not record the filament voltages
of an a.c. set, and, therefore, the double-pole
switch should be open when making the tests.

The tools and replacement equipment which
it is necessary for a service man to carry with him
in the field vary in different territories. In dis-
tricts where it is necessary to travel a consider-
able distance from the shop it is essential that the
equipment be very complete, and it should
consist of a large assortment of tools as well as
a number of replacement parts, such as trans-
formers, condensers, etc. On the other hand,
when working in a city a large kit of tools and
parts is not essential. However, it is always ad-
visable to carry a complete assortment of tubes
and a few minor parts such as grid leaks, fixed
condensers, amperites, etc. For city work the
tools which the average service man considers it
necessary to carry are a pair of long-nose pliers,
a screw driver and an electric soldering iron.

AN EFFICIENT TOOL KIT

1"HE writer was once employed as field en-
gineer for a well-known manufacturer, and,
while acting in this capacity, he covered
most of the southern states. In this work
it was necessary for him to carry a very
extensive kit of tools in order to be able to >•'

meet all conditions. In this particular case
the tools were carried in a case with straps
for each tool, thus making it difficult to
lose equipment as each strap had to be
filled before leaving the scene of work. A
list of the tools included in this kit is as
follows: flat-nose pliers, long-nose pliers
round-nose pliers, diagonal pliers, slip-
joint pliers, long-nose angle pliers, duck-
bill pliers, ratchet screw driver with three
blades, screw driver with screw-holding
attachment, set of Stevens Spintite
wrenches with detachable handle, Yankee
push drill, American Beauty soldering

A HANDY
METER AND

iron, bits, reamers, and counter-sink bits. The
miscellaneous equipment consisted of solder,
soldering flux, tape, saddle tacks, lead-in strips,
ground clamps, etc.

1 \i

SHORT-CIRCUIT TROUBLES

•ROUBLES caused by open circuits, which
were discussed at length in last month's
issue, are only one of the many types of
difficulties experienced in the repair of radio
receivers. Short circuits cause the service man
nearly as much worry as the former, and they
will be considered in the following paragraphs.
Short circuits, it might be explained, occur in
accessories as frequently as in the set itself, and
they frequently are the result of carelessness or
inexperience on the part of the person installing
the receiver.

Short circuits due to carelessness or inexpe-
rience frequently are found in the Radiola Super
Eights and the Radiola 2o's. In these receivers
six dry cells are connected in series parallel, and
in installing these batteries a short circuit is often
caused by two negative terminals touching
each other, thus shorting one or more batteries
and causing the set to go "dead." Much time is
lost and often another set of batteries is
ruined before the real cause of the trouble is
discovered.

Another rather baffling short circuit, which is
encountered frequently, is found in the older
types of reflex receivers, such as the de Forest.
Although there are few of these sets in city
homes, many of them are still being used in
outlying farm districts. This particular short
manifests itself as an open circuit, and, when the
set is tested with a set checker the results in-
dicate a burned-out transformer. However,
further testing will show that the transformer is
perfect; the trouble being caused by a shorted
condenser in shunt with the primary or second-
ary winding of the transformer. When the faulty
condenser is removed the set will function but
had distortion will be experienced until the
condenser is replaced.

I Push Button to place high Scale
' of Meter in Plate Circuit

• Conn, for external Bat in series
-I with low side of Meier

Conn for separate
use o' Veter

DPDTS. To cut Voltmeter

•To reverse polarity from Circuit
of Voltmeter

_ J To Filaments of
--• (Test Socket Phig

FIG. I. DIAGRAM OF THE TEST SET

An interesting short circuit of a peculiar na-
ture was discovered recently in a Radiola 18.
The receiver provided entirely satisfactory re-
sults for several months before the trouble
developed, and then the owner reported that the
only way it was possible to obtain reception
was to remove the first r.f. tube. The service
man who was sent on the job went prepared to
replace the tube which he considered defective.
However, it was found that fnis was not the
cause of the trouble. Upon removing the set
from the cabinet after the usual test failed to
indicate the fault, it was discovered that when
the tube was inserted in the socket, instead of
fitting into the contact springs, one of the fila-
ment prongs pushed its contact spring against
another lead, thus causing a dead short circuit
through the first r.f. coil and making the set in-
operative. A simple operation with a pair of
pliers shifted the spring to its proper position
and corrected the difficulty.

One of the simplest receivers ever placed on
the market developed one of the most unusual
difficulties which has ever come to the attention
of the writer. The set referred to was one of the
• old bread-board-type Atwater Kent receivers,
and because of the simplicity of the receiver no
complications were expected when the call for
a service man was received. The owner stated
that the set performed perfectly on the previous
evening, but that he was now unable to coax it
into operating. When the service man arrived he
discovered that the set would play satisfactorily
when held in the hand, but as soon as it was placed
on the table it was inoperative; this was indeed
an unusual condition. After carefully checking
the set it was found in perfect condition, and
then the surroundings were examined. Finally
the trouble was found to be caused by the table
cover on which the set had been placed; the
cover being made of metallic tinsel, it caused a
number of short circuits in the exposed wiring of
the receiver. Then a little judicious questioning
disclosed that the lady of the house had placed
the table cover under the set during the day,
and this explained why the set performed on the
previous evening.

Table covers having metallic tinsel in their
make-up have been the cause of more than one
set being inoperative. A few days after the
experience described above a service man was
asked to repair a Radiola 18 which was found
to have a short circuit caused in this way. In
this particular case the location of the short
circuit was never discovered, but removing the
table cover corrected the difficulty.

CARELESSNESS IN SOLDERING

INNUMERABLE short circuits are caused by
carelessness in soldering at the factory, and a
recent example of this was found in a-Radiol.i
JOA. This set had been operating perfectly for a
period of three months when it suddenly stopped,
and all the usual tests failed to dis-
close the cause; even the continuity
test of the manufacturer did not show
anything wrong, still the set remained
perfectly silent. Finally, it was dis-
covered after a careful inspection, that
a thread of solder, which was so fine
that it was barely visible to tli
was across the antenna and ground con-
nections. This thread of solder probably
was caused by the iron being slid from
one connection to another when the
set was wired and it did not cause a
short circuit until the vibrations from
the >peaker caused it to sag. However, in
the proper position, a thin piece of solder
is as effective in stopping the operation of
a receiver as a piece of No. 14 bus bar.

The Service Man's Corner

«HE number of radio service men appears
to be increasing with great rapidity.
Many service men, both old and new are
readers of RADIO BROADCAST and quite naturally
look to these pages for information of specific
help to them in their daily problems. It is true,
in a sense, that every radio article is of some
help to those working in the field, but articles
prepared with the problems of the service man
chiefly in mind are badly needed.

Regular features now found in this magazine
are designed to be of general help to those work-
ing in this field. In this classification fall the
RADIO BROADCAST Laboratory Data Sheets,
RADIO BROADCAST'S Service Data Sheets on
Manufactured Receivers, "Strays" from the
Laboratory, "Our Readers Suggest . . .," and to
a certain extent, RADIO BROADCAST'S Home
Study Sheets. And, beginning with the Novem-
ber issue, we started a series of general articles
on radio service problems by B. B. Alcorn.

The chief problems encountered in the field by
service men are classified in order of their im-
portance by Mr. Alcorn as follows:

1 . Dead tubes.

2. Run-down batttries.

3. Open circuits.

4. Defective parts.

5. Defective grounds.

6. Use of various "gadgets."

7. Defective antenna.

8. Misconnections.

9. Short circuits.

10. Lightning arresters.

A few of these problems were discussed in Mr.
Alcorn "s November article. In later articles, he
treats of the others. Do men actually facing the
problems of curing sick radio sets in the field
agree with this estimate? If not, in what order do
they list the troubles? What interesting short-
cuts to the work in hand have they evolved?
What simple test-sets have they built for their
own use? What small and persistently annoying
little problems have been solved in practise?
Wouldn't a short description of any one of these
pet ideas prove helpful to others doing the same
sort of work? "The Service Man's Corner" will be
a regular feature of RADIO BROADCAST from
this issue on, and contributions from service
men and professional set builders are welcomed
and will be paid for at regular rates.

WHAT ONE SBRVICE MAN SAYS

"THE data that I feel is most needed by the
* service man is along the lines of test appara-
tus that he can construct himself, and so know
the whys and wherefores of what is happening
when he uses the test sets, 1 have found a number
of service men who can use the more elaborate
test sets put out by
several concerns.
But few of them
understand what
the different read-
ings show them
about the condi-
tion of the receiver
under test. These
really fine pieces of
apparatus are not

serving their purpose unless they are in the
hands of one who really knows how to use
them." This service man concludes: "The ser-
vicing of radio receivers is a profession that no
one need be ashamed of and the radio industry
would certainly suffer if all service men were to
be removed from the field."

The radio service man will not depart from the
field, for 'he is too valuable a part of the present
radio structure. "The Service Man's Corner" in
RADIO BROADCAST will, we hope, be of help in mak-
ing his work easier, by affording a medium where
ideas and comment useful to him can be ex-
changed.

Field Suggestions

\ A /HEN I received a letter from the editor of
* ' RADIO BROADCAST, suggesting that sug-
gestions from service men would be appreciated
and asking if I had anything to say, I felt much
as I would if anybody had hinted that 1 might
improve Bobby Jones' drive. And — in case you

WITH this issue, we start a regular page
for the practising service man. This
department will, we hope, he a forum where
service men can discuss their problems, get
their pet ideas into print, and see now and
then a hint which will he useful in their daily
work. Contributions, which preferably should
be short, to the point and typewritten, are
solicited and will be paid for if used. Address
your articles to the editor, "The Service Man's

Corner."

— THE EDITOR.

«©*

don't know — my golf drive looks like a sine wave.
I have fooled around with radio receivers for
some time, starting just after "ham" radio
opened up after the war, so I can safely say 1
know nothing about radio. (You know, when you
first start, you know it all). I've picked up a few
kinks, most of which are doubtlessancient history
to real service men. But I am glad to. write about
them in the hope that it will help bring to light
some really good information on the subject from
others. There seems to be a surprising dearth of
information on the subject; I can't find any book
that covers it.

Use of a Set-Analyfer: The service man should
have in his kit a good set analyzer, of any good
make. Working without one makes the job
harder than necessary, and the use of one has a
valuable psychological effect on the set owner,
which is worth something. This unit should be
capable of measurements on both battery- and
a. c. -operated sets, and its use should be
thoroughly studied.

Another tool almost as useful is a modulated
oscillator that will cover the broadcast spectrum,
which can be modulated for balancing sets using
neutralized circuits. 1 have one that is small, self-
contained, and does the job. Single-control sets
are now in wide use, and if the condensers in the
set under test do not gang or do not follow very
closely, the receiver cannot give its best per-
formance. One of these oscillators will permit the

101

service man to adjust the condensers to a nicety.
Unless there is an adjustment provided, you do
it by bending condenser plates, of course. The
plates will get out of line in shipping, even when
every care is taken at the factory. That, we are
sorry to say, is not always the case. (A modulated
oscillator was described on page 90, June, 1927,
RADIO BROADCAST).

A tool 1 want is a tube checker working off
the a.c. light socket to handle any of the usual
tubes. They are on the market, but the price is
too high for me and I am hoping RADIO BROAD-
CAST will tell us how to build one. [A description
of such a tube-checker as constructed in our
Laboratory will appear in "The Service Man's
Corner" in an early issue. — Editor.]

— H. J. GODDARD, Ellendale, N. D.

MISCELLANEOUS SERVICE SUGGESTIONS

\Jl R. GODDARD has noted down some other
• service suggestions, which follow:
Line-voltage control: The advent of a.c. sets
has shown up the floppy condition of the average
commercial power line as nothing else could.
Many a set is hooked to a line that varies from
105 to 125 volts. The latter condition is especi-
ally hard on tubes. Every service man should
see to it, if the line voltage is high, that it is
reduced to normal before it reaches the set. A 50-
ohm resistor that will handle around 40 watts
will do the trick. I like them variable particularly
if they can be mounted on a bracket in a console,
out of sight. They are cheap and will pay for
themselves in no time. I'd like to see some reputa-
ble company build such a resistor that can be
plugged-in between the outlet and the set plug
so that a turn of the cap will permit adjustment.
There should be a big sale for them. [A number
of resistors which can be used for this purpose are
now on the market. Although none of them have
precisely the features which the writer desires,
the adjustable units are satisfactory for the pur-
pose.— Editor.]

Excessive plate voltage: Don't forget that a high
plate voltage is almost as hard on a tube as high
filament voltage. Watch that plate voltage if
you want your tubes to last.

Blinking a.c. tubes: Every rlow and then you
find an a.c. tube that is a blinker. These tubes
start all right when cold, but when heated, a small
break in the heater filament separates and for
all practical purposes, the tube goes dead. When
it cools a bit the ends come together and the tube
starts again. I found a bad case of "fading"
due to this very thing. Watching the tubes and
noting which filament goes black of course local-
izes the trouble. [See p. 428 RADIO BROADCAST,
April 1928, where this point was discussed at some
length. — Editor.]

Caution: Ever
notice the instruc-
tions to keep your
hands out of a set
when the a.c. is on?
Manufacturers do
that because they
are sore at the
mortician.

AS JHh KK

AS IKK ShhSTT

Information in the SounckMovie Field

IN ACCORDANCE with our recently an-
nounced intention to broaden the scope of
this department, while not departing from
its original purpose of serving the broadcaster,
we are going to print considerable material on
sound movies, of the same general type as the
articles which have already appeared in "As the
Broadcaster Sees It." For those readers who are
interested professionally in talking pictures
additional references will be of interest. I have
prepared these in the form of a haphazard, in-
formal bibliography — haphazard because sound
movie articles giving varying degrees of technical
information are appearing in great numbers and
in widely scattered publications, so that any
sort of complete collation is out of the question,
and informal because it contains comments and
information not usually included in the austere
files of bibliographies. The list:

Transactions of the Society of Motion Picture
Engineers. Vol. XII, No. 33. The current issue of
this publication contains "A System of Motion
Pictures with Sound," by H. B. Marvin of the
General Electric Company; and "Some Remarks
on the Acoustical Properties of Rooms," by J. B.
Engl. Marvin's article is a description of the
General Electric system as of April, 1928. With
modifications and additions from other sources,
this has become the R. C. A. Photophone system.
The discussion is interesting, in that a good many
questions are asked and answered which are
likely to occur to almost all students of sound-
movie technique. Copies of this issue may be
secured from the Secretary of the Society, Mr.
L. C. Porter, jth and Sussex Sts., Harrison, N. J.,

at §2.50 each. Earlier issues have also contained
sound-movie material.

Motion Picture Projection, by James R. Ca-
meron. Cameron Publishing Co., Inc., Manhat-
tan Beach, N. Y. The fourth edition of this hand-
book contains over 1200 pages, of which 124,
starting with page 699, are devoted to sound
movies. There are descriptions, mostly of the
"hand-out" variety, of Movietone, some of
Hoxie's pre-Photophone equipment, Vitaphone,
Vocafilm, and Phonofilm. The paper on the last
named is ascribed to Dr. De Forest. Following
these general outlines there are detailed instruc-
tions for the operation of Western Electric
sound-picture apparatus. Apparently this stuff
is reprinted from the manufacturer's bulletins.
It includes general layout wiring diagrams. The
whole book is $6.00.

Bell System Technical Journal, Vol. V, No. 2,
April, 1926. Published quarterly by the American
Telephone and Telegraph Co., 195 Broadway,
New York City. $1.50 per year; 50 cents per
copy. This issue contains a treatise on "The
Alkali Metal Photo-electric Cell," by Dr. Herbert
E. Ives. The general characteristics of central
cathode and central anode cells are given, to-
gether with a consideration of the effect of such
factors as gas, polarization and wavelength of
the incident light, the nature of the emitting
material and the surface coated with it, tempera-
ture, etc. A selected bibliography is included.
For those who can follow scientific expositions
articles of this and the following type are very
valuable.

General Electric Review, Vol. 31, No. 7, July,
1928. Published monthly by the General Electric

RADIO FIELD STRENGTH
CONTOUR MAP

WASHINGTON D.C.ANO VICINITY

Illustration courtesy Institute of Radio Engineers

MAP OF WASHINGTON, D. C. SHOWING FIELD STRENGTH OF SIGNALS FROM WRC

102

Company at Schenectady, N. Y. $3.00 per year;
30 cents per copy. This issue contains an article
by Dr. L. R. Roller on "The Photo-Electric Cell."
1 1 is of the same scholarly type as I ves' paper and
contains valuable curves. A few references are
given as footnotes.

The American Cinematograpber, Vol. I X, No. 6,
September, 1928. A camera man's magazine
published monthly by The American Society
of Cinematographers, Inc., at the Guaranty
Building, Hollywood, Calif. Yearly subscription
is $3.00; single copies, 25 cents. Material on sound
pictures is printed quite regularly. The issue
cited contains a story by Delmar A. Whitson on
his system, and a discussion by the editor on
"Who Invented Talkies?" The material is uneven
and often not free from mistakes, but should
prove informing to readers who do not know
much about the field and want to acquire semi-
technical knowledge.

The Motion Picture Projectionist, Vol. I, No. 1 1,
September, 1928. This magazine is published
monthly by the Craft Publishing Co., 45 West
45 Street, New York City; its readers are mostly
motion-picture operators and the material is
semi-technical, about on the same level as that
in the Cinematographer. The September, 1928
issuecontainsa leadingarticle on "Light Sensitive
Cells" by Samuel Wein, a discussion by Friend
Lescaboura on "Just What May We Expect of
Television?" some Electrical Research Products
material, a reprint from the Electrical Workers'
Journal by Prof. C. M. Jansky, "How a Rotary
Brush of Light Paints Pictures from Afar" (pic-
ture transmission) and various little items of
interest. Like most of the movie-trade journals,
it carries a lot of audio picture stuff.

