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/T^if DEPARTMENT * 2 2 MAY 19 4 4 


TM 11-455 

Tkh nuttbdf nptrit&j TM 11-455, 1? J%iy 1941 




Uniitd States Gmrnmtnt Priniiiit OJirr 

Wathintton : 1944 

(~* I,, Original from 



Washington 25, D. € rf 22 May 1944, 

TM 11-455, Radio Fundamentals, is published for the information 
and guidance of all concerned. 

[ A,G. 800 ,7 (28 D« 48 } t ] 

By oboes of the Secret art of War; 

4 '" Ck C. MARSHALL, 
. J Chief of Staff. 

Official: m ■ * t ' f 

J. A, ULIO, jf *\ \ 

Major General t i * * x 

The Adjutant Qemral* 

Distribution : 

Aa prescribed in paragraph 9a, FM 31-6, IR 1, 2, 4, 5, 7(3) ; I Bd 1-8. 5-8. 11, 17-19. 

44(b); 10 1-8, 11. 17-10, 44(10). 
IR l: T/O 4 E 1-113, Hq, Bomb Ops, HrorM; I -2529, Hq Ren Op (Spec); 1-TG2S. 

Hq Photo Gpft h Mapping (Spot), 
IR 2: T/O 2-11. C*v Regt, H ; 2 71. C*t Ragt, Mecc, 

IR 4; T/O 4-51, CA Regt (155 mm Gun) <Mbf}; 4 81, CA Rwt, HD. Typo A. 
IK 5: T/O A K 5-511S, Engr Boat and Shore Unit. 
IR 7: T/O 7-81 1 Iiii Fchat RegL 
I Bn I: T/O ft E 1-47, Fight** Control Sq; 1-50-1 S, Hq ft Hq Sq Fighter Comd 

(Sptt); 1-A7. Night Fighter Sq; 1*100-13, Hq ft Hq Sq, Bomber Conid (Spec); 

1-110-1. Hq ft Hq Sq, Bomd Wr; 1-117, Bomb Sq. Hf: 1-127, Bomb Sq, Med; 

l-BiiT Tactical Ren Sq; 1-310-1, Hq A Hq Sq, Tr Oarr Wg; 1-412, Hq ft Hq Sq, 

St Gpe; 1-547, Air Support Conlh Hq; 1 550-1, Hq ft Hq Sq, Air Def Wjj ; 1-637. 

Waa Ren Sq, M; 1-0379, 8th Air way ■ Bq Sp; 1-757, Photo Ren Sq; 1-768, Photo 

Mapping Sq; 1-759, Photo Charting Sq; 1-780-1, Hq ft Hq Sq, Photo Wg; 3-767. 

C Mapping Sq; 1-779, Photo Tech Sq; 1-800-lSRS, Hq ft Hq Sq, APRS; 1- BOO- 10 -ST. 

Hq ft Hq Sq, AF SF; 1-801-1, Hq 4 Hq Sq AF Comp Comd; 1-987. Emergency 

Rescue Sq; T/O 1-137, Bomb Sq, L- 
I Bn 2: T/O ft B 2-28, CaF Ren Sq; Meci Sep Sq Sq in Annd Dif, 
I Bn 3: T/O ft B 3-25, Oml Bel, llti. 
IBdG: T/O ft R 5 15, Bngr Bn. 
1 Bn G: T/O fl-75, FA Oban Bn. 

1 Bn 7: T/O ft E 7-15, Inf Bn; 7-25, Armd InC Bn* 
T Bn B: T/O ft E 8-75, Med Bn, Ami, 
I Bn 11 : T/O ft E U-15, Sig Bn: T/O 11-95 Sig Opn Bo. 
I Bn 17: T/O ft E 17 85S, Tank Bn, Med {Spech 
T Bn 19: T/O ft E 19-55. UP Bn. 
1 Bn 44: T/O ft B 44-135, AAA LS Bn. 
10 1; T/O ft B l-47 t Fighter Control Sq; 1-312. Hq, Tr Carr Op; 1-727, Radar 

Calibration Dot. 
10 2; T/O 210 1, Hq ft Hq Tr, Car Brig { Horae } ; 2-79. Support Tr, CaT Regt 

Waci; T/O ft E 2 22, Hq ft Hq Tr, CaF Gp, Mots; 227, C*F Ren Tr {Mecz). 
T C 3: T/O ft E 3-17. Wpna Co, Oml Mta Kftgt or Cm! Mti Bq, Sep; 8-287, Cml Smoke 