Exhibitors Herald and Moving Picture World.
This trade weekly carries a monthly supplement,
Better Theatres, which contains semi-technical
sound-movie articles of varying degrees of relia-
bility. The New York office is at 565 Fifth Ave-
nue. Subscription costs $3.00 a year. The Septem-
ber i issue carried a theatre architects' sympo-
sium, the conductor of which announced as one
of the major conclusions, "Corrective work will
be necessary in houses where no acoustical prop-
erties now exist," while some of the contributors
were responsible for such illuminating state-
ments as, "Will probably require the use of loud
speaker equipment" (in answer to the question,
"What effect will the synchronized pictures have
on the acoustics in building the theatre of the
future?)" and, "Yes, where this has not been
considered, sounds and tones are more sharp,"
the question being, "Will it be necessary for pres-
ent day theatres to reconstruct so as to provide
for the proper acoustical properties in the audi-
toriums?" Although the number of such inane
answers was large, the idea of the symposium
was good and a few searching comments were
elicited. A comical piece in this issue was an
illustration of a section of the dome of a theatre,
showing treatment with acoustical felt, and cap-
tioned, "Acoustics in the Fox Theatre, Detroit."
In the September 29 issue F. H. Richardson,
who edits the "Better Projection" department of
the Exhibitors Herald and Moving Picture H or/J
had a discursive but fairly informing article on
" 1 liePick-Up,"coveringmethodsofgettingaudii)

DECEMBER, 1928

BROADCAST STANDARDIZATION

103

input to the amplifiers from film and disc sound
records. The illustrations were excellent. Rich-
ardson talks on paper, so to speak, which is fine
in fostering informality but uses up a lot of
words when carried too far. The August 25 issue
of the same magazine carried a description of
the RCA Photophone system, under the title of
"How RCA Photophone Times Synchronism."
Except for the title and a few mistakes in the
text, the article was informing enough.

The combined bibliography and review printed
above will give some idea of the variety of pub-
lications in which articles on or pertaining to
sound pictures are to be found. It includes only
those periodicals in which such material appears
more or less regularly.

Broadcast Standardisation

THE National Electrical Manufacturers'
Association (NEMA) has a transmitter
Section which deliberates occasionally
on the subject of what broadcast transmitters
should be like and in what terms it is valid to
talk about them. The last meeting, in June,
1928, discussed a number of technical subjects
especially pertinent in view of present develop-
ments.

The following methods for adherence to as-
signed frequencies by means of automatic master
oscillator control are specified:

a. Quartz crystal

b. Standard clock with harmonic amplifier

c. Tuning fork

d. Magnetic striction bar

In any case it is specified that the master oscilla-
tor is to be arranged to be independent of ex-
ternal changes in humidity, temperature, barom-
etric pressure, or loading.

Under the allied subject of frequency monitor-
ing the Section adopted as a standard the use of
an oscillating or heterodyne frequency meter
whose frequency is held constant by one of the
methods above, and so constructed that it can
be shipped periodically to a primary standardiz-
ing laboratory.

In rating the coverage of a broadcasting sta-
tion the population contained within the area
over which the field strength is 5000 microvolts
per meter, or more, is considered basic. Beyond
this, under favorable transmission and reception
conditions, it is permissible to add the population
within a circular area having a radius four times
the mean radius of the basic area. In determining
the distances corresponding to the 5000 micro-
volts per meter field strength, measurements
are to be made during the daytime on not less
than 10 radii spaced at approximately equal
angles around the station.

All this is, of course, empirical, but it is cer-
tainly effective in bringing down estimates of
broadcast coverage from the blue sky to the
solid earth. Applying the method to a specific
case, we may use the Radio Field Strength
Contour Map of Washington D. C., and Vicinity,
presented as Fig. 9 in the paper by Bown and
Gillett: "Distribution of Radio Waves from
Broadcasting Stations over City Districts,"
(Proceedings I. R. E., Vol. 12, No. 4, August,
1924). This map was based on measurements
made on the old WCAP 5OO-watt transmitter,
which is no longer in existence. The contour
lines in the case of Washington are quite close to
circles, the transmitting conditions being favor- '
able for urban conditions (few high buildings,
and a general distribution of low buildings and
open spaces). The 5 millivolt per meter contour
is a circle with a radius of about 14 miles around
the transmitter. The population within this
circle would have been the basic population

AN OLD MAP SHOWING RADIO FIELD-STRENGTH CONTOURS OF WEAF
SUPERIMPOSED ON AN AERIAL PHOTOGRAPH OF NEW YORK CITY

served by WCAP. Under favorable conditions
WCAP would have been credited with the popula-
tion within a radius of 56 miles (four times the
mean radius of the basic area, in this case four-
teen miles). The half-tone on these pages
shows the radio field strength contours for WEAF
superimposed on an aerial photograph of New
York City.

In rating the audio-frequency characteristics
of a broadcasting station the NEMA Trans-
mitter Section prescribed the following method:
The number of octaves transmitted above 800
cycles (the mean speech frequency) and those
transmitted below 800 cycles, with a deviation
not to exceed plus or minus I TU, measured
from the microphone input terminals to the
rectified antenna output, shall be counted, and
the smaller of these two numbers multiplied by
two. The resulting number shall stand as the
audio-frequency characteristic rating of the
station.

On this basis a transmitter with a frequency
characteristic flat within I TU between looand
7000 cycles would receive a rating of 6, since it
transmits 3 octaves both above and below 800
cycles. If it only went up to 4000 cycles its rating
would drop to 4, since it would be based on the
two octaves above 800. Even if it went down as
low as 50 cycles it would receive no extra credit,
since the method of rating requires a balance
between the ability to transmit high and low
notes. About the highest rating within reach is 8,
entailing flat transmission up to 12, 800 cycles on
the high end, and 50 cycles on the low. Appar-
ently no one can get credit for going down below
50 cycles. If loud speakers are improved this
point might be criticized, and likewise the i TU
tolerance is open to question, since it cannot be
detected by ear. A 3 TU tolerance might be pre-
ferable in practice. The general method, how-
ever, seems excellent.

Under"Modulation Capability" the committee
specifies a single-frequency sine-wave audio
input, to the maximum degree of modulation
possible without "noticeable distortion," the
analysis being on the basis of rectified radio-fre-
quency output.

For the purpose of supervising modulation the
Section specifies the use of a standard volume

indicator, on the scale of which the following
relative limits are to be allowed:

Constant testing tone 30 divisions

Music peaks 30

Piano peaks 20

Speech 15-20

These values correspond to standard practice
in chain broadcasting. The piano is more sensitive
to slight overloading and so is given more margin,
while speech is kept down to a value where an-
nouncements will not break into the music with
obtrusive loudness.

Regarding microphone set-ups for broadcast-
ing the committee decided that, in "view of the
present relatively undeveloped state of this
portion of the art," this subject should be tabled.
This was no doubt a prudent move, since as
things stand there are as many microphone
set-ups for a given aggregation of musicians as
there are musicians, announcers, engineers,
musical directors, acousticians, program man-
agers, commercial sponsors, studio supervisors,
and advertising experts in the room, every
man is sure he is right, and nobody can prove
anything one way or the other.

Under "Standard Reference ('Zero') Level for
Broadcasting Use," the NEMA group laid down
the following specification: "It shall be standard
for broadcasting use, to regard the term 'reference
level' ('Zero level') as referring to a power of 10
milliwatts, corresponding to a current of 4.17
milliamperes flowing through a resistance of 600
ohms, or 2.47 volts across 600 ohms." This defini-
tion should put a stop to the endless wrangling
about "zero level" which has been going on
among the broadcasters.

Transmitter name plates, says the NEMA,
should contain the following data: (a) Power
rating in kilowatts; (b) Radio-frequency range
over which the set will deliver full power; (c)
Characteristics of the antenna for which the set
is designed. That the power rating is to be in
terms of power delivered to the antenna should
have been specified, since in many countries
power to the plates of the radio-frequency tubes
is the basis of rating.

If the Transmitter Section continues its work
on this plane it will become one of the most
influential agencies in this branch of the industry.

Some Experiments With Band-Pass Filters

By KENDALL CLOUGH

Engineering Department, Silver-Marshall, Inc.

FOR month* ur hair bcoi trying io secure
quantitative data on band-pass selectors
and filters for use at broadcast frequencies.
Not only does Mr. Clough, who is Chief
Engineer of Silver Marshall, Inc., give the
result of bis laboratory work, but be gives some
idea of how the home experimenter may play
with the circuit for himself. Mr. Clough
promises more interesting haw-to-do-it ma-
terial for an early issue.

— THE EDITOR.

CONSIDERABLE amount of material
has appeared in the engineering press on
band-pass filters for radio-frequency
tuners. Principal among these is the circuit dis-
cussed by Dr. Vreeland in the Proceedings of the
Institute of Radio Engineers, March, 1928. In
his paper Dr. Vreeland points out very com-
pletely the advantages of the use of a band-type
filter in the tuner of the receiver, but for the bene-
fit of those who have not had access to this paper,
these advantages are redescribed here.

If we were to connect a stage of radio-fre-
quency amplification, as shown in the circuit of
Fig. i, and run a resonance curve at 1000 kc.,
we would find that the circuit responded at and
about resonance as shown in curve A of Fig. 2.
Now in receiving a signal from a transmitter at
looo kc., we would find that, in the course of
modulation, frequencies varying from 995 kc.
to 1005 kc., had been combined with the carrier.
Obviously, if the reproduction is to be of the
best, a band of frequencies must be transmitted
from the antenna to the loud speaker with equal
amplitudes rather than the single carrier wave
only. Just how wide this frequency band should
be has been the point of many discussions, some
contending that we need to regard only a band
5 or 6 kc. either side of resonance, while others
believe that a band 10 kc. wide each side of the
carrier is necessary for perfect fidelity of repro-
duction. The finest audio equipment manufac-
tured to-day is designed on the 5 kc. basis, so it
seems superfluous to consider a band of greater
width than this. We are not concerned here with
the actual band width, however. The fact re-
mains that, whichever stand one wishes to take,
the resonance curve A of Fig. 2 does not permit
the free passage of a band of frequencies of any

M-20uh
FIG. I

A transformer u-ith a tuned secondary constitutes

the coupling device used between one radio-frequency

amplifier tube and another in the vast majority of

present-day receivers.

THIS PICTURE SHOWS THE EXPERIMENTAL BREADBOARD RECEIVER
WHICH THE WRITER CONSTRUCTED TO TEST THE BAND-PASS PRINCIPLE

appreciable width. It will be seen in the curve
that a frequency 5 kc. off resonance is amplified
only 8} per cent, as greatly as the carrier, and a
frequency 10 kc. off resonance only 62 per cent.
as great.

RESULTS WITH THREE STAGES

an interference standpoint the single
stage of amplification would be far from
adequate for modern conditions, so we have
shown the resonance curve B in Fig. 2 which was
obtained by cascading three of the circuits. It
can be seen that the selectivity to an interfering
station is greater, while the 5 kc. amplification is
only 60 per cent, of normal and the 10 kc. ampli-
fication 25 per cent, of that of the carrier. The
operation of this receiver would be equivalent
to the use of a tuner with a perfect band pass
and an audio amplifier having good amplifi-
cation of the bass notes and falling to 60 per
cent, of the bass amplification at 5000 cycles,
and 25 per cent, at 10,000 cycles. It should be
remarked that the receiver having the resonance
curve B of Fig. 2 would not be considered a
particularly selective receiver, so the reader can
judge for himself the side-band cutting that is
going on in the high-grade selective outfits. The
ear is a tolerant device, and never seems to miss
that which it has not heard.

Now, it may be demonstrated that the reso-
nance curve shown is a definite geometrical shape.
By this we mean that there is no adjustment
(such as the resistance of the coil, the primary
coupling, or the L/C ratio) which will cause the
circuit to admit, say, a 5 kc. band with more
facility without admitting an interfering station
10 or 20 kc. off resonance with corresponding
facility. Thus, the only circuit of the usual reso-
nant type which would provide perfect fidelity
would be a circuit infinitely broad, a mathemati-
cal fiction which would be worthless in reality.
This indicates that an entire change in the shape
of tuner response would be desirable.

The dotted-line rectangular curve of Fig. 2
would be the theon-tica] ideal shape. This shape
is not capable of attainment, but there is a cir-

cuit, old in the art, which under proper condi-
tions will produce a response approximating this
curve more or less closely.

Dr. Vreeland has discussed a similar circuit
(Fig. 3) in detail in the paper mentioned, but it
can be shown that this circuit is the analytical
equivalent of the circuit with which we are to
deal, Fig. 4, and which has been covered theoret-
ically in all standard texts. So thoroughly has it
been discussed that there is little we can add to
the treatment other than to present curves and
observations made in the laboratory. It is hoped
that certain readers will find sufficient material
and interest in these notes to enable them to go
on with the experiments in this interesting field
of band-pass filters which is far from a state of
perfect practical application.

The theory of this circuit indicates that, when
the coils, coil resistances, and condenser capaci-
ties are identical in each circuit, both circuits are
tuned to the same frequency (due to the identical
construction) when operated independently.

-25 -20 -15 -10 -5

*10 +15 «20 *

FIG. 2

// the voltage across the condenser in Fig. i (<•;)
were measured as the frequency of the voltage input
to the preceding lube was changed, a curve similar
to "a" in this graph would result. If three stage-.
used, the selectivity would he greater, as shtncn hy
the decreased response at point': Jar a-^-ay irnni
resonance in curve "b."

SOME EXPERIMENTS WITH BAND-PASS FILTERS

105

However, they actually tune to two separate
frequencies when operated with coupling be-
tween the two circuits as shown. In other words,
if the coils and capacities are of the proper size
to tune both circuits, LI, Q, and U G>, to a
frequency of FI kc. independently, the combina-
tion will not have any resonant peak at FI kc.,
but will have two resonant peaks at other fre-
quencies. One of these peaks will be Fm kc. above
the frequency of resonance, the other will be
Fm below the frequency of resonance and this
interval of Fm is defined by the equation:

This equation says in simple terms that, if the
mutual inductance between the coils is A per cent.
of the inductance of either coil, there will be a
peak of resonance A/2 per cent, either side of the
frequency of resonance of the circuits considered
independently. Thus, in the circuit of Fig. 4, if
we select two coils of 230 microhenries each, tune
them independently to 1000 kc., and then
couple them with a mutual inductance of 2.3
microhenries, there will not be a resonant peak

FIG. 4

Mr. Clough chose to study this variation of the
land scheme — he uses the mutual inductance be-
tween two coils as the coupling impedance.

at looo kc., but there will be two other peaks,
one at 995 kc., and the other at 1005 kc. Antici-
pating that this arrangement would approximate
ideal selectivity, the circuit was set up for meas-
urement and curves were run.

ANALYSIS OF CURVES

THE circuit used for these curves is shown in
Fig. 5. Two commercial coils of very low-loss
construction, the specifications of which are given
on the circuit, were chosen for the tuner. Two
mechanical placements are given in Fig. JA
which resulted in a measured mutual inductance
of i per cent, of the coil inductance. In order to
compensate the tube and other capacities of the
circuit, the tuning of the condensers was accom-

72TumsNo.25P.E.wire-32
turns per inch

LI -LI

(DIAGRAM A)

+ 45

FIG. 5

How the circuit actually was set-up to be measured. A screen-grid tube fed the vacuum-tube voltmeter by

means of the band-pass coupling mechanism. Diagram A — Two possible methods uf obtaining one per

cent, coupling between the two Silver Marshall coils used by Mr. Clough.

FIG. 3

Dr. l/reeland's circuit started all the discussion regarding
band-pass tuning for broadcast-frequency amplifiers. It con-
sists of two identical coils and condensers tuned to the same
frequency and coupled together by an inductance.

plished by disconnecting th> primary condenser,
Q, and tuning the secondary to resonance with
the desired wave, then disconnecting the second-
ary condenser, Q, and tuning the primary. It
was necessary to use a strong signal from the
oscillator while tuning in this manner as the
transfer from one coil to the other was very
low. After tuning each circuit independently,
both condensers were connected and the curve
for a particular frequency measured. These
curves were taken at 600, 1000, and 1500 kc.,
and are shown in the full lines of Fig. 6. It will be
noted that they check very well with the theory
of the circuit, for in the 600 kc. curve Fig. GA,
we have the two resonant humps lying very close
to 3 kc. off resonance on either side while in the
1000 kc. curve, Fig. 6e, they are 5 kc. off the
normal resonance of the individual circuits. The
1 500 kc. curve did not turn out as well, although
the separat'e resonances can still be distinguished.
It will be noted that the curve at 1000 kc. com-
pares very favorably with the ideal curve as the
amplification varies very little in the 5 kc. pass
band either side of the carrier. Attention is called
to the variation with frequency in the width of
the band passed in the three curves, which was
predicted in equation (i) to which we will refer
later.

The low amplification obtained with the screen-
grid tube in the above curves is of no moment for
our discussion. It was due to the low value of
coupling used between the tube and primary
circuit (20 turns at the base of the coil), which
was employed in order to prevent the tube cir-
cuit from affecting the coil circuit until the curve
shapes were assured.

In order to compare the shapes of the band
curves of Fig. 6 with the performance that
would be obtained when using
the same tube, coupling, and coils
in Fig. i, the secondary circuit
was removed and the measure-
ment repeated with the single
circuit shown in Fig. 7. These
curves are plotted in dotted lines
in Fig. 6 so they may be compared
with the band-pass filter perfor-
mance. It will be noted that at each
measured frequency the selectivity
of the band circuit is greater than
the ordinary resonant circuit and
that the amplification is about
on a par, one with the other.

Other curves were checked us-
ing the whole primary coil for
coupling to the tube. It was
found that the curve shapes re-
mained substantially the same
as shown in the full lines of Fig. 6,
but the amplification went up to
an average of 30 per stage.

NEW FILTER UNIT

kTHER studies of this circuit indicate that
the coil resistance must be kept very low
in order to maintain the desired shape of curve
and amplification for the stage. A similar type of
band-pass filter having two sections has appeared
recently upon the market and the writer had an
opportunity of running a curve on the selector
feature. The size of the coils was such that a high
resistance could be predicted and the resulting
curve is shown in Fig. 8. It will be noted that the
amplitude varies very badly with the frequency
and the band effect, while better than the aver-
age tuner, is far from the desired shape.

To check by actual observation the effect of
the band passed on the reception, a receiver was
made in breadboard style with two of the band
circuits and screen-grid tubes. Feeling that some
readers may desire vo hear this circuit for them-

-50 -40 -30 -20 -10 0 +10 +20 +30 +40 + 50
KC OFF RESONANCE
FIG . 8

A commercial receiver uses a system somewhat
similar to the one described in this article. The re-
sult obtained in measuring the ratio between output
and input voltages of such a band-pass selector is
shown in this figure.

106

RADIO BROADCAST

DECEMBER, 1928]

112-A

OTOA.F

FIG. 9

The complete schematic diagram oj the experimental breadboard receiver which is pictured at the head of

this article. Note the thorough filtering of all the screen-grid and plate leads, the volume control which varies

the screen-grid voltage, and the absence oj Melding except the single metal plate.

selves, the complete circuit diagram of this set-
up is given in Fig. 9, as well as a photograph
from which the physical considerations can be
seen. One coil of each of the pairs was mounted
in slots so that the coupling could be varied, thus
varying the width of the pass band. The distance
between coils used in making the curves was
marked so that it could be referred to during
operation of the receiver.

This receiver was operated for
two evenings in conjunction with a
Silver-Marshall type 682-250 pack
with remarkable results from a
quality standpoint. Several un-
biased observers stated that it was
the finest quality of reception they
had ever heard, noting particularly
the excellent timbre of the high
notes of the piano and organ. It
would be even safe to say that
some of the curse is removed from
soprano solos when the overtones
are freely admitted by a band
filter.

By placing a milliammeter (0-3)
in the plate circuit of the detector,
the double hump of the curve could
be noted at the lower broadcast fre-
quencies. Selectivity was ample for Chicago
conditions, good clear spaces being obtained be-
tween the local stations in a location where local
field strengths were very great. In these clear
spaces on the dial two out-of-town stations could
be heard weakly, but the amplification of the
system was not sufficient to provide a good signal.
The receiver in the form indicated could be called
an excellent local receiver of the highest quality.
To extend the scope of this model beyond local
reception, two possibilities present themselves
for the future. The first would be the addition of
band-pass filter stages. This does not appear
feasible except for those who can bear the expense
and the necessary difficulties attendant with the
matching of the large number of stages which
would be involved. The other possibility is the
use of a broad amplifier having no manual ad-
justments and equally responsive over the whole
broadcast band. Many will recall the amplifiers
of this type used in the early days of broadcast-
ing when the tuned radio-frequency receiver
came to the rescue. Perhaps with more general
knowledge of electrical theory better success
could be obtained with this type of circuit than

in the past. The two tuned circuits above present
sufficient selectivity to form very fine receivers
with a good "untuned" amplifier of this type.

CONCLUSIONS

\A7H1LE this is not intended to be a "final

• * hearing" on the subject of band-pass

amplifiers, it would appear that sufficient ma-

FIG. 7

The dotted curves in Fig 6 were taken with this circuit, which is

an auto-transformer in which part of the coil is used as primary

and all of it as secondary.

terial has been presented to arrive at the follow-
ing conclusions with regard to circuits of the
type discussed:

(i) That the coils must be carefully matched
and of low-loss construction in order to obtain a

good band-pass filter effect. Good coils are
usually large physically, so good band-pass filters
can be expected to have a considerable amount ol
bulk in their finished form.

(2) That with good coils, better selectivity
per stage can be attained than is to be had with
the same coil operating as a conventional radio-
frequency transformer. In general, it can be saic
that this increase will not be great enough to
compensate the greater cost of the band-pass
tuner stage, which is at least double that of the
single circuit.

(3) That the width of the band passed wil
vary with frequency when attempting to tune the
broadcast band by means of fixed coils and vari-
able condensers. This follows from the theory o
the device, and was confirmed by the curves which
show that with a i per cent, coupling the
band passed was 10 kc. wide (total) at 1000 kc.:
6 kc. wide at 600 kc. This constant percentage
relationship prevents the attainment of the
correct band width except over a small range ol
frequencies without changing the coupling be-
tween the coils. A similar situation has been long ;
tolerated in radio-frequency transformers, how-
ever, so this is not a serious consideration.

(4) That it is not possible to cascade sufficient
of these stages to obtain the degree of amplifica-
tion that is usual in sensitive radio receivers
without prohibitive cost and constructional
difficulties.

(5) Far superior tone quality can be obtained
by the use of a band-pass device than from the :
use of simple resonant circuits. It is undoubtedly
on this last point that the band-pass filter will
find a prominent place in the radio art.

LIST OF PARTS

THH apparatus used by the writer in making
this investigation is given below. There is
nothing special in any of the parts, and similar
apparatus would perform as well. The coils, as
mentioned above, must be of low-loss construc-
tion.
The list follows:

Li — i S-M r.f. choke coil, No. 275;

Lj, La, Li, L6 — 4 S-M inductances. No. 140

(minus primary);
Q, Q, Q, Cr— i Four-gang condenser,

0.0003 5-mfd.;

Q, Q, Cn — 3 Parvolt condensers, 2-mfd.;
C?, Cs — 2 By-pass condensers, o. i mfd.;
C> — i Carter condenser, o.oooi 5-mfd.;
Cio — i Condenser, o.ooi-mfd.;
Ri — i Filament resistor, lo-ohm;
RI — i Potentiometer, 3000-0(101;
Ra — i Filament ballast, lo-ohm;
R< — 2 Filament ballasts, lo-ohm;
Rt — i Durham grid leak, i-megohm;
8— -Fahnestock clips;
VT,, VT? — 2 Screen-grid tubes;
Del — i Detector tube, 312-type.

FIG. 6

The result — note that in each case the dotted fiirve, which represents a conventional transformer
with only the secondary tuned, has poorer selectivity than the double-tuned transformer.

DECEMBER, 1928

RADIO BROADCAST

107

O No. 11

Radio Broadcast's Home Study Sheets December, 1928

Resonance in Radio Circuits

Part I

"XTITHENEVER one tunes his radio receiver or transmitter he performs
»* one of the most interesting and most fundamental experiments in all
electrical science; he demonstrates a phenomenon that underlies prac-
tically all radio work. This is the phenomenon of resonance which occurs
in an a.c. circuit under certain conditions of inductance, capacity, and
frequency.

To study, experimentally, this phenomenon of resonance, we will need
the following:

LIST OF APPARATUS

1. A simple radio- frequency generator consisting of a vacuum tube

connected to a coil and

100,000 w

FIG.

condenser as in Fig. 1
The plate potential of
150-180 volts may be
supplied by a standard
B-power unit.

2. A meter which will
read radio-frequency
current. The Weston
Model 425 thermo-
galvanometer is a good
t example. It will measure
i 115 milliamperes at
M • radio frequencies, has a

ao resistance of 4.5 ohms,

S and costs $18.50. This
2 is rather expensive, but,
in view of the number
of uses to which it can
be put, it is found in
every well -equipped
laboratory. Other meter
manufacturers make
similar meters.

3. A coil. The one used
in taking data for this
experiment was part of
a Brown ing- Drake Kit and had the following dimensions: number of
turns, 46; length of winding 1-11/16"; diameter, 2-11/16". The wire
was about No. 24 and was spaced about the diameter of the wire.

4. A calibrated variable condenser — a good one is a General Radio Type
247-E in which the capacities are engraved on the dial.

PROCEDURE

Start up the generator, and if possible, measure its frequency or wave
length. This is not essential, however. Connect the coil, the condenser,
and the current meter in series. Couple the coil loosely to the generator
inductance, and slowly tune either the generator or the tuning condenser
until some current is read on the meter. Tune through "resonance," in-
dicated when the current is a maximum, making sure that the meter
does not go off scale. Now use as loose coupling as possible to the genera-
tor, and plot the current (or deflections of the meter) against condenser
degrees and condenser capacities, as the tuning condenser is varied
through resonance with the generator. A specimen "resonance" is shown
in Fig. 2. Add a 10- to 30-ohm resistance in series with the circuit and repeat.

Calculate the inductance of the coil from the formula given in Home
Study Sheet No 2 (August RADIO BROADCAST) and from the formula con-
necting wavelength, inductance and capacity,

Cwavelength):!=3.54 X L X C
where C is in mmfd.

L is in microhenries
wavelength is in meters.

DISCUSSION

What is happening that the current in such a combination of apparatus,
known as a series-resonant circuit, increases at first slowly, then more
rapidly, then decreases sharply, and finally falls off to a very low figure?

The answer may be found in Home Study Sheets 7, 8 and 10. In these
sheets the effect of a capacity, and an inductance upon the a.c. current
in a circuit was discussed. Thus, in an inductive circuit

T- JL
"XL

and in a capacitive circuit,

I=JL=Ex6.28xCXf

and when L, C, and R all exist in a series circuit, the current I is

— Xc)=

where

Z is called the impedance of the circuit

<d is equal to 6.28 X f

f is the frequency in cycles

X is the reactance of L or C

Now an inspection of this formula for current shows that the capacity
reactance is to be subtracted from the inductive reactance to get the
total reactance in the circuit which, combined with the resistance, forms
the impedance which controls the flow of current. If, therefore, we add
sufficient capacity reactance to the circuit, so that it is equal to the in-
ductive reactance, the two, when combined by subtracting their values,
add up to zero, and the impedance then is composed of the resistance only.
In other words, tuning the condenser changes the capacity reactance,
thereby decreasing the total reactance, decreasing the impedance, and
increasing the current.

This is exactly what was done in the above experiment. We balanced
out the inductive reactance, which is determined by the coil and the
frequency, by changing the capacity reactance, which is determined by
the condenser and the frequency. When the two reactances are equal in
value but of opposite effect, the total impedance offered to the flow of
current is very low, consisting of R only at this value of L, C, f, and the
current is a maximum.

In a series resonant circuit the current may become very high although
the driving voltage, which is across the entire circuit, may be fairly small,
and although the individual reactances of the coil and condenser are
large.

VOLTAGE IN CIRCUIT

As in all circuits the voltage across any part is the product of the cur-
rent through it and its impedance. Thus the voltages across a resistance,
inductance, or condenser in such a circuit are:

ER=IXR

EL=! X XL=ILfc>
Ec-IXXc-i

and since the current at resonance may become very high — it is governed
by the voltage and resistance only — the voltage across the coil and con-
denser may become very high. For example, if 100 milliamperes flow in a
circuit at a resonant frequency of 1000 kc. when the inductance is 200
microhenries, the voltage across the coil is

EL = I X L<J=(100 X 10-3) x (200 X HX>) X (6.28 X 1000 X 10»)
=125.6 volts

although, if the resistance in the circuit is 10 ohms, the impressed
voltage necessary to drive 100 milliamperes through it is only one volt.

A series circuit may be tuned to resonance by varying either the ca-
pacity as is usually done— or the inductance, or the frequency. Below the
resonant frequency the principle reactance is capacitive. The inductance
offers little reactance at low frequencies. At frequencies higher than
resonance, the major reactance is the inductance, because the condenser
reactance steadily decreases with frequency. At the resonant frequency
the two reactances are equal, and hence the voltages across them are
equal. This occurs when XL = Xc, or when

. 159200 . ,
f= ,? — • when L=
V'Lxc

u.h, C=mmfd. f =kc.

Thus a series-reson-
ant circuit is a kind of
voltage multiplier. A
small driving voltage
across a low-resistance
("Low-loss") circuit
will cause a high cur-
rent flow at resonance
and a large voltage to
appear across the in-
ductance and coil.

PROBLEMS

1. Assume L = 200
microhenries, C = 500
mmfd, R=10 ohms.
Calculate the react-
ances, impedance, cur-
rent, resistive, and in-
ductive and capacitive
voltages in the circuit
when E = 10 volts. Plot
all these against fre-
quency from 400 to 600
kc. If the experiment
outlined under Proced-
ure has been carried out,
use the values of L
and C obtained there
and assume R=10

ohms. Since the current is known, calculate the voltage across the circuit
at resonance.

2. How do the two calculated inductance values check?

3. Does the current lag or lead the voltage below the resonant fre-
quency? At the resonant frequency what happens to the phase angle?
What above the resonant frequency?

4. In an amateur transmitter tuned to 40 meters, the antenna current is
one ampere. This flows through a series tuning condenser of 100 mmfd.
capacity at resonance. What voltage must the condenser stand?

5. In Problem 1, what is the ratio between the current at resonance and
at 20 kc. below resonance? What would be this ratio if R were doubled, or
halved? Do you see the importance of low-resistance circuits?

430 440 450

460 470 480 490

Cmmf

FIG. 2

500 510

108

RADIO BROADCAST

DECEMBER, 1928

No. 12

Radio Broadcast's Home Study Sheets December, 1923

Resonance in Radio Circuits

Part II

RESONANCE may occur in a radio cir-
cuit in one of two ways. The series
resonant circuit was discussed in Home
Study Sheet No. 11. Suppose, instead of
having the voltage impressed in series with
the inductance and capacity, it is impressed
across the condenser and inductance con-
nected in parallel, as in Fig. 1. What
happens, as the frequency is changed?

DISCUSSION

S 22

5 20

300 320 340 360 380 400
FREQUENCY IN KC.

Unfortunately the experiment to show
just what happens to the various currents jg

and voltages in such a circuit is difficult to
perform. It is simple, however, to calculate
what happens and to plot it. Instead of ig

going into the laboratory for this experi-
ment, then, we shall rely on the slide rule
and graph paper to delve into another in-
teresting radio phenomenon known as
parallel resonance.

In a series circuit the same current flows
through all the units, but the voltages

across these units may differ. In a parallel circuit, the same volt-
age exists across the branches, but the currents through them differ.
The total current, I, flowing out of the generator, in Fig. 1, is the sum
of the currents flowing in the two branches. Since, however, a capacity
reactance is considered as a negative reactance, the current through such
a reactance may be considered as having an algebraic sign opposite to that
of a current through an inductive branch. The total current, then, is the
difference of the currents, i.e.,

I = lL-Ic

and from previous Home Study Sheets, the currents in these branches

may be calculated if the reactance
ajid the voltage is known.

The formula above, which gives
the generator current) flowing in-
to the parallel circuit, shows that
if the current in the capacity
equals the current in the induct-
ance, the difference, or generator
current, becomes zero.

IMPEDANCE OF
PARALLEL CIRCUIT

The impedance of any device
or circuit may be defined as the
ratio between the voltage across
it and the current through it.
Thus, the impedance of a parallel
tuned circuit is

7-E
Z I

and if, at resonance, the current,
I, is zero, the impedance must
be infinitely high, because cur-
rent would not flow out of the
generator no matter how high its
voltage.

A series-resonant circuit, looked at from the viewpoint of the generator
is a very low-impedance circuit at resonance. The current it draws from
the generator is large. On the other hand, at resonance the impedance of
a parallel-resonant circuit, looked at from the generator, is very high,
and the current fed into it from the generator is very small. Series circuits
are used when low impedances are desired; parallel circuits, when high
impedance circuits are needed.

If we measure, or calculate, the current flowing in each of the two
branches in Fig. 1, and the total or generator current as well, we shall
obtain curves similiar to those in the accompanying figures. These are
theoretical curves and do not take the resist-
ance of a circuit into account. Resistance
usually exists in the inductance of such a
circuit, but in well-designed radio circuits it
is small compared with the inductive react-
ance of the coil. If this is true, the imped-
ance into which the generator feeds current
at resonance is equal numerically toL2(*>2
where R is the resistance of the coil. ~~R~~
As the frequency of the generator is
changed, the current through the inductance
decreases and the current through the
capacity increases as in Fig. 2. At very low
frequencies there is a large difference be-
tween the two currents— the inductance cur-
rent being the larger — and so a large current
flows from the generator. At very high fre-
quencies the capacity current is much
greater than the inductance current and so a
large difference current, i.e., the generator
current, flows as in Fig. 3. For this reason,
below the resonant frequency the generator
views I he circuit as inductance shunted by a
i <>nd< -riser whose reactance is so high it takes
but little current. At frequencies above reson-
ance the inductance has little effect upon
the generator current and so the circuit is
said to be capacitive. At the frequency
which makes the inductive and capacitive

FIG. 2

reactances equal, the currents are equal,
the circuit as a whole is neither capacitive or
inductive and, therefore, must be resistive
only, so far as the generator is concerned.
At this frequency, then, the circuit looks
like a resistance to the generator, and the
expression above for its impedance is
properly called its effective resistance.
Thus,

RH,F- t£ or ^

where L = coil inductance

<•> = 6.28 x f
C = capacity required to

resonate the coil
R = series resistance of coil
For example if L = 200 microhenries,
R = 10 ohms and f = 1000 kc., RIFF =
160,000 ohms.

PROCEDURE

1 . Assume L = 200 microhenries, C = 500

mmfd., E = 10 volts. As the frequency varies from 400 to 600 kc..
calculate; a. reactance of inductive and capacitive branches, b. currents
in these branches, c. total current flowing from generator, d, impedance
presented to generator;

2. Plot all of this data against frequency.
PROBLEMS

1. If the coil in Procedure has 15 ohms resistance at 300 meters, and
the grid and filament of a vacuum-tube amplifier is connected across the
circuit, what impedance does the tube work out of? Suppose it is in the
plate circuit of a power tube

which feeds 100 milliamperes into g

it. What is the power required
from the tube if P = I- REFF

2. An antenna-ground system
has a capacity of 0.00025 mfd.
An inductance is to be put in
series with it so that the entire
circuit will be resonant to 400
meters. Calculate the total circuit
inductance. If adistant station im-
presses across this system causes
voltage of 100 microvolts, what
voltage is across the inductance
at resonance?

3. An interfering station work-
ing on 600 meters also sets up
across the antenna a voltage of
100 microvolts. If the system is
tuned to 400 meters, what voltage
at the interfering frequency will
appear across the inductance?

300 320 340 360 380 400
FREQUENCY IN KC.

FIG.