Gen Oo. 
10 4: T/O 4-104, CA Btry, Mine Planter (Cable Ship). 
I C 5: T/O ft E 5-lft, Hq ft Hq Serr Co, Engr Bn; 5-87. En*r J, Pon Co; 5-192 

Ilq ft Hq Co, Enp C Op; T/O 5-227, Pent Co, A B Engr Bn; T/O ft Jfl 5-218, Co, 

Armd Etibt Bn (Bridge) ; 5-500, Engr Surv Orgn. 
I C ft: T/O ft E fl-10-1, Hq ft Hq Btry. DiF Arty, Inf or Mtz DIt ft Hq ft Hq Btry, 

PA Brig; 612, Hq ft Hq Btry, Mta FA Qp; 6-2fi, Hq ft Hq Btry, Mta, FA Bn 

(Trk-Dr or Tr-Dr); 8-29, St Btry, Mte FA Bn. 105 mm How, Trk D ; 8-38, Hq 

ft Hq Btry, Mti FA Bn. 155 mm How or 4,5 Quo Trk-Dr o* Tr-Dr; fl-39. Sf Btry. 

Mt* FA Bn r 155 mm How, or 4.5 Gun Trk-Br. 
10 7: T/G ft B 7-19, Inf AT Co; T/O 7 37, Inf Rifle Co, Pent Bn, 
I C 8: T/O ft E B-27 T M«d Collecting Co Sep; T/O 8 -37 r A/B Med Co, 
I O 11: T/O ft E H-T. Sig Co, Int Hif; 11500, Sig Sf Orgoj U-517S, Big Co 8p«; 

11-537S, Sig Co. Engr Sp Brig; 11-557, A/B Si* Co; 11-587. Sig Baoe Maint Co; 

11 5BS, Hq ft Hq Co, Sig Babb Dop; 11 597, Sig Base Dcp Co; T/O ll-47 T Sic Tr. 

CaF DiF; 11-77, Sig Bad Int Co; 11-97, Big Opn Co; 11107, Slfi Den Co; 11-127, 

H\f Rep Co; 1 1-200-1, Hq, Sig St, A; 11-327; Sig Port Qv Co; 11 547. Si* Ctr T*am, 
I C 17; T/O ft E 17-22, Hq ft Hq Co, Armd Gp; 17-9B3, Sep Tk Co. Hv. 
I C IB; T/O ft B 18-101, Hq ft Hq Oo, TD Go; T/O 18-27, TD Co, 
I C 19: T/O ft E 19-38, Hq ft Hq Det. MP Bn. A. 
I C 44: T/O ft E 44 10a, ITq ft Hq Btry, AAA Brig: 44-12, Hq ft Hq Btry, AAA Op; 

44117, AAA Gun Btry, SM; 44-127, AAA Anto Wpra Bn. SM; 44-130, Hq ft Hu 
. Btry, AAA SL Bn; 44 327 h AA Bin try Btry. VTjA. 

For explanation o£ symbols see PM 21-6. 

H "r>- ** ■ «■'-* 

Original from 



rs i | 
• • 

Helton I. 


















Introduction to radio ........... , 

Circuit elements and symbols 

Tuned circuits * , 

Vacuum tubes , , . . , 

Vacuum-tube detectors ....... 

Vacuum-tube amplifiers , . . . . .. 

Tuned radio-frequency receiver 

Superheterodyne receiver , 

Power supplies »*....•. 

Vacuum-tube oscillators . , 

Continuous-wave transmitters , 

Modulated, transmitters , 

Frequency modulation , 

Antennas, radiation, and wave propagation 
Yery-bigh- frequency communication 

CathodQ-ray tube 

Radio direction finding 

Care and maintcDunco of radio equipment. . 

■■■ + ■ 

■ * + ■ 

■ * ■ ■ 


. J 9-32 
. 33-48 
. 49-57 
, SB-GO 
♦ 67-74 
. 75-84 
- 85-95 







IV m 




Index .... 

Ktulio abbreviations ................ 

Glossary of radio terms ............. 

Summary of formulas .............. 

KM A radio color tiodes 

Multiples irnd gubmultiplcB , . + 

Kilocydc-metcr con version ......... 

Induetaiice-ettpucituueG product values 
Squares and square roots 

Bibliography .. ........... 

Itevicw questions ..,...,.,...,...... 

J . . . 



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cf by C glC 

Original from 

This manual supersedes TM 11-455, 17 July 1941. 



L General 

a. The success or failure of any military mission, depends on the 
efficiency of its communication system. In these days of high-speed 
warfare, rapid positive communication is more vitally important than 
ever before. The advent of mechanized warfare, made possible by the 
development of motor vehicles, airplanes and tanks, has created a 
demand for a fast, flexible, and mobile communication system, and 
radio is the only means of communication which will meet these require- 
ments. The successful coordination of all the units which constitute a 
modern righting force would be virtually impossible without radio com- 
munication. Hundreds of thousands of radio sets are used by the Army 
to direct our tanks in battle, detect the approach of hostile airplanes, 
bring our fighter airplanes into contact with the enemy, and direct our 
bombers to their targets and bring them safely home. Small portable 
sets provide instant communication for troops on foot, and powerful 
fixed stations transmit orders to commanders in the field. All of these 
sets must be properly operated and maintained ; otherwise they will be 
worthless- The failure of but one radio set in the field may cause the 
failure of a mission and the loss of many lives, 

b. Radio is not difficult to learn if the fundamentals are mastered 
step by step. A thorough knowledge of these important fundamentals 
enables a radio operator or technician to understand the equipment he 
handles and to obtain the best results from its use. Abbreviations 
common to radio communication work are used throughout this manual 
to accustom the reader to those terms which are used frequently in all 
radio publications. A list of these abbreviations and their meanings is 
given in appendix L 

2. Electrical Background 

a. The basic laws which govern the electrical phenomena in radio 
communication systems are much the same as in ordinary power sys- 
tems. A discussion of these basic principles of electricity is presented 
in TM 1-455, including a study of the current and voltage relationships 
in elementary direct-current (d-c) and alternating-eurrent (a-c) cir- 


(~* I,, Original from 


cuits, with application! to power equipment and to measuring iiistru 
ments. It is assumed that the student is thoroughly acquainted with the 
material contained in TM 1-455. Basic electrical principles are men 
tioned in this manual only to the extent necessary to show their 
application to the fundamentals of radio. 

6. An elementary principle of radio transmission can be more easily 
understood if it is compared to the action of a transformer. (See 
TM 1-455.) If two eoila are coupled together magnetically* and an 
alternating current is applied to one of the coils (known as the primary ) , 
a similar alternating current appears in the second coil (known as tho 
secondary), even though there is no direct physical or mechanical 
connection between the two coils. In radio transmission a high-frequency 
(h-f) alternating current, which is known as radio-frequency (r-f) 
current, is applied to a wire known as the transmitting antenna. The 
r-f current flowing through this wire sets up a h-f magnetic field around 
the wire. If a second wire, known as the receiving antenna, is placed 
somewhere in the magnetic field of the transmitting antenna, r-f current 
will flow in this second wire. Thus the transmitting antenna cor- 
responds to the primary of a transformer, and the receiving antenna 
corresponds to its secondary. The effect of the transmitting antenna 
on the receiving antenna is similar to the effect of the primary on the 
-secondary of a transformer. 

3. Frequencies of Communication 

a. An a~c wave makes a number of complete cycles every second. The 
number of cycles per second (cps) determines the frequency of the 
wave. The frequencies which can be used for communication purposes 
may be divided into two broad groups: audio frequencies and radio 

d. Audio frequencies are those frequencies between about 15 and 
20,000 cycles per second to which the human ear normally responds. 
Sounds which occur at frequencies below 20 cycles per second (such as 
the staccato tappings of a woodpecker) are recognizable more as indi- 
vidual impulses than as tones. The frequencies that are most important 
in rendering human speech intelligible fall approximately between 200 
and 2,500 cycles per second. The fundamental range of a pipe organ \h 
from about 16 to 5,000 cycles per second, and the highest fundamental 
note of the flute is about 4,000 cycles per second. Speech and music actu- 
ally consist of very complicated combinations of frequencies of irregular 
and changing shape. These are harmonics, or overtones, which are 
multiples of the fundamental tone, or frequency, and give individual 
characteristics to sounds of the same fundamental frequency coming 
from different sources. Thus, a violin and a piano both emitting a 
1,000-cycle tone would not sound alike, because of the presence of 
characteristic overtones. It has been determined by experiment that 

(~* I,, Original from 


the human ear responds best to sounds of about 2,000 cycles per second. 
Sound waves around 15,000 cycles per second and higher, such as those 
Bet up by very high-pitched whistles, are likely to be inaudible to the 
average ear. Audio frequencies are used to operate telephone receivers, 
loudspeakers, and other mechanical devices to produce sound waves 
which are audible to the ear. Although the audio frequencies cannot be 
used directly for transmission purposes, they play a large part in 
radio communication. 

c , Radio frequencies extend from about 20 kilocycles (20,000 cycles) 
to over 30,000 megacycles (30,000,000,000 cycles). Since different 
groups of frequencies within this wide range produce different effect* 
in transmission, radio frequencies are divided into groups, or bands, of 
frequencies for convenience of study and reference. The bands used 
for military purposes are shown in table 1, 

Table I 


Low-frequency (]-f) 

Medium-frequency (m-f) ... 