4. Suppose in series with the

antenna is connected a circuit like F IG. 3

that of Fig. 1 and tuned to 600
meters. Assume the resistance of

the coil is 10 ohms. Calculate the impedance this circuit would offer to
the 400-meter and the 600-meter signals. Then calculate roughly the
ratio of wanted to unwanted voltage across the inductance L. The com-
plete antenna system is shown in Fig. 1. Incalculating the impedance
neglect the effect of L and C.

5. If doubling the inductance of a coil doubles its resistance, loo, what
effect upon the effective resistance of a shunt tuned circuit has doubling
the inductance? Of course, a smaller condenser would be used to reach

the resonant condition. How much smaller
would the condenser be?

6. A plate-supply device has considerable
120-cycle hum in its output. Suppose a
parallel tuned "trap" is placed in the positive
lead. A 30-henry inductance is available.
Calculate the size of condenser needed. If
the inductance has 500 ohms resistance what
miix-dance will the trap offer the 120-cycle
current?

7. The maximum voltage gain that may be
secured from a screen -^rid tube may be calcu-
lated from Gm X RKKF, where Gm is the mu-
tual conductance of tin- tubr and KKKF is the
imjH'dance of the tuned circuit into which the
tube works. Assume that Gm =300 micromhos,
L=200 microhenries, wavelength=300 meters
and calculate the maximum resistance that
can IM- tolerated in the coil and condenser
which tunes it to permit an amplification of
60. If the resistance of the coil is doubled.
what happens to the gain of the tube and coil-
condenser combination?

Note: Reader may send their answers :o these
questions to the Editor to be checked.

LOUD SPEAKERS-A DEBATE

Dynamic Versus

Type

AT THE end of an average day, the waste-
paper baskets in the Editorial Office
of RADIO BROADCAST are almost filled
with press releases written about every known
kind of a radio equipment. It is a rare release
which contains information which can be used
in this magazine. The truth is, that it is a rare
release which has any information of use to
anyone. Occasionally, however, the mail

clerk brings one which promises a great t*n^^^^——^^—^—^—^^^^^——^-^ai.
deal.

The following statements are taken ~D ADIO folk feel keenly that, out of the mass of near

•*•*- accurate statements and downrieht mis-statement

trical input. We have stated already that, if a
speaker could be made 10 TU more efficient than
the best of our present devices, we could obtain
sufficient volume of sound from it without using
expensive power apparatus.

The fact that the efficiency of the unit which
the release mentions differs at various power
levels indicates, according to the engineer, "con-

from a release dated August 2gth, titled
"Little-known Facts About Well-known
Cone Speakers," and signed by the Director
of Research of a cone-type loud speaker cor-
poration. The replies to these statements
were made at our request by an engineer
who has no connection with the loud-
spi-jker industry, but who, in our opinion,
is equipped with as much unbiased data
as any engineer in the country. His in-
terest, as shown in his statements below,
is the interest, of engineering truth. We
believe this little debate may give our
readers more information than a much
longer article written in conventional "®*
style and form.

The release: "It is freely claimed that the
efficiency of the usual rocker-type driving unit
for cone speakers is from 4 to 7 per cent., and
that its power rating is strictly limited to that
for which it is set. The Western Electric unit,
for example, is set for I watt, the BBL for 2.5.
watts, while the Stevens unit will take 3 watts
and still maintain an efficiency of 7 per cent.,
although at 0.5 watt its efficiency is only 2 per
cent "

Our engineer replies: "The so-called wattage
that can be put into a speaker varies with the
frequency. The Western Electric cone handles
ut ii watts at the lower end of its response
without striking the pole pieces. Because
"t its rather greater efficiency than most
cones this gives as much sound output as
a cone 3 TU less efficient capable of han-
dling 3 watts."

We should like to put in a word here.
I The average listener need not worry about
1 the power-handling ability of his loud
i speaker. What difference would it make if
I his speaker could handle 100 watts of
t energy without smoking, blasting, or
: hitting the pole pieces? What he should
3 be interested in is the amount of sound he
I gets out of the speaker with a given elec-

accurate statements and downright
about topics which may at the moment be uppermost, there
must be some generally accepted facts to which they may
cling. This is usually so, but the search for simple and real
truth is not easy. In the comparative merits of different
types of loud speakers, the flow of inaccuracies has re-
cently been especially strong. The short discussion here of
the comparative merits of the magnetic vs. the moving-
coil loud speaker is as technically accurate as we know how
to make it, without ascending to the rarefied atmosphere
of higher physics. We hope it proves interesting and
valuable to our readers.

— -THE EDITOR.

siderable lack of linearity in response. When the
ratio of input to output is not linear, the har-
monics are very bad."

EFFICIENCY OF DYNAMIC UNITS

CREAKING of moving-coil loud speakers, the
^ release states, "The very best design avail-
able develops an electrical efficiency of only 50
per cent., and this figure is attained only by
employing a push-pull transformer which has
been designed specially and constructed of very
expensive material.

"The electrical efficiency of some of the most
popular makes of electrodynamic speakers is a

scant 30 per cent., when actuated by push-pull
amplifiers. When, however, they are actuated by
ordinary transformers, and from a single power
tube, the efficiency drops to as low as 20 per cent,
with some distortion. In some of the cheaper
makes, the efficiency is further impaired by
mechanical losses due to the cone suspension."
Our engineer says: " The push-pull element has
nothing to do with it. Equal efficiencies
n5v> can be obtained with either single or push-
pull amplifiers and transformers. An ef-
ficiency of 30 per cent., is probably high
mis-statements for even the best of the dynamic speakers.
Any speaker worked from improper con-
ditions will perform unfavorably."

The release: "These figures, let it be
understood, are electrical only, and to
obtain the overall efficiency, we must
subtract the cone losses. It is quite true
that in cones of the same size — the 1 8-inch
size, for instance — the efficiency of a cone
actuated by a dynamic unit is greater
than that of one actuated by an electro-
magnetic unit. This is due to the greater
power available for flexing the material,
provided, of course, that the source is
capable of furnishing the energy. On the
other hand, it is well known that the

^-^

^— -»_

\*~~~~

-^ \

MAGNAVOXRJ

500
FREQUENCY

FIG. I. FREQUENCY RESPONSE CURVE
OF A TYPICAL MOVING-COIL SPEAKER

ICK)

amplitude of the apex of the cone varies inversely
as the square of its diameter, plus a logarithmic
constant for air slippage."

POWER VS. EFFICIENCY

'"PHE engineer: "The power available has
^ nothing to do with efficiency; the losses are
what reduce it. Dynamic speaker* may be made
to have less eddy current and hysteresis losses
than the rocking-armature units. In the latter
type the armature itself is saturated rather
heavily. In addition, mechanical masses are sup-
ported better and distributed so that imped-
ances, both mechanical and the resultant elec-
trical, are more uniform and permit of
better matching in the dynamic type.

"This is true for a constant radiating
power at a particular frequency, and pro-
vided the whole moves as a unit (piston)
without interference between the front
and back faces. This latter is not the case
for the Western Electric type of cone,
and is true for the dynamic only when the
baffleboard is sufficiently large (diame-
ter), massive and damped."

The release: "In practice this works
out so that if an 18-inch cone requires
o.oo8-inch amplitude down to 100 cycles,

110

RADIO BROADCAST

DECEMBER, 1928

then a 7-inch cone will require 0.062 inch
down to the same frequency. If, however, a
dynamic speaker could reach only 100 cycles,
it would not be satisfactory; and to reach
48 cycles, a stroke of plus or minus J inch
or J inch is necessary. It is, of course, theoreti-
cally possible to make a dynamic unit with this
stroke, but at the present time there are only two
commercial speakers of this character, and both
are sold complete with amplifier and power plant.
The very best dynamic unit for 7-inch cones has
a possible stroke of plus or minus 3% inch. Others
have a possible stroke of I's inch. One type ex-
amined in our laboratory had as little as a *V
inch stroke."

The engineer: "This depends upon how much
energy must be radiated. Under any reasonable
conditions it will be superior to the Western
Electric type of cone in radiation (area x ampli-
tude) at frequencies below 300 cycles because
of the baffleboard's effect."

ELECTRICAL TROUBLES

T HE release: "Electrical troubles are caused
' by the following factors: To begin with,
dynamic units are not provided with a spring
resilience as are the electromagnetic units. Push-
pull actuation is therefore more essential. Again,
in all cases an output transformer of special
design is required, and unless made of special
alloy cores, which are quite costly, it can intro-
duce quite as much distortion as a third stage
of audio amplification."

The engineer, replying on the spring resilience
of the cone-type speaker: "That is the reason
dynamic units are so very much superior to
balanced-armature speakers from the standpoint
of harmonics generated. Since they are always
actuated by a.c., theoretically no restoring
force should be necessary. The current itself will
always return the moving system to the start-
ing position. Push-pull actuation is obtained by
the r.c., not by any push-pull feature of the
'amplifier system. The currents fed to a speaker
by a push-pull amplifier can in no possible man-
ner be told from those fed by a single-tube
amplifier as long as neither is overloaded and
the d.c. component is eliminated by a trans-
former or blocking condenser."

The release: " Many makes of dynamic speak-
ers now on the market are provided with cheap
transformers, so that the distortion caused by
combinations of capacities, chokes and resist-
ances — including cut-off filters intended to cover
up poor design — tends to cut down the efficiency.

The engineer: "This is equally true of other
types."

The release: "The greatest electrical loss, how-

• Permanent Magnet

F1G. 2. CONSTRUCTION DETAIL OF A
BALANCED-ARMATURE MAGNETIC UNIT

ever, occurs in the movable coil itself. The im-
pedance of the movable coil is not matched with
the secondary of the output transformer; and
even when it is nearly matched, the 12R loss
of the coil is enormous. The reader can obtain a
really practical picture of this loss if he stops to
consider that in a correctly designed step-down
transformer whose ratio is 50 to I, the secondary
wire is No. 18 while the dynamic coil is wound
with No. 30 or 32! This means that the dynamic
coil is being overloaded from 300 to 400 per
cent."

The engineer: "The losses (I2R) in the coil are
fairly large compared to those in certain other
parts in the system, and account largely for the
30 to 50 per cent, efficiencies instead of 100 per
cent. It is still better than the 3 to 7 per cent,
efficiency obtained by other types. The moving
coil cannot be considered as "overloaded" unless
it is mechanically or electrically 'Jikely to be
destroyed."

FIELD COIL EFFICIENCY

HpHE release: "The electrical efficiencies dis-
*• cussed above are those of the signal system
only. In addition, we must consider the efficiency
of the field, which is from 101025 percent. When
a field for the signal of a 1 12-type tube was re-
quired, this was quite immaterial. To obtain a
field for the signal from the2io-or2jo-type tubes
in push-pull, however, requires enormous power.

"Dynamics of the kind having a signal effi-
ciency of about 20 per cent., with the field rated
at i ampere at 6 volts were found to take 6 watts
at 6 volts, while those having a signal efficiency
of 30 per cent., required 16 watts. Those with a
signal efficiency of 50 per cent, demanded a 50-
watt field. The latter field requirements are far
above the usual output of eliminators. Hence the
field cannot be used as the eliminator filter choke,
but must be fed with a separate rectifier."

The engineer: " Proper design of the magnetic
circuit can reduce greatly this loss and apparent
inefficiency."

In this connection, we understand the Vita-
phone loud speakers in factory production have
an average efficiency of 35 per cent., and that
the field of these speakers consumes about 10
watts.

The release: "In conclusion, it should be noted
that the deep bass notes developed by many
dynamics are additive resonance, and if one likes
this effect one can readily obtain it by much
cheaper and simpler means. The deep resonance
is caused by the fact that most dynamic cones
are fastened to a metal frame which, in turn, is
bolted to a large wooden baffle acting in the
capacity of a diaphragm. This serves to accentu-
ate certain low frequencies. To prove this, let
the reader actuate a dynamic unit thus mounted,
with an organ record of low pitch, and he will
hear clearly a bass drum accompaniment. If this
effect is desired, the same results can be produced
with an impregnated cloth cone (of curved angle)
glued to the same kind of baffle and actuated by
a high-power electromagnetic unit."

The engineer, " No loud speaker is quite so free
from the effects of resonance as the dynamic with
a piston-type paper cone or properly designed
moving system and horn. The reproduction of
low notes is not due to resonance. The wooden
baffle does not act as a diaphragm. In fact, the
less it moves the more effective it becomes."

OUR DECISION

VA/E HAVE enjoyed this little argument.
• ' The cone designer is correct in stating
that good cone-type speakers can deliver excel-
lent quality; the engineer, whose feeling in favor
of "dynamic" speakers is so evident, is correct

Leads to output coil.'
of power stage output
transformer

FIG. 3. CONSTRUCTION DETAIL OF
A MOVING-COIL SPEAKER UNIT

too — for the best possible reproduction, a good
moving-coil speaker is superior to a speaker of
the Western Electric type.

There is only one more statement we hope
everyone will understand — all speakers are
dynamic speakers in that some of their parts
move. All dynamic speakers, however, are not
"moving-coil loud speakers" which are the
type the engineer favors, and what nearly every-
one thinks of when the word dynamic is men-
tioned. Up to the present time there is no type of
speaker generally available which is the equal of
the better grades of moving-coil loud speakers.

More Data on Loud Speakers

HpHE above argument will be followed by addi-
' tional data on dynamic speakers, as the Editors
have made arrangements with Joseph Morgan,
of the International Resistance Company, for
the preparation of another of his articles on loud
Speakers — the first appeared in August RADIO
BROADCAST and was called "All About Loud
Speakers." The second of Mr. Morgan's articles
will be "All About Dynamic Loud Speakers,"
and he has been instructed to take every type of
dynamic speaker now easily obtainable and put
them through their paces in his laboratory. His
article will contain curves showing how the vari-
ous speakers respond to the necessary audio fre-
quencies, what field currents they require, how
much audio power is necessary, what their ad-
vantages and disadvantages are, and where the
magnetic type of speaker stands. in the path of
progress toward better and better radio recep-
tion.

The advantage the second of Mr. Morgan's
articles will have over other dynamic-spr.iker
descriptions is that his remarks will be b.ised
upon laboratory measurements and actual tests,
not upon matters of opinion. Together with
the present article made up of the remarks of
two engineers, it ought to equip any serious radio
thinker with sufficient data to decide for himself
and advise his friends on the question, "Is it
worth while to invest in a dynamic speaker?"

— THE EDI ion.

A TTPICAL WELL-ARRANGED AMATEUR STATION

Removing Nonsense from Short -Wave

Transmission

THE transmitter described in the last two
installments of this series will supply its
owner with a considerable amount of
pleasure, whether he be experimenter, scientist,
engineer, tinker, or friendly " rag-chewer." It is
not intended especially for the highly specialized
message-handler, but is quite adapted to the
uses of that group as well.

Just how it may be made to serve these vari-
ous purposes may be told best after we have
pushed aside certain very widespread hoaxes.
That it is really necessary to do this can be made
plain by recalling that grotesque hearing at
Washington wherein a large group of gentlemen
demanded all the short waves — and more — from
the Federal Radio Commission. Not only did
these representatives have in mind purposes
that ranged from worthy to silly, but a goodly
proportion of them were filled with the most
amazing illusions as to the possibilities of short
waves. Surely Mr. Average Citizen is at least as
badly off. In order to give him a fair start one
must surely equip him with something approach-
ing the truth.

The fault lies with the evergreen enthusiasm
of the reporter and the fish stories told him by
the station owner. Between them they produce
an illustrated story about some amateur station
which is claimed to " talk to every country in the
world and be in touch with Australia every
night." (The quotation is genuine.) Probably
neither one recognized this as a plain ordinary
lie — yet a lie it is. No amate'ur (or professional)
station can "talk to every country in the world"
for the good reason that not all countries have
stations which can reply. Again, one is perfectly
safe in saying that no amateur station in this
country has ever maintained daily contact with
Australia (or any other foreign country with the
exception of Canada or Mexico) for even one year.
Any attempt to claim that a shorter demonstra-
tion is a proof can be set down as an admission

By ROBERT S. KRUSE

that the speaker is not familiar with the seasonal
and climatic vagaries of short waves. There are
many cases on record to show that quite good
international amateur contacts may exist for
several months, only to disappear completely
with no assurance of recurrence twelve months
later.

International amateur contacts are a post-
graduate activity in any case. The beginner may
make an occasional contact of this sort but as a
rule the signals are weak, the interference is

BRASS-pounders, message-handlers, rag-
cbewers — these are terms dear to the heart
of all amateurs. In this article Mr. Kruse
puts such divisions of the amateur lists into
their proper places, and points out that the
true experimenter is a combination of all
three — and has lots more fun. In the bargain,
Mr. Kruse gives some directions about
operating the master-oscillator code-and-
phone transmitter already described by him in
RADIO BROADCAST.

— THE EDITOR.

strong and the contact at best is fleeting, even
with good operating skill. Where a contact is
recurrent it is almost invariably due to a sche-
dule, careful hunting and breathless listening —
plus some "filling-in" by the receiving operator.
Working in that manner certain operators, such
as Clifford Himoe, who just returned from the
McMillan Greenland Expedition, and Fergus
McKeever of Lawrence, Kansas, have accumu-
lated extraordinary strings of "calls-exchanged"
and not a few international contacts have per-
sisted for weeks or months. These feats have the
same relation to the results obtained by an

I! I

ordinary operator that Will Rogers' rope spinning
has to my attempts — or yours.

THE LOW-POWER MYTH

AT ONE time there existed a state of near-
anarchy in amateur radio, occasioned by
the installation of high-power broadly tuned
transmitters by those amateurs who could afford
them. The rest of the congregation was allowed
to sit and listen. This was combatted by legisla-
tion and by a campaign for low-power records.
The expected result of such a campaign followed
in the form of a low-power cult which was sur-
rounded by as much exaggeration as the interna-
tional contacts. We read of this or that station
which operates a receiving tube (replaced how
often?) at 350 volts and is in "constant" (all
newspaper radio contacts are constant) com-
munication with amateurs all over the country
(even where there are none). Because superb
operators like Mason and Waskey maintained
Wilkin's lane of communication with sets of the
one-mouse-power class we are asked to believe
that the range of a battery-driven set using a
receiving tube is 500 or 1000 miles. But, nothing
is said of the frequent failures, of the painstaking
repeat-repeat-repeat, of the nerve-straining
listening, and of the weary hours spent in search-
ing for a lost whisper.