High-frequency (h-f ) 

Very-high-frequency ( v-h -f ) 
TTltra-high -frequency (u-h-f) 

.Frequency range 

30 to 300 kc. 

300 to 3,000 kc- 

3,000 to 30 mc, 

30 to 300 mc. 

300 to 3,000 me. 

Since these frequency bands have certain transmission characteristics, 
it is convenient to note the approximate results which may be expected 
from the use of various frequencies under normal operating conditions. 
These results are shown in table II. 

Table II 




H~f (3 to 10 mc). 

H*f (10 to 30 mc.) 



Medium , . 




Long . . . 
Long . . . 


to long 
Short , , . 
Short . . . 


Very high . 
High to 

medium, . 
Medium . ♦ 



Antenna length 



Very short 

Long range: over 1,500 miles. Medium range; 200 to 1,500 miles, 
range : under 200 miles. 


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4. Relationship Between Radio Frequency and Wavelength 

(t. When r-f current flows through a transmitting antenna, radio waves 
are radiated from it in all directions in much the same way that waves 
travel on the surface of a pond into which a rock has been thrown. It 
has been found that thee© radio waves travel at a speed, or velocity, of 
186,000 miles per second (equal to 300,000,000 meters per second). 
Radio waves are produced by sending a h-f alternating current through 
a wire. The frequency of the wave radiated hy the wire will therefore 
be equal to the frequency, or number of cycles per second, of the h-f 
alternating current. 

h. Since the velocity of a radio wave is constant regardless of its 
frequency, to find the wavelength (which is the distance traveled by 
the radio wave in the time required by one cycle) it is only necessary to 
divide the velocity by the frequency of the wave. This is an important 
relationship of radio communication. 

300,000,000 (velocity in meters per second) Wavelength (in 
Frequency (in cycles per second) meters). 

Example? To find the wavelength of a radio wave with a frequency 
of 100,000 cycles per second: 

300,000,000 QA _ 

^-iooTooo = J '°° meters * 

This same relationship can be expressed in another way. If the wave- 
length is known, the frequency can be found by dividing the velocity 
by the wavelength. 

300,000,000 (velocity in meters per second) Frequency (in 

Wavelength (in meters) cycles per second). 

Example: To find the frequency of a radio wave with a wavelength 
of 150 meters: 



= 2,000,000 cycles per second (or 2,000 kc). 

(i. Radio waves are usually referred to in terms of their frequency. 
Since the frequencies employed in radio transmission extend from 
several thousand to many hundreds of millions of cycles per second, 
it is more convenient to refer ta them in terms of kilocycles per second 
(kc) and megacycles per second (me)* 

1 kc = 1,000 cycles per second, 

1 mc — 1,000 kc = 1,000,000 cycles per second. 

5. Elements of Radio Communication 

a. In order to transmit messages from one location to another by 
radio, the following basic equipment is required. (See fig. 1.) 

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Figure 1, Block diagram of baau* radio equipment. 

(1) A transmitter, to generate r-f energy waves. 

(2) A key, or microphone, to control these energy waves. 

(3) A transmitting antenna, to radiate r-f waves. 

(4) A receiving antenna^ to intercept some of the radiated r-f waves, 

(5) A receiver, to change the intercepted r-f waves into a-f waves. 

(6) A loudspeaker, or headphones, to change the a-f waves into sound. 

b. The simplest possible radio transmitter (fig. 2) consists of a 
power supply and a device known as an oscillator, for generating r-f 
alternating current. The power supply may consist of batteries, a 




— g KEY 

Figure £, Block diagram af simple r&lia transmitta, 

generator, or an a-c-opeTated power source. In order to tune such a 
transmitter to the desired operating frequency, the oscillator must 
contain a tuned circuit. It is also necessary to have some method of 
controlling the r-f energy generated by this transmitter, if messages 
are to be sent by this means. The easiest way of doing this is to use a 
telegraph key (which is merely a type of switch for controlling the 
flow of electric current) connected in series with the power supply and 
the oscillator. When the key is operated, the power applied to the 
oscillator to establish a flow of current is turned on and off for varying 
lengths of time, to form dots or dashes of r-f energy. Since the output 


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power, or ivf energy, generated by this oscillator is normally not great 
enough to permit transmission over long ranges, it is seldom used alone 
as ft radio transmitter. In order to increase, or amplify, the output of 




Figure $ k Block diagram of oaciUator-&mpUfter transmitter. 

the oscillator, a device known as a r-f amplifier is generally used in 
modern radio transmitters. The addition of this stage is shown in 
figure S. Such a transmitter is entirely satisfactory for practical pur- 
poses where only radiotelegraph or code transmission is desired. In 
order to transmit messages by voice, however, it is necessary to find 
some way of controlling the output of the transmitter in accordance 
with the voice frequencies (audio frequencies). In modern radiotele- 
phone transmitters this is done by means of a modulator, which in- 
creases or decreases the output of the transmitter in accordance with 









MICROPHONE r t .*.„ 

Figure 4. Block diagram of radiotelephone transmitter. 

the voace frequencies generated when speech enters a microphone* This 
process is known as modulation, and a r-f wave affected in this manner 

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is known as a modulated leave. Figure 4 shows the addition of the 
modulator and microphone required to change the radiotelegraph 
transmitter into a radiotelephone transmitter. 

c. The radio receiver operates in a manner different from that of 
the transmitter just discussed. Consider the diagram of a simple radio 
receiver. (See fig. 5.) Radio-frequency waves (from a transmitter) are 
reaching the antenna of the receiver. If a pair of headphones (headset) 
is connected directly to the receiving antenna in an attempt to receive 
the incoming radio waves, the attempt would not be successful, because 
the human ear will not respond to radio frequencies- A method is 
therefore needed whereby intelligence in the form of audio-frequency 
waves can be extracted from radio-frequency waves, and converted into 
sound by a headset. The circuit which is used in radio receivers to 




Figure 5. Block diagram of simple radio receiver. 


accomplish this is known as a detect or t since it actually detects the 
incoming signal (radio wave). Since it is known that the radio trans- 
mitter is sending out radio waves of a certain frequency, the receiver 
must have some means of tuning in, or selecting the frequency of the 
desired radio wave. This is necessary to avoid receiving many radio 
signals of different frequencies at the same time. That part of a 
detector which is used to tune in the desired signal is called a tuned 
circuit. Because a radio signal diminishes in strength, or amplitude, 
at a very rapid rate after it leaves the transmitting antenna, it is 



Future tf. Block diagram of detector and a-f amplifier. 

(~* I,, Original from 


seldom possible to use a detector alone to tune in the desired signal. 
The greater the distance between the transmitting and receiving 
antennas, the greater will be the reduction, or loss, in aignal strength. 
By the time it reaches the receiver, the signal may be bo weak that the 
sound in the headset is too faint to be understood. The actual r-f signal 
voltage picked up by a receiving antenna in normal communication 
work is usually only a few microvolts, or millionths of a. volt. In order 
to increase the level of the a-f output of the detector to obtain satis- 
factory headset operation, an a-f amplifier is used in most radio sets, 
Figure 6 shows an a-f amplifier added to the simple radio receiver. 
If it is desired to increase the sensitivity (ability to receive weak sig- 
nals) of the receiver still more> it will be necessary to amplify the r-f 
signal before it reaches the detector. This is done by the use of an r-f 
amplifier. Since the r-f amplifier , like the detector, is provided with 









Figure 7. Block diagram of complete radio receiver. 

one or more tuned circuits, so that it amplifies only the desired signal, 
the addition of an r-f amplifier to the receiver gives not only greater 
sensitivity, but also greater selectivity (ability to separate signals). 
The essentials of a modern radio receiver are shown in figure 7, 



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6. Simple Circuits 

a. It has been shown that radio transmitters and receivers are made up 
of a number of circuits, each of which has a definite job to do in the opera- 
tion of the whole. The failure of one part in any of these various 
circuits can cause the failure of the entire radio set. It is therefore 
necessary to study carefully sueh circuits and their individual parts. 