If, indeed, these tiny transmitters were capable
of reliable work over distances, then we must
suspect the Western Electric Co. of being very
badly informed; a joo-mile transmitter manu-
factured by this company occupies most of a
standard boxcar when it is shipped.

It all boils down to this, the range of a short-
wave transmitter varies with time, weather,
operator's skill, location, interference, and
adjustment. The total variation that results is
certainly at least as large as 10,000 to i, and it
is quite possible for the same small set to com-
municate with its antipode with fair signal and

112

RADIO BROADCAST

DECEMBER,

on another occasion to fail entirely to " get out of
town." There is also another factor which multi-
plies the above by apother 10 or so. This is the
fact tha( "you can always hear a signal on
schedule" if it is even partly readable, through
that same signal may have been begging vainly
for attention for months past.

THE FRIENDLY "RAG-CHEWER"

CEVERAL years ago when engaged in the
"^ process of exploring the wave band of 1 5 to
loo meters a group of experimenters became very-
much disgusted with the existing fad for extreme
curtness toward any radio activity other than
the handling of messages. After listening to a
large number of the messages, and handling
many of them ourselves, we became convinced
that they were mainly of no consequence and
could be replaced with profit by tests and
friendly conversations. F. C. Beekley, QST's
Advertising Manager, and L. W. Hatry, well-
known to readers of this magazine, conceived
a purely paper organization to be called the
" Rag Chewer's Club." In order to become
a member of this organization one had to
furnish written proof of a jo-minute non-
message-handling radio conversation with an-
other amateur station, whereupon there was sent
out a membership certificate made up as a
burlesque of the I.A.R.U. and A.R.R.L.
certificates. It was hoped that, if amateurs could
once be induced to assume a human friendliness
toward their radio contacts, they would never
again lapse to the same machine-made routine.
That hope was fulfilled to such a degree that
the mailing of the "certificates" became almost
a full-time job for one person. Even now, after
a number of years, the idea is still alive and one
may derive considerable pleasure from a friendly
conversation with other transmitting amateurs
in various parts of the United States.

— AND THE CALL CARD

C EVERAL years ago _Don Hoffman of Akron,
^ Ohio contributed to this picture the idea of
sending a postal card as friendly acknowledge-
ment of a radio conversation. His postal card

Lightning Switch
required by ..-••
Insurance
Company

had the radio call, 8ux, printed across its face in
large letters, and other information was pen-
written across it.

Within a year the call-card was a fad and
every conversation wound up with " Pse send
card," or simply "QSLL". Shortly after this the
owners of active stations discovered that the cost
of the cards, and the time to write them, would
shortly compel abandonment of radio activity —
and as a result they sent fewer of them. Then
ensued a violent argument which has raged these
three years with no conclusion arrived at. The
amateur across the Atlantic especially is not at
all pleased with the remarkable lack of considera-
tion shown by his American neighbor who fails
to reply to all of the apalling flood of cards and
letters from European listening posts which
have no transmitters. Meanwhile the varieties of
cards grows, but no more colors are available,
the limit having been reached in Lawrence
Mott's famous Catalina Island series, termina-
ting with an eagle-trimmed card in five colors
and gold. Beyond that even the Southern
California imagination has not gone.

THE EXPERIMENTER

I PROPOSE to give a considerable space to the
' experimenter because he is in the most in-
teresting of radio fields, also because he is least
organized and least catered to. Let us begin by
explaining the seeming contradiction in the
preceding sentence.

The experimenter, whether he uses a micro-
phone or key, must say something with it, and
unless he uses an outright automatically sent
test signal there is no easy way to avoid "rag-
chewing" or "message-handling." This causes
him to be included in one class or the other al-
though he has no primary interest in message
totals or call cards, does not gain any satisfaction
from the activites of either of the groups, and
intends to stop sending as soon as he has worked
out the problem that happens to be under way —
which is likely to be anything under the sun
from an antenna test to an investigation of the
electron distribution in the upper atmosphere
during an eclipse of the moon. In addition to this

To Antenna
To Counterpoise

Ground

Separate
Antenna

FIG. I. ARRANGEMENT OF APPARATUS

i>>lf ibouU be about 29 to )O inches high If much operating is done the leg room should be entirely
<hek-es. batteries or other obstructions. The location <;/ the key is optional, but the transmitter switch
should />,• loni'fiiienl /«' the left l\n:J.

the experimenter works best in small groups
which break up and re-form about various prob-
lems, making it very hard to keep track of their
performances or to give them any of that entirelv
proper publicity which will call in new aid.

By tradition organization has centered about
message-handling, and personnel or cash has
never been placed in back of an attempt to create
a coherent experimenter class or a clearing house
for information. This has been done for other ac-
tivities which, in some cases, have received sup-
port extending as far as the employment of
laboratorians to solve problems by proxy!

All of these things contributed to an unhappy
state wherein the experimenter was an outsider,
compelled to seek out his own co-workers,
handle all his own correspondence without aid
and then, if he had energy left to do any effec-
tive work, to take his reward in personal satisfac-
tion. The experimenter is usually neither a pub-
licity seeker nor a shirker, but even he resents
such an unfair situation. Several years ago, in an
attempt to even matters up, 1 formed the," Ex-
perimenter's Section, A.R.R.L!" This organiza-
tion was as loosely put together as the " Rag
Chewer's Club" and the motives were not en-
tirely different. The "Section" proposed to issue
at intervals lists which would tell all members
what work was being done, and by whom, thus
facilitating inter-member contacts. To this were
to be added occasional outline-suggestions and
the results, which were to be circulated in mimeo-
graph form or made into magazine reports when
of sufficient interest. Although this work never
received more than one fourth of one man's time
it really made surprising headway. From it
developed the Official Wavelength Station
scheme of Don Wallace, the Standard-Frequency
transmissions of IXM and gxL-wcco under
Lansingh and McCartney, some really worth-
while information on transmission r.f. chokes,
a variety of circuit improvements, the fine
General Electric and amateur "April tests."
much article-material and a considerable contri-
bution toward a changed attitude of the amateur
as to experimental work.

IS IT WORTH WHILE?

ON 1C may question with perfect justice the
value of amateur experimental work, since
the professionally equipped laboratory seems so
much better able to cope with questions than
the amateur.

I his may be answered either by logic or from
the record. First of all. the logic may be taken
from(l think) Josh Billings who said, " It's better
not to know so much than to know so much that
'aint so." That is the handicap of the trained
man seeking new trails — he is too sure of many
facts that are not facts, too certain that a whole
variety of things cannot be done, too inclined
to reason out his course. Furthermore, he can-
not escape this tendency, for he is always under
the eye of his associates who feel likewise, and
usually under the surveillance of an impatient
production department which does not want
e\erything tried and but one thing finished

The amateur is not so; he is not required to be
logical, or to know any theory from which the
result can be predicted. Therefore, he blather-,
around cheerfully with just the faintest contact
with established knowledge, and often he falls
n\er the most amazing and fundamental dis-
coveries. Later the engineer and the physicist
and the mathematician 'will refine and make UM--
ful these discoveries. However, it is a fact that a
good share of the fundamental things in radio
have been discovered with the poorest of equip-
ment and in the face of contrary opinion.

This is, of course, not a suggestion that all •
established information is wrong. It is. however.

REMOVING NONSENSE FROM SHORT-WAVE TRANSMISSION

113

an earnest suggestion that the experimenter
always be inquisitive and ever ready to try those
things that seem to be vague or incorrect. What-
ever the conclusions may be which these tinker-
ings and testings give forth, they should be
aired. Progress comes from comparison and
from the matching of results.

It seems that sooner or later there must exist
an agency which may act as a clearing house for
the needs and questions of the experimenter in
a manner somewhat as adequate as the provision
now made for other activities. This will be a
rather difficult matter to frame, for it will be re-
quired to deal with all manner of interests from
the most serious and important to the case of the
misguided boy who is trying earnestly and
persistently to determine whether Hartley or
Armstrong invented the "best sending cir-
cuit."

THE INNER URGE

1'HAT this agency does not exist now will not
matter greatly to a real experimenter; when
the genuine radio experimenter is interested
in a problem time, cash, and correspondence deter
him no more than a violent golf fiend is deterred
by a pneumonia fog. Similarly I also suspect
that such stubborn persistence does produce the
best work. At any rate the genuine radio ex-
perimenter need not read this discussion, nor
will I convert anyone who is not already an
experimenter of some sort. The thing is organic,
like red hair, and if artificially produced is
equally temporary.

THE MESSAGE-HANDLER

AS HAS been suggested above, the handling
of amateur radiograms seems to carry less
opportunity for developing something worth-
while than do other uses of amateur radio.
In the main it serves only to develop operating
skill which can then be diverted to worthy ends.
The messages handled meanwhile are mostly
worthless in the same manner that a copybook is
worthless after one has learned to write.

The applications of that operating skill have
been advertised so widely that it seems almost
useless to mention them. Radio contact with
exploring expeditions, occasional emergency
work when wires are down, application to mili-
tary situations, have all been mentioned many
times. There exists an Army-Amateur scheme of
cooperation as well as a Navy-Amateur scheme,
the activity of each differing materially in vari-
ous territories. The object of these systems is
that in case of war there will exist a partly
trained reserve.

Recent hearings before the Federal Radio
Commission suggest that if no better agreement,
can be made between the various interests who
"must" have radio for emergency contact we
may well consider the possibility of a public-
property or corporation-operated radio emer-
gency net whose operators may keep in practice
by talking to amateurs. A preliminary tryout
of such an idea was instigated some years ago by
i i. L. Bidwell of Washington and operated by A.
L. Budlong for the Pennsylvania railroad. It
gave a very good account of itself and I (who be-
gan as a skeptic) feel that, with somewhat
stronger support, it would have expanded easily
into the scaffolding on which a permanent
system such as suggested could be erected.
Certainly the tryout would have been made
easily with amateur stations — and can to-day.
It would be a most unhappy matter if instead
we were to have a horde of privately owned
stations working for their separate owners, hav-
ing no contact in normal times and hampered by
financial affiliations so as to be unable to work in

Antenna

110V.

~ Gnd

FIG. 2. SWITCHING ARRANGEMENT

Switch l changes the antenna so that various bands may be employed. Switch 2 shorts or opens the an-
tenna loading coil used to operate on the i8o-meter band. Switch ) turns on and off the a.c. power input to
the transmitter. Switch 4 turns on and off the A battery — ;/ used. It must be kept away from Switch ), ami
if a.c. is used in place of the A battery switch 4 is not necessary. Switch 5 is the lightning switch required by

the fire insurance company.

the free-handed manner that a good emergency
net must base upon.

The suggestion just made, is that certain but
ill-defined military value seems to be the main
reason for the message-handling game. Expedi-
tions will not continue indefinitely to depend on
amateur stations, though they will continue to
use amateur operators because they are able to
work under limitations of apparatus. Interna-
tional message handling is involved just now
in a great bog of diverse opinion from which it
may not emerge for a long time, if indeed it
emerges.

TEST TRANSMISSIONS

I CANNOT resist a paragraph regarding test
' transmission. These sendings are typified by
the "test number one, test number two, test
number three" of the man making circuit ad-
justments or by the dreary machine-sent "Test
IXAM" or "Test IOA" that has gone out so many
times during these years of short-wave explora-
tion. Such transmissions are meaningless to
those who have not been informed in advance,
except when supplemented by hand-sent or
spoken information, but this does not in any way
mean that they are out of the way or to be con-
demned. On the contrary, they deserve a hun-
dred times as much respect as the senseless call-
ing of station after station for the purpose of
hastily asking for a card and then jumping to the
next station. Such foolery teaches nothing, does
not provide satisfaction to the stations so curtly
dismissed, and may well step aside for even a
machine-sent "test test test" which at least re-
presents an attempt to learn.

CONCERNING THE STATION

CVERY man is free to choose which of the
*-• foregoing activities he cares to indulge in,
but he will find .that certain conventions relating
to operating and station arrangement apply
in each case. One must comply obviously with
the legal provisions as to manners of calling and
signing to some degree. A certain use of abbre-
viations is also most helpful and in any case is
forced upon one if key operation is employed.
These abbreviations fall into two classes, the
absolutely arbitrary ones, which are matter of
international agreement, and those which are

merely butchered words such as "Xtal" and
"Xmitter" for "crystal" and "transmitter" or
"Wx" for "weather." Picking these up is not as
painful as it sounds. Another class of abbrevia-
tions occasionally met is that sort made by
phonetic spelling or by dropping vowels, as
"sine" for "sign" and "tmrrw" or "Tmw"
for "to-morrow." These too dawn on one soon
enough.

Station arrangements are suggested in the
illustrations herewith and usually a wide depar-
ture from these is not advisable. It is really
surprising how much more one can accomplish
when things are arranged conveniently. Es-
pecially one should avoid placing apparatus in
cabinets or against walls in such a manner that
alteration or inspection requires disconnecting
wires. The transmitter as here shown is without
a case and this is my preference since a dust-
cover made of khaki cloth serves all the useful
purposes of a cabinet. If a case is desired it should
be made so that it may be removed without
disturbing wires. Mere opening of the lid does
not suffice.

Other than these generalities it seems destruc-
tive to give advice. Such matters as the exact
antenna arrangement, the construction of the
loading coil, etc. can be decided best by the
owner, and useful experience is gained at the
same time. If the antenna must be made shorter
than was described, perhaps the counterpoise
may be made longer, or a different combination
altogether worked out to cover the various
wavebands. If a loading coil is necessary no
exact dimensions need be followed. One may
start with the first thing handy — a Dutch
Cleanser box for example — and wind it with
ordinary annunciator wire. A little loop should
be brought out and twisted together every
5 turns until perhaps 40 have been wound.
As soon as the loops are skinned one can
connect to the lower end of the coil, clip the
antenna to one of the loops and, by trying vari-
ous wavelengths and noting the condenser
setting, one can arrive quickly at the correct
number to load to the desired 2o-meter wave-
length. This is quite as effective and much
more educational than a set of ready-made
directions which might not fit the antenna. At
the writer's station the process was timed
and it required 20 minutes.

"Our Readers Suggest-

Readers Suggest — " is a clearing bouse
(or short radio articles. There are many
interesting ideas germane to the science of radio
transmission and reception that can he made clear
in a concise exposition, and it is to these abbrevi-
ated notes thai this department is dedicated. H bile
some of these contributions are from the pens of
professional writers and engineers, we particularly
solicit short manuscripts from the average reader
describing the various "kinks," radio short cuts,
and economies that he necessarily runs across from
time to time. A glance mer this "Our Readers Sug-
gest" will indicate the material that is acceptable.

Photographs are especially desirable and will he
paid for. Material accepted will be paid for on pub-
lication at our usual rates with extra consideration
for particularly meritorious ideas.

— THE EDITOR.

An A. C. Screen-Grid Booster

IN THE August RADIO BROADCAST there ap-
peared the description of "An Extra R.F.
Stage For Any Receiver," employing a d.c.
screen-grid tube. 1 was very much interested in
this arrangement, but as I desired to use it in
conjunction with an a.c. receiver, I thought it
logical to redesign the circuit for the Arcturus
a.c. screen-grid tube. The altered diagram is
shown in Fig. i. The following is a list of the
parts used in the construction:

2 — Frost ux-type sockets;

i — broadcast-range plug-in coil;

i — Variable condenser, 0.0003 5-mfd.;

i — Frost fixed condenser, o.ooo25-mfd.;

2 — Silver-Marshall r.f. choke coils;

3 — Tobe by-pass condensers, i-mfd.:

7 — Eby binding posts;

i — National dial;

i — Front panel, 7" x 9";

Baseboard or sub-panel;

Corwico Braidite for wiring;

Miscellaneous hardware;

i — Power Clarostat;

i — Step-down transformer, ij-volt;

i — Frost strip resistor, 500-ohm;

i — Arcturus screen-grid tube, 22-A.c.-type.

The unit is wired to antenna, ground and re-
ceiver as indicated. The output post is led to the

antenna post on the receiver. An Transformer
Arcturus 22-type tube is plugged
into the socket. The ground is con-
nected both to the receiver and to
the booster.

The dial on the booster functions
as an additional tuning control.

The power Clarostat is placed in
series with the primary of the step-
down transformer and is employed
to regulate the output to fifteen volts
for the Arcturus tube. The voltage is
best determined by means of an a.c.
voltmeter, however, if this instru-
ment is not conveniently available,
the voltage may be adjusted by
noting the time lag between the turn-
ing on of the current and the heating
of the tube to its normal operating
temperature. When the voltage is correctly ad-
justed, the lag will be approximately 30 seconds.
PETER L. JONES, Boston, Mass.

Work Bench Clamp

THE presence of wires indiscriminately strewn
about the test bench is hardly conducive to
efficient work, to say nothing about the possibility
of short circuits, wrong connections and general

I O To Ant.

•-- Post on
Set

FIG.

FIG. 3

lack of order. I have found it decidedly worth
while to equip my test bench with several simple
clamps designed to hold the various wires
to the table.

A simple clamp for this purpose
is shown in the picture, Fig. 3. It
was assembled from odds and ends
out of the junk box.

A small strip of thick wood pro-
vides the base. Near one end a
recess is filed out for the cable.
The latter is held in position by a
narrow strip of ebonite which acts
as the upper jaw.

Slightly off the center, a counter-
sunk hole is drilled in the wood
block for a A-inch screw, and the
ebonite strip is forced down by a
small wing nut. Although a spring
washer is used between nut and
ebonite strip, the latter will have
a tendency to turn when the nut
is tightened. To prevent this, the
other end of the ebonite strip has

114

FIG. 2

been slotted; in this slot lies a stop — a nail
driven into the wood block and snipped off
about |" above the surface of the latter.

C. A. OLDROYD, Lanes, England.

Push- Pull with Standard
Transformers

UOR clear and undistorted output, with abun-
^ dant volume a well-made push-pull amplifier
is the standard of comparison.

The transformers specially made for such cir-
cuits have a center-tapped secondary winding
in the first stage, and a center-tapped primary
winding in the output stage. Yet a standard
audio transformer may be used with excellent
results if an external center-tap is provided, as
indicated in Fig. 2.

Two ioo,ooo-ohm resistors are connected in
series across the secondary. The grid bias is
applied at the central junction of the two resis-
tors. The value of the resistors is not critical,
and slightly higher or lower values may be used if
the experimenter happens to have them at hand.

The only other special part needed for a
push-pull amplifier is the output transformer or
a center-tapped output choke through which the
B current reaches the plates of the power tubes.

A commercial choke may be used in this posi-
tion, but the same results can be secured by
using two standard chokes connected in series.

The inner turns of the two chokes are joined
by a connecting lead, to which the B-plus lead
is wired. Two fixed condensers are connected as
shown in Fig. 2 to keep the loud-speaker wind-
ings at a low potential.

C. A. OLDROYD, Lanes, England.

Home-made Soldering Lug

D UNNING out of soldering lugs in the mid^t
1^- of a radio construction job, 1 twisted the
loose ends of the stranded hook-up wire together
and bent them in the form of a hook. To reinforce
the stiffness of the lug it was tinned thoroughly.
While the solder was still soft, the hook was flat-
tened with pliers, thus providing a better contact
surface.

This lug is no harder to make than it is to
solder the wire to a conventional lug — and it
costs nothing.

EDWARD PIKANIAN, Philadelphia. Pa.

The New A, C. Screen-Grid

Drake Receiver

THE Browning-Drake Kit-Set has enjoyed
a continued popularity since its introduc-
tion a number of years ago, doubtless due
to the efficiency of the radio-frequency trans-
formers, combined with the simplicity of the
circuit in which they were used. During this time
some slight improvements have been made.

With the introduction of the screen-grid tube,
the problem of designing a one-stage tuned
radio-frequency amplifier was attacked again
from an analytical standpoint and an extremely
efficient transformer was developed for this type
of circuit. The problem of obtaining selectivity
and gain in a radio-frequency amplifier employ-
ing a screen-grid tube differs considerably from
that encountered when using a 199- or 201 A-type
tube. This is due to the inherent electrical
characteristics of the tubes themselves. For
instance, the 201 A-type tube has a plate im-
pedance of approximately 12,000 ohms, and an
amplification factor of 8, while the screen-grid
tube has approximately 400,000 ohms of imped-
ance with an amplification factor of from 150 to
300. With the 20 1 A-type maximum gain could be
obtained easily by the proper number of turns
on the primary, together with a normal co-
efficient of coupling which was about 0.5. This,
together with a low-resistance secondary, re-
sulted in a gain of about 12 to 15 per stage.
However, when using this tube careful neutral-
ization, even in a single-stage amplifier, was
necessary to obtain the best results. The primary
purpose in the design of the screen-grid tube
was probably to make the capacity between
plate and grid so small that neutralization was
usually unnecessary. In interposing the screen-
grid between control grid and plate, high amplifi-
cation in the tube itself resulted as well.

PROBLEMS OF DESIGN

THERE are two ways of obtaining radio-
frequency amplification under the new con-
dition imposed by the screen-grid tube, i e., by
an auto-transformer (tuned impedance), or by

By GLENN H. BROWNING

Browning Drake Corporation

the usual tuned radio-frequency transformer,
consisting of a primary and secondary winding.
These have been discussed at some length in a
previous article in this magazine and will not be
dwelt on here. It is sufficient to say that tuned
impedance has the advantage of slightly more
amplification per stage while the transformer
gives greater selectivity. The design of such a
transformer, however, is not a simple matter by
any means. With the increase of plate resistance
that the screen-grid tube has over the 201 A type,
the turns on the primary of the radio-frequency
transformer should be increased a great deal for
maximum gain, or the coefficient of coupling
must be increased, or both. Unfortunately, there
is a very definite limit to the number of turns
which may be used on the primary of the trans-
former. This limit is determined by the dis-
tributed capacity and inductance of the winding
itself, coupled with the capacity placed across it
due to the plate to ground capacity of the tube
used as the radio-frequency amplifier.

These two capacities tune the primary to a
definite wavelength, and if this wavelength is
200 meters or above, the transformer as a whole
will tend to pass a signal coming in on this wave
no matter where the secondary is tuned. In
designing a transformer for the screen-grid, the
high plate resistance means that primary turns
should be increased, but the plate to ground
capacity is increased over the 201 A type of tube
by a factor of three or four times, due to the
proximity of the screen grid to the plate. There-
fore, it is essential to increase the coefficient of
coupling as much as possible.

Some months ago the writer started to de-
termine just how this coefficient of coupling
could be increased from its normal value
of about 0.5 to as great a value as possible (the
maximum theoretical value is i). The result
was that, with a short winding length for the
secondary and a slot wound primary placed in
about J" from the low-potential end of
the secondary, the coefficient of coupling in-

creased to 0.91. Thus, with this coefficient of
coupling and placing as many turns on the
primary of the radio-frequency transformer as
possible, consistent with keeping its natural
period below 200 meters, a transformer for the
screen-grid tube was developed, which has an
extremely good gain. However, it might be
stated that as far as the writer has been able to
determine, it is impossible to get the maximum
theoretical gain at broadcast wavelengths from
the screen-grid tube because of the limitations
imposed on the number of turns on the primary
winding of the radio-frequency transformer.

THE SELECTIVITY

THE selectivity of the transformer under
discussion, as well as the amplification, is
considerably better than in the case of using the
2oiA-type tube as a radio-frequency amplifier.
There are two reasons for the increase in selec-
tivity. First, other factors being equal, the
selectivity is better in a radio-frequency trans-
former when the gain is below maximum.
Second, for a given amount of gain the higher the
coefficient of coupling the greater the selectivity,
provided the resistance of the secondary winding
of the transformer is approximately the same in
both cases. This later statement will probably
not be evident but can be proved mathematically
or can be shown readily in laboratory measure-
ments.

The 1929 Browning-Drake Assembly employs
one stage of tuned radio-frequency amplification
with either a.c. or d.c. screen-grid tubes with
the transformer described. Tickler feed-back
is used in the detector as previously. No neutral-
ization or shielding is necessary for efficient
operation. The antenna system has been changed
to use an untuned primary. This is because the
coefficient of coupling between primary and
secondary has been increased to 0.91 so that a
primary is as effective as direct coupling and aids
somewhat in making the kit absolutely single
control. The o.oooi-mfd. condenser (Ci) is con-

116

RADIO BROADCAST

DECEMBER, 1928

VIEW OF RECEIVER SHOWING ARRANGEMENT OF APPARATUS

nected in series with the primary in the antenna^
circuit so that the primary is never tuned by
the capacity of any antenna length within the
200-550 meter band.

Three stages of resistance-coupled audio
amplification are recommended, although other
types may be used if desired.

It is unnecessary here to give constructional
details for the kit-set as these may be obtained
directly from the Browning-Drake Corporation,
who manufacture the 1929 Browning-Drake
A.C. Shield Grid Kit.

The Kit-Set has been carefully designed by
mathematical and laboratory methods, and the
writer feels that it is the best Browning-Drake
ever presented to the public. Not only does it
outperform all previous models from the stand-
points of selectivity and sensitivity, but its tone
quality is as nearly perfect as can be obtained
at the present stage of the radio art. Coupled
with this is the feature of simplicity of operation,
due to its being absolutely single control.

LIST OF PARTS

A COMPLETE list of the apparatus employed
** in the construction of the A.C. Screen-Grid
Browning-Drake Receiver follows:

AI, A2 One Browning-Drake 1929 kit (a.c.
screen-grid type);

LI, L2 Two radio-frequency transformers;

RI Three Browning-Drake center-tapped re-
sistors;

Ra Three Aerovox or Durham resistors, 0.05-
megohm ;

Ra Two Aerovox or Durham resistors, o. i-
megohm;

Ri One Aerovox or Durham resistor, o.5-megohm;

RI, One Aerovox or Durham resistor, 8-megohm;

Re One Frost volume control and I lo-volt switch;

R? One Yaxley resistor, looo-ohm, type 71,000;

Rs One Aerovox resistor, zooo-ohm, type 992;

Ci One Aerovox or Sangamo moulded fixed con-
denser, o.oooi-mfd.;

G;, Cs, C« Three Aerovox or Sangamo moulded
fixed condensers, o.ooi-mfd.;

C6 One Aerovox or Sangamo moulded fixed con-
denser, o.ooooy-mfd.;

Ce One Aerovox or Parvolt by-pass condenser,
i-mfd.;

CT Three Browning-Drake special amplifier cou-
pling condensers, o.i-mfd.;

C8 One Aerovox moulded condenser, o.j-mfd.;

Two Eby vacuum-tube sockets, uv-type;

Four Eby binding posts (Ant., Gnd., Speaker
and Speaker);

One Browning-Drake Foundation Unit consist-
ing of drilled front and base panels with ampli-
fier sockets and resistor mounts, twisted a.c.
filament wire, connecting cable and all hard-
ware.

The total cost of the parts in the above list is
$59.45.

The additional apparatus required for plac-
ing the receiver in operation follows:

One screen-grid tube, a.c. -type;
One detector tube, UY227-type;
Two amplifier tubes, ux226-type;
One power tube, lyiA-type;
Filament transformer, B-power unit, aerial,
ground, loud speaker, etc.

FIG. I. COMPLETE SCHEMATIC DIAGRAM

Book Reviews

"A TREATISE ON 25 TESTING UNITS FOR SERV-
ICE MEN." Published by Radio Treatise
Company, New York City, 26 pages.
Price — $1.00.

TH E title of this book defines clearly and defi-
nitely its scope and purpose. In the descrip-
tion of the various testing units the author has also
given considerable general data on how to test
receivers. The service man, endeavoring to serv-
ice a radio receiver by simply following instruc-
tions, has before him a task comparable to that
which Diogenes had. To successfully service a
set one needs, besides the mechanical tools of the
trade, a clear understanding of how receivers and
set-testing devices work. The educated service
man will not hesitate to try this strange food —
for an understanding of why the wheels go round
is strange to many service men.

I he testing devices described in this book are
many and include the following:

Tube Reactivator and Filament-Emission
Tester.

Vacuum-Tube Bridge, by which one can
measure the amplification constant and plate
impedance of a tube.

lube Tester, for measuring the electrical
characteristics of all types of a.c. and d.c. tubes.

Voltage Tester, for measuring the A, B, and C
potentials in any modern receiver.

Oscillators, both audio- and radio-frequency,
to be used as local sources of signals in servicing
sets or in making laboratory tests.

Laboratory Oscillators, producing frequencies
throughout the audio range and up to about 300
kilocycles. This is useful in measuring audio-
frequency apparatus intermediate-frequency
transformers, etc.

Indicating Devices, which include descriptions
of several vacuum-tube voltmeters.

On page 17 is described a vacuum-tube volt-
meterof thedesign generally known as the "slide-

back" type, in which the unknown voltage is
balanced against the C-battery voltage. It is
not generally the case, however, that the un-
known voltage is equal to the change in grid
voltage, although the author slates that such is
the case. When very large voltages are being
measured the slide-back method can be used
with quite a small error, but with small voltages,
such as one frequently must deal with in am-
plifiers, the slide-back method, in the reviewer's
opinion, is quite inaccurate. However, the service
man generally is interested in qualitative rather
than quantitative values and in such cases this
type of vacuum-tube voltmeter probably can
be utilized satisfactorily.

As we indicated previously, the appeal of this
book is confined generally to the dealer or serv-
ice man, although many set builders should
find the constructional data on laboratory instru-
ments very helpful

— H. K. K.

An A. C. Band-Pass Screen-Qrid* Receiver^

The Master "Hi*Q 291

By WILLIAM E. BARTON

llammarhtnd'Roberts, Inc,

FR as many moons as the
jldest radio editor can remem-
oer, writers and engineers
have deplored the fact that one
cannot have both selectivity and
fidelity — and yet nothing much has
been done about it. On the one
hand, we have receivers which may
get alt the audio notes in proper
proportion, but which, so far as
selectivity is concerned, are as broad
as the proverbial barn door. On
the other hand, we have receivers
which, to use an advertising phrase,
are as sharp as a knife blade, but
which — and advertising writers say
nothing about this fact — get few
notes above 3000 cycles. And there
you are. You may pick and choose,
but you can't have your loaf and
eat it.

A radio receiver first of all must
select the program you want to
listen to, and then must amplify
the audio tones to the level desired,
whether it be for head-phone re-
ception or full loud-speaker volume,
usually the latter. But if, in the
process of selecting, half of the
audio tones you want to hear are
lost, no amount of audio amplifica-
tion will bring them back again in
their proper proportion. There will
be plenty of "lows" to be sure, lots
of the bass drum — unharmonious
instrument — but few of the human-
like notes of the violin.

And so in a congested district
where broadcasting stations are
placed far enough apart not to bother each
other, but close enough together to pre-
vent any "getting out," people built receiv-
ers which were not very selective, and so the
quality was good. Fans outside the large
cities, however, had a different problem. Sur-
rounded by stations, all over 100 miles away, the
receiver had to have selectivity enough to cut
out a geographically near-by station which
might be poor in quality in order to receive a
good station only 10 kc. away from the nearer
station. No wonder side bands were clipped.
No wonder few notes above 3000 cycles were
received.

Up until about a year ago such a dilemma as
this existed in every constructor's mind — should
he build a selective receiver, one that would get
out, or should he be satisfied with local broad-
casting and build a broadly tuned receiver? In
most cases a compromise was difficult to effect.
Then, at a meeting of the Institute of Radio
Engineers, Dr. F. K. Vreeland gave his paper
of band-pass tuning which promised not only
more selectivity but greater fidelity of response
as well. This started many engineers thinking
and remembering their text books and wonder-
ing why they hadn't thought of Dr. Vreeland's
scheme themselves. For the truth must be
told, Dr. Vreeland called to mind the old system
of tuning two circuits to the same frequency and
then coupling them closely enough together that

THE Hi-Si IN A COMB/NATION
PHONOGRAPH-RADIO CABINET

THE theoretical background of the 1929
model of the Hi-Q receiver, as outlined
in October RADIO BROADCAST, is amplified
in this article from the. Hi-Q organization.
In the Laboratory, as tested under average
conditions, the receiver seemed to have con-
siderable r.f. amplification, tie selectivity
was good, and the fidelity of response was ex-
cellent. The completeness with which the r.f.
circuits are filtered probably has much to do
with the stability, and simplicity of opera-
tion. There are no trick adjustments.

There is one interesting point which is not
mentioned in this article, and about which we
hope to present data soon. This is the fact
that the shape of the response curve depends
upon the frequency — that is, the curve will be
one thing at 500 kc, something else at 1500 kc.
— when the circuits are coupled by induct-
ance, capacity, or mutual inductance. Just
what this effect is, in the present receiver, was
not apparent in the Laboratory. At the top
and bottom of the broadcast-frequency
spectrum good response was obtained. Per-
haps the receiver bad been adjusted somewhere
in the middle of the band so that at the two
ends it still had a band-pass circuit cbaracter-
estic. — THE EDITOR.

117

the response curve no longer looked
like a steep mountain, but like twin
peaks side by side. It no longer
had gently sloping sides down at
the interference-frequency region,
but a sharp cut-off.

And what good is such a curve,
you may ask? Why have a broad
top and steep sides. The answer is
the reply to the prayers of engineers
and those who want more selectivity
and more fidelity of response. At
the top of the curve — where the
audio tones are — a broad flat plateau
exists, at the sides of the curve —
where interfering stations are —
there is a steep precipitous drop in
response.

BAND-PASS FILTER CURVES

LET us look at Fig. I which rep-
resents engineering data on the
Hammarlund Roberts "Hi-Q 29," a
receiver making use of the time-
honored method of obtaining the
flat-topped response curve described
above. In the case of the single coil
and condenser tuned to 1000 kilo-
cycles, the top of the curve is peaked
markedly, and if, as is usual, a
little regeneration creeps into the
amplifier, this peak becomes even
more marked. The dotted line rep-
resents the response or resonance
curve of a circuit with a resistance
of about 5 ohms at 1000 kc. a low-
loss circuit. At 2000 cycles the curve
is beginning to droop and at 5000
cycles the response has been reduced
to only 60 per cent, of the response at, say, 100
cycles. Now look at the full-line curve which
represents the band-pass tuning arrangement.
This is laboratory data on a single r.f. stage of
the "Hi-Q 29" receiver. At 2000 cycles the curve
has not even begun to drop, and at 5000 cycles
the loss is only 10 per cent.

At the bottom of the curves in Fig. i other
interesting things may be noted. The dotted
curve shows a response at 20 kc. off resonance
of 20 per cent. In the case of the Hi-Q stage,
however, the loss is 90 per cent, which, in a two-
stage affair, where the loss is squared, gives a
response of i per cent, instead of 4 per cent, for
the simpler circuit.

So far so good, but how is it possible for a re-
ceiver to be selective and still have good fidelity
of response? Fig. 2 is the diagram of a single
transformer-coupled stage of r.f. amplification
which has the proper electrical characteristics to
give a curve like that of Fig. I (dotted-line curve).
The less the resistance in this circuit, the greater
the amplification, and the greater the loss to
the high audio tones. Now let us contrast this
circuit with the more complicated one in Fig. 3,
which is the arrangement used in the Master
"Hi-Q 29" receiver. Here, again, we have a
transformer-coupled stage of r.f. amplification,
'but both the primary and secondary windings are
tuned — and they are tuned to the same fre-
quency. In fact the primary and secondary coils

118

FIG. 2

FIG. 3

and condensers are identical. The lower the
resistance of the coils, the greater the amplifica-
tion, but the side-band clipping depends upon
the coupling between the coils which can be ad-
justed mechanically.

Theory states, and if you care to look it up
you will find it in Pierce's Electrical Oscillations
and Electrical Waves, pages 73 to 85, or More-
croft's Principles of Radio Communication,
pages 119 to 136, that when two such circuits
are tuned to the same frequency, and coupled
electrically to a sufficient degree, the circuit no
longer responds to the frequency to which the
individual circuits were tuned but to two new
frequencies which are displaced from the single-
circuit frequency a certain amount, depending
upon the coupling. In Fig. 4 may be seen two
extremes of coupling. In one case (A) the
coupling is very loose, so that a single sharp
peak shows up, and with this adjustment little
energy is transfered from primary to secondary.
In the other case (B) the coupling is too close.
Two peaks arise with a sharp dip between.
Somewhere between these two degrees of coup-
ling is found the type of curve we want, broad
at the top and steep at the sides.

COUPLING ADJUSTMENT

THE coupling, then, is the important thing,
and fortunately it can be controlled and ad-
justed until the desired type of response curve
results.

So much for the theory of the Master "Hi-Q
29" receiver, It employs a band-pass filter type
of r.f. amplifier using screen-grid tubes (a.c. or
d.c.) which selects and amplifies but which clips
side bands far less than the conventional low-
loss coil transformer of the type used in nearly
all present receivers. The detector of the re-
ceiver is conventional — a grid leak and con-
denser type. The audio amplifier is composed of
two stages coupled with high-grade audio trans-
formers. The last tube is a 171 type, and, as may
be seen in the picture, sufficient room is allowed
on the chassis for mounting an output device,
or if desired a push-pull amplifier.

RADIO BROADCAST

This chassis is made of steel,
plated with cadmium which
prevents rust, and has almost
the same dimensions as last
year's model. The individua!
stage coils and condenser are
housed in aluminum boxes. The
tubes themselves are between
the shields and shielded leads
make the connections from the
control grid to the apparatus
within the boxes The vol-
ume control governs the volt-
age on the screen grids. The antenna stage
is tuned separately from the other circuits,

20 15 10 5-0+5 10 15 20 25 30
KC OFF RESONANCE

FIG. I

and an additional tapped arrangement on the
coil permits some range of selectivity con-
trol in the antenna stage. Careful shielding is
necessary in this circuit where the primary
windings of the radio-frequency
transformers are tuned. Any feed-
back coupling would introduce seri-
ous difficulties. For this reason the
stage shields used are tight fitting,
and the wires which connect coils to
the tubes are enclosed in screening
which is supplied as part of the
Hi-Q kit.

So much for the electrical and

mechanical properties of the new re-
ceiver. Complete data on how to
build, adjust, and operate the set are
obtainable from RADIO BROADCAST or
from the Hammarlund-Roberts organization. The
list of parts on the next page is the list specified
by the manufacturer. The coils and mountings
are special, and difficulty would be had in at-

DECEMBER, 1928

tempting their construction. Their essential di-
mensions are given in the complete circuit dia-
gram, however, so that if the constructor desires
he may try his hand at it.

What does the receiver do on the air? Just as
an automobile manufacturer sends his product
out over the road to see how long it will run with-
out falling apart, how fast it can go, or other tests
which the user probably never will desire to
make himself, so must the radio set manu-
facturer make his receiver go through a "road
test." The following is a report of such a test
made for the Hi-Q organization.

In a small town on Long Island, about 8 miles
from WEAF, 30 miles from wjz and 10 miles from
WABC we set up the model using Arcturus a.c.
screen-grid tubes in the r.f. circuits, Arcturus
type 46 and 48-tubes in the detector and first
a.f. circuits, and a Cunningham type 371 A
tube in the power a.f. stage. The antenna was
used ordinarily for a 4O-meter amateur trans-
mitting station and was about 6o-feet long with
the lead brought in from the middle. The two
ends were about 45 feet above ground, and the
set was operated in a second-story room. The set
was not grounded — this was accidental, not in-
tentional— and the selectivity and sensitivity
might have been improved if proper grounding
had been looked after. The night was October
13, the beginning of the winter season of heter-
odyne notes and ether jamming.

The stations whose calls were identified defin-
itely came in with a more than ample volume for
a large Peerless dynamic speaker in a three-
foot baffleboard. WLS and woo in Chicago were
very strong. WIP in Philadelphia was very loud —
ordinarily he is difficult to hear on Long Island —
and the old stand-by's, KDKA and WGY were roar-
ing in. WHAM at Rochester was easy to get,
so were WBZ, WTAM, and one station between
WEAF and wjz was separated easily from these

FIG. 4

REAR VIEW OF HI-Q AND POWER UNIT

two near-by stations. It was probably WRC in
Washington. All in all, the writer had an enjoy-
able evening and predicts much fun for the owner
of such a receiver.

OTHER MODELS

THERE are two models of- the Master Hi-Q
receiver. One is designed for d.c. tubes, and
thecircuitdiagram for it was published in October
RADIO BROADCAST, page 343. The other is for
a.c. tubes and the diagram is published herewith.
This receiver uses Arcturus screen-grid tubes
which require a ij-volt filament supply. The
fhordarson power-supply equipment illustrated
in Fig. 5 supplies this voltage as well as the other
filament plate and grid voltages for the operation
of the entire receiver.

The foundation unit supplied by the Hi-Q
organization includes the resistors R«, Rr, and
Rs which are the center-tapped 5O-ohm units for
the first tube. And small fixed resistors used to
filter the screen-grid circuits.

The picture of the receiver which appears on
the next page shows several interesting features of
its mechanical construction. One of the sides of
the middle shit-Id has been removed to show how

DECEMBER, 1928

the two coils of the transformers are located,
one above the other and on opposite sides of the
insulating strip on which they are mounted.
The position of the coils is fixed in the factory
so that the desired coupling is attained. The
double-condenser system, one condenser for
each coil, is easily discernible, as well as the fact
that all of the condensers, except the antenna
tuning capacity, are controlled by a single dial.
The antenna stage is, of course, in a separate
shielded compartment, and consists of a high-
gain, low-loss coil with taps on it so that an-
tennas of various lengths can be accommodated.
In operation the proper antenna tap should be
found by trial and then the top of the compart-
ment screwed down tightly, thereby reducing
unwanted coupling to near-by fields to a mini-
mum, and effectively sharpening the tuning of
this stage when operated in the vicinity of
several local stations.

The list of apparatus employed for the con-
st ruction of the Master Hi-Q a.c. receiver follows:

Ai, As, As One Hammarlund Hi-Q 29 coil set;
Ci to C6 Five Hammarlund midline condensers,

o.ooo35-mfd., type ML-iy;
C« One Sangamo fixed mica condenser, 0.00025-

mfd.;
C7 One Sangamo fixed mica condenser, o.ooi-

mfd.;
Cg to CM Six Parvolt by-pass condensers, 0.5-

mfd., series 200;
CnOne Parvolt by-pass condenser, 4-mfd., series

200;
Li, Li, La Three Hammarlund r.f. choke coils,

type RFC-8s;

Lt One Thordarson choke coil, type 196;
Ri One Carter "Hi-Pot" Potentiometer, 100,000-

ohm, Type 1 1 ;
Rj One Durham metallized grid resistor, 1.5-

megohm;
Ra, R4 Two Durham metallized resistors, 0.25-

megohm;
1 1. Tj Two Thordarson a.f. transformers, type

R-3Oo;

THE MASTER HI-Q 29

119

VIEW OF HI-Q WITH SHIELD COVERS REMOVED

Y One Yaxley cable and connector plug, 12-
wire;

Two Hammarlund knob-controlled drum dials,
typeSDW (Walnut);

Five Benjamin Cle-Ra-Tone sockets, type 9040;

Two Eby binding posts, engraved;

One Hi-Q Foundation Unit containing one drilled
and engraved Westinghouse Micarta panel,
three complete aluminum shields, one drilled
steel chassis, shafts, one binding-post strip,
Fahnestock clips, fixed resistor units Re, R?
and R8, resistor mounts, brackets, clips,
wire, screws, nuts, washers, and all special
hardware required to complete the receiver.

The total cost of the parts required for the
construction of the receiver is $109. 45.

The following is a list of the parts used in the
construction of a power unit for the Master
Hi-Q a.c. receiver:

series

Cis One Parvolt by-pass condenser block,

3-mfd.;
C« One Parvolt filter condenser, i-mfd.,

200;
Cn One Parvolt filter condenser, 2-mfd.,

200;
Cis One Parvolt filter condenser, 4-mfd.,

200;
Cn One Parvolt filter condenser, 2-mfd., series

400;

P One Thordarson power compact, type R-iyi ;
R6 One Electrad "Truvolt" resistor, Hi-Q-type;
Tj One Thordarson filament transformer, 15-

volt, type T-26io;
S. S. One Pair of Yaxley insulated phone-tip

jacks, type 422.

The total cost of the apparatus employed in
the construction of the power unit for the Hi-Q
is $41.60.

2ndA.F.
171-A

110V60-

Y12 34 56 78 9 10 11 12

— i

— 1

180 i

\ '•

- f \

? f

-*o

p? ""

JRect.

— [ 0000 v

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U u

— ' 11 !

135

/*5^k

0
<=>

JO "^

\s/ — i

- 1 j

i 45

T-±F- n

"c"

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•

c,.

-1.0
-4.5

-40.5

p J

-i

FIG. 5. THE COMPLETE SCHEMATIC DIAGRAM OF THE MASTER HI-Q RECEIVER TO-
GETHER WITH THE CIRCUIT OF A SPECIAL POWER UNIT DESIGNED FOR THE RECEIVER

New High- Voltage Metallic Rectifiers for

By J. GEORGE UZMANN

THE rectifier described in this article
is a new device designed for use
in Majestic, Thordarson R-iyi. or
other similar types of B socket-power
devices. This new unit was made possible
only after many months' research and ex-
periment with metallic (electronic) rec-
tifier systems by Harry Shoemaker, Chief
Engineer of Elkon, Inc., New York City.
While the underlying principles of contact
rectification (into which class falls this
rectifier) perhaps are not new, still the
pitfalls were many, particularly when
dealing with potentials in the order of
350 volts many problems arise that
must be solved.

Any means of high-voltage rectification
must be comparable in performance to
that produced by modern high-vacuum
filament-type rectifiers. This new rectifier,
known as the new Elkon type EBH, is the
first of a series of high-voltage rectifiers.
The pictures show the new Elkon EBH
rectifier in assembled and partially com-
plete forms; it is 5$ inches in height,
has a diameter of i| inches and weighs
approximately 16 ounces. The use of a
standard tube base permits its use
in the same way as any other ordi-
nary type of rectifier — gaseous or thermionic.
Since the unit is nearly all metal and contains no
glass envelope or supporting structure it is obvi-
ous that little or no damage can come to it.
The outer aluminum casing serves essentially as
a heat radiator.

The actual rectifier consists of a large number
of couples made up of cupric sulphids in contact
with an aluminum-magnesium combination.
These coupling elements have all the appearances
of a large number of washers, and are 7s-inch
in diameter. In proper combination they are as-
sembled into four stacks, and then by means of
clamping collars are forced together hydrauli-
cally to a predetermined pressure. The four main
sub-assemblies are then inter-connected so as to
fit electrically the circuit for which the rectifier
is intended; thus the base-plug permits supplying

FIG. 2

How the rectifier compares in
sije Ttith a screcn-prid tube

FIG. I

CT"HE article on this page presents the description
•L of a new higb^roltagc metallic rectifier which
may exert a great influence on the design of power-
supply devices of the future. The data were pre-
pared by J. George Ufmann at the request of
Elkon, Inc. — THE EDITOR.

the high voltage raw a. c. to the coupling units,
and finally taking off the full-wave rectified d.c.
output component.

The process of manufacture, treatment and
aging, around which the device evolves, makes a
story in itself , but space does not permit a lengthy
description. In the complete assembly 240 pair of
"couples" are employed, and, since the maximum
impressed voltage per anode is 350 r. m. s. (700-
volt total), it is evident that the couples are do-
signed for a potential pressure of approximately
3 volts r. m. s. per pair of couples. The operation
of the EBH rectifier is based upon the physical
fact that when relatively high electropositive and
electronegative bodies are brought into proper
contact, and current passed so that an electro-
chemical reaction takes place at their junction,
an asymmetrically conducting film is formed
at the junction which permits the passage of
current in one direction only. These tilnis
can be formed and continuously maintained
when proper electrical and physical conditions
prevail at the junction.

The load characteristic of a typical B-po\\er
unit using the Elkon I-.BH rectifier is given in
Fig. I. It should be noted that the slope is
very uniform from no load to maximum load.
with but a slight curvature at the extreme ends.
For gas tube rectifiers the curve would show the
output "oltage rising abruptly within the no
load area. The output voltage of an EBH recti-

12(1

fier is about 20 volts lower, over the useful
ranges, than that of a gaseous rectifier.

An important feature of these new
metallic rectifiers, according to the manu-
facturer, is that they have a life of
approximately 5000 hours as compared
to 1000 for other types of rectifiers.

The Elkon EBH rectifier may be used
in constructing new power units or may
be used as a replacement rectifier in
existing power units using a gaseous
rectifier. In using it as a replacement
rectifier no circuit changes are required;
simply remove the present gaseous recti
fier and place the EBH rectifier in the
socket. The EBH rectifier cannot be used
to replace rectifiers of the filament type
without making changes in the wiring
of the power unit.

The electrical characteristics of the
Elkon EBH rectifier follow:

Use: In full-wave rectifier circuits of
B-power units.

Base: Standard ux Base. The anodes
are connected to the two filament prongs
and the cathode to the plate prong.

.Maximum permissible a.c. plate voltage
per anode: 350 volts r. m. s.
Maximumd.c. output current: 125 milliamperes.
Maximum overall height: 5! inches.
Maximum overall diameter: \\ inches.
General: This rectifier is designed for use in
the construction of B-power units to supply
sets requiring plate voltages not in excess of
about 180 volts. The prongs on the tube's base
are so wired that this rectifier may be used as a
replacement rectifier in all types of B-power units
originally designed to use a gaseous-type rectifier.
The Elkon Rectifier type E8o has similar charac-
teristics to the type EBH except that the base
is arranged to replace 28o-type filament recti-
fiers in power units.

160.

I !(,. 3

i/'i;a.i mechanical unislruc-
<f the new metallic rectifier

DECEMBER, 1928

RADIO BROADCAST

121

No. 13.

December, 1928.

Radio Broadcast's Service Data Sheets
The A. C-66 Dayton Receiver

THIS data sheet is devoted to a
A discussion of a receiver that util-
izes the screen-grid tube as an r. f.
amplifier. There are three r. f. stages
in the set, the a. c. screen-grid tube
being used in the first stage and
226-type tubes in the second and
third r. f. stages. The detector cir-
cuit uses a 227-type tube, the first
audio stage uses a 226-type tube and
the power stage employs a 250-type
tube. This receiver can be considered
unique in that it is one of exceed-
ingly few manufactured receivers
that utilizes the screen-grid tube in
the r. f. circuit and a 250-type tube
in the output circuit.

TECHNICAL DISCUSSION

1. Tuning System

The four tuning condensers are
ganged to a single control. Across the first tuning
condenser is placed a midget variable condenser so
that this circuit may be tuned to exact resonance. An
antenna of ordinary length is connected directly
to antenna terminal No. 1 ; a long antenna is con-
nected to terminal No. 2 so that the signals are
compelled to pass through a small fixed condenser,
Ci, which has a capacity of 0.00025 mfd. No neutral-
izing or stabilizing devices are necessary in conjunc-
tion with the 222-type tube. To suppress oscillations
in the 226 circuits 900-ohm fixed resistors are con-
nected in series with the grid of these two tubes.

2. Detector and Audio System

In the grid circuit of the detector is placed a 2-
megohm grid leak and a 0.00025-mfd. grid con-
denser. The detector is a 227-type tube and the
output of the detector is bypassed to ground with a
0.001-mfd. fixed condenser, G>. The audio amplifier
contains two transformer-coupled stages. A 0.0025-
mfd . fixed condenser is connected across the
secondary of the first audio transformer to improve
the high-frequency response. The 250-type output
tube feeds into a choke-condenser combination
located in the power unit.

3. Volume Control

The volume control in this receiver consists of a
100,000-ohm resistor, Ri, connected across the
secondary of the first tuned circuit. By adjusting
this control it is possible to regulate the amount qi
energy fed into the r. f. amplifier. In this way the
possibility of overloading in any of the tube cir-
cuits is prevented.

4. Filament Circitits

Filament current for the various tubes in the
receiver is obtained from several windings on the
power-supply transformer located in the power
unit. The 222-type a. c. screen-grid tube obtains its
current from a 2.5-volt winding, and 2.5 volts for
the detector tube filament is obtained from a wind-
ing on a separate filament transformer. The r. f.
tubes and the first audio tubes are supplied from a
1.5- volt winding and the 250-type power tube from
a 7.5-volt winding. The 1.5-volt winding, is shunted
in the receiver by a 20-ohm potentiometer, Ri,
with two 0.5-mfd. by-pass condensers connected
across it, their center point being connected to the
125-volt lead from the power-supply unit. A 65-ohm
center-tapped resistor, Ri, is connected across the
filament circuit of the 250-type power tube. The
20-ohm potentiometer is adjusted at the factory to
the point of minimum hum in the loud speaker.

MODEL A.C.-66

. Plate Circuits

The screen grid of the 222-type tube and the
plate circuit of the detector tube are supplied with
125 volts through a 35,000-ohm fixed resistor, R?,
which serves to reduce the potential to approxi-
mately 45 volts. The plate circuit of the first audio
tube is supplied with 125 volts through a 10,000-
ohm fixed resistor, R<, which serves to reduce the
potential to about 100 volts at the plate of the tube.
The plates of the 226-type r. f. tubes and the plate
of the screen-grid tube are all supplied with 125
volts. The 250-type tube is supplied with 350 volts
from the power unit.

6. Grid Circuit

The grid bias on the grid of the screen-grid tube
is 1.4 volts, obtained by connecting a 900-ohm
resistor, Rs, in series with the plate circuit of this
tube and then utilizing the drop in voltage across it
for grid bias. The 900-ohm resistor, Re, supplies 9
volts of grid bias to the 226-type r. f. tubes and the
first audio tube. There is no bias on the detector
tube. A 1500-ohm C-bias resistor in the power unit
supplies 63 volts to the grid of the 250-type power
tube.

7. The Power Supply

The power supply, not shown in the circuit dia-
gram below, is placed in the cabinet with the radio
receiver. The power unit is of conventional design,
supplying all the d. c. and a. c. voltages required
for the operation of the set. A 281 -type tube is used
as the rectifier.

The following data was supplied by Mr. R. S.
Copp, Chief Engineer of the A-C Dayton Com-
pany:

"The a. c. screen-grid tube is rather new to the
public and has only been available to manufactur-
ers a comparatively short time, and, therefore,
there is not a great deal of data available as yet.
Our Engineering Department has been giving quite
a bit of time in the laboratory on this new tube and
we have found out a few things which might be
of interest to the readers of RADIO BROADCAST.

"The a. c. screen-grid tube receiver, known as
Model AC-66, uses one of these tubes in the first
radio- frequency circuit only. It is placed in the
first r. f. circuit in order to gain sensitivity, es-
pecially on inefficient antennas. This tube is then
followed by two tuned stages of radio- frequency
amplification, using the 226-type a. c. tube. The
227-type tube is used as detector, the 226-type tube
as first audio and then for the last stage, we are

using the new 250-type super-power
amplifier in order to give the best of
tone quality with the increased vol-
ume obtained.

" We are using r. f. transformer
coupling on the screen-grid tube with
a ratio of one to three. This system
is employed in preference to impe-
dance coupling in order to obtain a
good degree of selectivity, and yet
not destroy the sensitivity which
this tube has.

" Inasmuch as this new tube is for
a. c. operation we obtain ourC bias
through a 900-ohm resistor in the
plate-supply lead which is in series
with the cathode and ground. This
gives approximately 1.4- volt bias on
the grid of the tube, with 125 volts
of plate potential with 45 volts ap-
plied to the screen-grid element.

"The heater element of the a. c.
screen-grid tube is the same as the one used in the
227-type tube, and the same methods are used as in
the 22 1 heater circuit. The center tap of the heater
circuit is grounded and is not connected to cathode
as shown in some circuits. By grounding center-tap
connection of heater winding the heater becomes
1.4 volts negative with regard to cathode which is
necessary in order to obtain maximum efficiency.

"Our experiments on screen-grid tubes have
shown us that this tube is very efficient. In fact,
so much so in some cases as to prohibit the use of
more than one of the tubes in a tuned r. f. set, with-
out decreasing selectivity to an undesirable de-
gree. Then again if several of these tubes are used
and coupled properly to obtain the highest order of
amplification, the sensitivity becomes so great as
to increase background noises which in the end does
not give satisfactory results, unless the volume
is reduced and then the efficiency is back again to
that of one tube, in the first r. f., followed by
regular tubes as r. f. amplifiers.

"The shielding of a screen-grid amplifier is very
important, otherwise, inter-coupling of circuits
will develop and cause uncontrollable oscillation.
Where only one screen-grid tube is used, it is not
necessary to employ extreme shielding, as is the
case where two or more are used in a receiver.

" With our form of construction and circuit de-
sign, we find a gain of approximately twenty in the
first stage as compared to approximately eight
in the second and third stages of the r. f. circuit,
therefore, our gain up to the detector tube is in the
order of about 1300, whereas the gain on a straight
226-type set-up of three stages is about 500. This
we believe is a sufficient increase over a 226-type
set-up, presenting a compromise of amplification
and selectivity.

" If three stages of screen-grid amplification were
used, a gain or twenty per stage could be main-
tained; the result would be 8000. However, this
gain is entirely theoretical and would be decreased
by several thousand in actual practice. Grant-
ing that we could count on a gain of 6000, what
would our chances be in using it all? Atmospheric
static and electrical disturbances in the average
community are such to-day that this enormous
amplification would cause a noise level in the
volume of a roar and in order to reduce this roar,
the volume control would have to be retarded
greatly, so we would be only bringing up something
we could not use and there is no object in this."

One Dial Control

2 meg

Speaker

THE RECEIVER CIRCUlf

122

RADIO BROADCAST

DECEMBER, 1928

No. 14.

December, 1928.

Radio Broadcast's Service Data Sheets

The Fada 50, 70, 77, 72 A. C. Electric Radio Receivers

THE Fada receivers, models 50, 70, 71 and 72,
all use the same chassis so that in operation
and in technical characteristics they are similar.
The model 50 is a table model, the 70 is housed
in a low-boy console, the 71 in a high-boy console,
and the 72 is contained in a console which also con-
tains a phonograph turntable and pick-up unit.
"~he receivers are of the single-control type, the
station finder being calibrated in wavelengths
and also in degrees. The sets may be operated on
either a loop or antenna. The models 70, 71, and 72
are equipped with a new Fada model-14 dynamic
speaker.

TECHNICAL DISCUSSION

1. Tuning System

The tuning system used in these receivers com-
prises four r.f. transformers and tuning condensers
and include of LiCi, LjCj, LaCj and L.Ci. The four
tuning condensers are ganged to one control and
are operated by a single drum dial. Between the
first and second r.f. tubes is placed a special un-
tuned radio-frequency transformer designed with
a gain-frequency characteristic essentially opposite
to that of an ordinary tuned radio-frequency trans-
former; the result of using such a transformer is
that the gain-frequency characteristic of the entire
receiver is made much more uniform than it would
otherwise be. All of the stages are neutralized by the
Hazeltine method. The neutralizing condensers are
Ci, CB, C? and Cs. Each radio-frequency trans-
former is enclosed in a shield in order to prevent
interstage coupling. The tuning condensers, Cj,
Ca and Cj, are shunted by small midget condensers
which are adjusted at the factory to bring each
stage into exact resonance.

2. Detector and Audio System

A grid-leak-condenser-type detector is used,
C» being the grid condenser and Ri the grid leak.
The output of the detector is bypassed to ground by
the fixed condenser Cio, and, in order to keep the
radio-frequency currents out of the audio system,
the r.f. choke coil, Ls, is placed in the detector plate
lead. The audio amplifier is a two-stage transformer-
coupled affair with a phonograph jack placed across
the primary terminals of the first audio transformer.
An interesting point about the audio amplifier is
that the d.c. plate current of the first audio tube is
kept out of the primary of the input push-pull trans-
former, T!, by means of the fixed condenser C» and
the audio-frequency choke coil,
L«. The removal of the direct
current from the transformer
winding eliminates the possi-

Loop

MODEL 72

bility of saturation in the core which would lower
the inductance and cause a loss in amplification
at low frequencies.

3. Volume Control

The volume control, Rs, is a variable high resistor
connected across the secondary of the untuned
radio-frequency transformer. In this position it
serves to control the amount of energy fed into the
remainder of the r.f. amplifier and detector system.

4. Filament Circuits

Since 227-type tubes are used in all the sockets
of this receiver with the exception of the power
stage only two filament windings are necessary on
the power transformer. One of these windings sup-
plies approximately 2.5 volts to the heaters of all
the 227-type tubes and the other winding supplies
current to the power tubes in the push-pull ampli-
fier. To prevent the r.f. currents in the amplifier
circuits from circulating around the various cir-

Detector Tube
C-327 or
-227

cuits, choke coils, LT, Lt, Ln. Lio, are placed in the
cathode leads to each of the four r.f. tubes.

5. Plate Circuits

Filters are used in the plate circuits of all the r.f.
tubes so that the r.f. currents are compelled to re-
turn directly to the cathodes of the 227-type tubes.
If the filter systems were not used these currents
would pass into the power system where common
coupling would result. The filter systems in the r.f.
plate circuits consist in each case of Ri, and Cn.
The filter system in the plate circuit of the detector
tube consists simply of Cia.

6. Grid Circuits

C bias for the various grid circuits is obtained by
connecting fixed resistors in series with the cathode
leads in the case of the 227-type tubes and in series
with the center-tap connection of the filament-
transformer winding in the case of the power tube.
For bias on all the r.f. tubes a common resistor, R«,
is used. It is bypassed by Cis. Bias for the first audio
tube is obtained from Rs and bias for the power
tubes is obtained from the resistor, Re, located
in the power-supply device.

7. The Power Supply

Two power-supply units are available for use
with this series of receivers. The type E-420 is for
use with 210-type power tubes and the type E-1KO
is for use with 17lA-type power tubes. The latter
power unit is illustrated in the circuit below. This
power unit consists essentially of the power trans-
former, T«, which supplies plate and filament vol-
tages for operation of a 280-type full-wave rectifier
tube and filament voltages for the receiver. The out-
put of the full-wave rectifier leads into the filter
system, consisting of the two filter choke coils,
Ln and Li2, and the filter condensers Ci3. The con-
denser, Cn, connects from side of the 110-volt line
to the ground circuit and this condenser serves to
bypass to ground any line noises which might other-
wise be audible in the output of the receiver. Plate
voltage for the power tube is obtained at the junc-
tion between the two filter choke coils, and plate
voltages for all the other tubes in the receiver are
obtained by connecting suitable resistors across
the output of the filter system. The primary of
the power transformer is arranged with two taps,
one for use with line potentials of 110 to 130 volts
and the other for use on line potentials of 90 to 1 10
volts. Power to the entire receiver is controlled by
the switch, SW. The power units E-420 and E-180
are both available in two models so that the re-
ceiver may be operated from power lines with a
frequency of 50-60 cycles or 25-49 cycles.

1st AF. Tube

4th R.F Tube

3rdRF.Tut» C -327 or
C-327or UV-227
UY-227r— — 1

Shielded

THE RECEIVER AND POWER CIRCUITS

Shading indicates areas covered by the
principal power net-works. However.all

power stations within these districts are
not always interconnected.

HOT

rnHC PROBLEM OF

above may be connected i/ necessary

Problems in Synchronizing Television

THE development of a system for keeping
the scanning disc of a television receiver
in exact synchronism with that of the trans-
mitting disc is one of the biggest problems in
radio at the present time. The simplest method
would be to have the disc at the transmitter on
the same shaft with the similar disc at the re-
ceiver. For a review of the principles of television,
this simplified arrangement will be considered.

In diagram A of Fig. i the subject, A,
at the left is being viewed by the elec-
trical eye, C, through the holes in the
rotating scanning disc, E. The varying
electrical impulses in C, produced by dif-
ferent intensities of light reflected from
light and dark parts of the face of A, are
amplified by a common vacuum-tube am-
plifier and cause the brilliancy of the flat
plate in the neon tube, D, to vary in step.
An observer, B, looks through spirally
arranged holes in disc F at this plate and,
although he actually sees it through only
one hole at a time, the rapid rotation of the
disc gives him the impression that image,
A, is on the flat plate in D. What B sees
is shown in diagram B of Fig. i, the dotted
line representing the margins of the scene.
Each hole in the disc passes across the
scene and traces the light-intensity varia-
tions on the plate below the path of the
previous hole until the last hole in the
spiral comes around when the process
is repeated.

By BOYD PHELPS

It will be evident what a mess would be made
of the picture if the receiving disc should slip
around on the shaft only half a picture width,
or if it was half a turn out of phase with the
transmitter disc. But such a difficulty would be
tame as compared to cutting the shaft between
the two discs and running the discs with separate
motors at different speeds ! A rain or black and
white dots or streaks with no trace of a picture

SYNCHRONIZATION is one of tbe major problems
J in the accomplishment of television. Although in ex-
perimental television, manual control may be used satis-
factorily, it is absolutely essential, if any' sort of practical
television is to be achieved finally, that some method of
synchronisation be evolved that will hold tbe received image
exactly stationary on the screen of the television receiver.
At one time or another many of us probably have thought
over the possibilities of synchronising the television re-
ceiver with tbe transmitter by tbe use of synchronous motors
operated from the power lines of tbe local power company.
A discussion of this method of synchronisation is the sub-
ject of this article.

The author, Mr. Phelps, after many heated arguments
with radio enthusiasts who felt that this method would work
satisfactorily, decided that the onlv way to settle the argu-
ment was actually to get tbe "dope" and tbe ingenious
method which he used to secure tbe data certainly makes
interesting reading.

—THE EDITOR.

would be the result. With these remarks as an
introduction the necessity for precision in the
maintenance of exact speed of the receiving disc
in television may be more apparent.

SYNCHRONOUS MOTORS

COR distance transmission where a shaft con-
necting transmitter and receiver is impracti-
cal, the first method of synchronizing that seems
obvious is to use synchronous motors op-
erating on the same alternating-current
supply. This works fairly well under some
conditions, but we shall speak more of
this later. An ordinary 1760 r. p. m.
squirrel-cage induction motor will run
synchronous at 1800 r.p.m. on 60 cycles
if four slots are cut in the rotor, and
similarly, motors can be made to run
3600, 1200, 900, etc. Fifteen complete
pictures per second means 900 r. p. m.
of the scanning disc and is about the
minimum speed permissible without
flicker. This speed is used at present by
C. F. Jenkins at his station )XK in
Washington, D. C., on 46.72 meters (see
Table n). Twenty pictures per second
means 1200 r. p. m. of the scanning disc
and is the speed used for receiving the we Y
pictures on 380 meters from Schenectadv
N. Y.

A frequency higher than 60 gives a more
accurate lining up of the edges of the pic-
ture, so in the Bell Telephone demonstra-

124

RADIO BROADCAST

DECEMBER, 1928

A variable-speed motor with a highly developed
speed control is used in Ms televisor

tions a 2ooo-cycle current was generated by the
motor turning the transmitter disc. This current
was carried by a pair of wires, or a separate radio-
wave channel, to the receiver where it was ampli-
fied and fed into a 2ooo-cycle synchronous motor.
This motor was aided in driving the receiving disc
by a larger motor adjusted quite closely to the
right speed. Rumors have it that the Baird sys-
tem scrambles a synchronizing signal in with the
picture frequencies and thus needs only a single
pair of wires or wave band for the whole tele-
vision operation. If so, and it is simple, it is a
great step in the right direction.

Then we have many semi-successful speed
regulators for variable-speed motors. Some are
highly developed mechanical governors similar
to the one shown in the picture. It is an interest-
ing diversion to put a slip of paper under the edge
of a phonograph record on the turntable and try
adjusting the speed to, say, 75 revolutions per
minute. After several minutes of timing and re-
adjustment notice if the slip of paper crosses the
starting position at the end of every minute.
Electrical equivalents often include a
resistor thatt may be short circuited
periodically with a button or key ma-
nipulated by the operator who must
pay close attention to the direction
the picture is slipping, bring it back
quickly, and not overdo it. The exact
speed adjustment may be found finally
with any of these devices, but at that
moment the disc may be part of a
turn around from where it should be,
so exact synchronism in every respect
is not easy. It is quite a ways from
throwing a switch and sitting in the
big arm chair for an evening's enter-
tainment as we now do with audible
programs (or until some advertising
program gets so nauseating we tune
in another station)

So far it might appear that a syn-
chronous motor connected to the home-
lighting current is the final answer, as
60 cycles is the standard frequency
supplied about 98 per cent., of the
homes in this country having public
electric service. Indeed, many believe
this is the only satisfactory answer,
and in most any radio store one can
get into heated argument on either
side of the question. The only tan-
gible evidence seems to be that

a brand of clock, known as a "Telechron,"
which has a miniature synchronous motor geared
down to the hands, seem reasonably accurate
when operated on a 6o-cycle circuit. The argu-
ments then proceed along lines of how much
deviation from 60.0000 cycles is permissible for
television, and if the clocks gain or loose a few
seconds per day how many times in so many
minutes will the pictures be null and void or
worse. Also, if the error accumulating during the
24 hours of the day is corrected in the space of
an hour by a worse error in the opposite direction
what will be the effect? The writer, being of an
experimental turn of mind, and thoroughly fed
up on such arguments which get nowhere, de-
cided to find out for himself how
the various so-called ob-cycle cur-
rents in different parts of the coun-
try compare, which data forms the
"meat" of this article. If anyone
else knew the answer, based on
measurements instead of hearsay
or guess, he has certainly kept it a
close secret.

A few oscillographs costing a few
thousand dollars connected to a
few leased long-distance telephone
lines and an army of engineers put-
ting local lighting current on the
end of these lines suggested itself
as the first solution. It never got
further than a suggestion, how-
ever, as every radio amateur has
a reputation to uphold, namely,
being able to get any result desired
from the stuff in his boxes of junk.
(That wasn't the only reason, but
it may get by.)

Now it so happens that when a
neon bulb (costing 5;c) held in the
hand is moved parallel with an an-
tenna lead of an amateur transmit-
ter it lights bright and dim in spots if a poor filter
is used in the transmitter plate-supply system. If
no filter is used at all there are spaces between the
bright spots that are dark, especially if a single
oscillator tube is used in a transmitter operating
on one half of the a.c. cycle. This lamp acts the
same way if connected through a transformer to
the output of several stages of audio-frequency

Table I. Comparisons of Lighting Frequencies

P'requency

59.546
59.615
ttl HK2
(« I**,
58.000
59.333
60.000
59.927
60.084
60.000
<«> im
59.933
60.378
60.000
1,0 X,\
60.308
60.274
60.400
60.323
60.000
60.008
59 98-t
60. 28T,
59.884
60.000
60.000
60.100
60.098
60.200
59.929

Mi:)*:',

60.000
60.096

62.000

amplification after an ordinary short-wave tuner
has been adjusted to similar signals from other
amateur stations. Now it was only necessary to
compare these flashes produced by the distant
station with those caused by the local lighting
current to measure their difference in frequency.
Here at last was found one desirable feature of
the class of transmitter which is most cursed by
broadcast listeners in its immediate vicinity!

The first measurements were made by passing
enough of the local house current through the
neon lamp to light it to about half brilliancy or
about the same intensity as the signal which was
passed through the same lamp. When the incom-
ing signal was in step with the local power the

Observer

•Time

Call

Location

Sec. per

Direction

Rev.