6. There are three general types of electrical circuits, known as series 
circuits, parallel circuits, and series-parallel circuits, depending on the 
arrangement of parts, (See fig. 8.) The principle of operation of these 




- ■•vAAA/V - 

— WWV- 


Series-connected circuit. 
* (§} Parallel- connected circuit* 
®, ® Bevies-parallel, combination circuits. 
Figure 8* Simple circuits. 

simple circuits is discussed in TM 1-455, A simple relationship, known 
as Ohm's law, exists between the voltage, current, and resistance in 
electrical circuits. The student should become thoroughly familiar with 
all three forms of Ohm's law, since it is very useful in determining the 
voltage, current, or resistance in an electrical circuit. When any two of 
these values are known, the third can easily be found. 

c, Ohm's law simply states that the current flowing in a circuit ix 
equal to the voltage applied to the circuit divided by the resistance. 

I (amperes) == 

E (volts) 

E (ohms)" 

This is the form that is used when the voltage applied to the circuit amt 
the resistance of the circuit are both known, and the value of the current 
flowing in the circuit is wanted. 



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Example: If 500 volte are applied to a circuit which has a resistance 
of 5,000 ohms, the current in the circuit will 

I= 5^00 = 15" = 01 Mp0t 

d. If the current and the reactance of the circuit are known, the volt- 
age applied to the circuit can be found by use of the second form of 
Ohm's law, which states that the current times the resistance equals the 

E (volts) ™ / (amperes) X R (ohms). 

Example: If a current of 3 amperes is flowing through a circuit hav- 
ing 70 ohms resistance, the voltage applied to the circuit will be — 

E = 3 X 70 = 210 volts. 

e. If the values of the current and voltage are known, the resistance 
of the circuit can be found by the third form of Ohm's law, which states 
that the resistance equals the voltage divided oy the current, 

R (ohms) = E * volta} 
I (amperes) " 

Example: If a current of 0.25 ampere flows in a circuit to which 100 
volts ib applied , the resistance of that circuit will be — 

R = -przrzr = 400 ohms. 

/, D-c circuits and a-e circuits are dealt with separately in TM 1~155, 
and no attempt is made to consider circuits iu which both direct current 
and alternating current are present at the same time. Since both direct 
current and alternating current are present simultaneously in most 
radio circuits, it is important to understand the manner in which the 
various parts of a radio circuit control the current flow. 

7. Circuit Bemen+t 


Any radio circuit is a combination of parts arranged to control the 
flow of current in such a manner that certain desired results - are pro- 
duced. These parts are called circuit elements. The three main circuit 
elements used in radio work are resistors, inductors, and capacitors, 

8. Reiittori 

a. A resistor is a circuit element designed to introduce resistance into 
the circuit, so as to reduce or control the flow of current. Resistors may 
be divided into three general types, according to their construction. 
These are known as fixed resistors, adjustable resistors, and variable re* 


(~* I,, Original from 


b. Fixed resistors are used to introduce a constant value of resistance 
into a circuit. Their size and construction are determined by the 


/ \AAA/ V 



Q) TFtr#-uNHind resist or. 
(g Low-wattage carbon testators 
(D High-wattage nor ho% rwistor*. 
Figure 9, Fixed re*i*tQT*» 

amount of power they must carry, For low-power requirements^ small 
carbon or metallized resistors are used; where heavier power must be 
carried, larger resistors o£ wire- wound construction are employed. Sev- 
eral types of fbced resistors are shown in figure 9, together with the 
symbol which is used to represent them on circuit diagrams. Fixed 
resistors are often provided with colored markings to indicate their 
resistance value and accuracy (tolerance). This system of marking, 
called the Resistor Color Code, is simple, and should be memorized for 
future reference. Table III shows the complete Resistor Color Code, 
and gives several examples of its use. When a color-coded fixed resis- 
tor does not bear either a gold or silver tolerance marking, it should be 
remembered that the resistor is only accurate to within 20 percent of its 
marked value in ohms. Large fixed resistors, for use in highpower cir- 
cuits, are found without the color coding, but the value in ohms gen- 
erally is printed somewhere on the resistor. 

c. Adjustable resistors are used where it is necessary to change or 
adjust the value of the resistance in a circuit from time to time. In its 


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usual form, the adjustable resistor is wire- wound, and has one or more 
sliding collars which may be moved along the resistance element to 
select any desired resistance value. It is then clamped in place. Figure 
10(T) shows an adjustable resistor. 



(F) Adjustable power rezistot. 
(a) Potentiometer fttr volume contrvL 
@ Potentiometer for power supply. 
Figure 10. Adjustable and variable remittor*. 

d. Variable resistors are used in a circuit when a resistance value 
must be changed frequently. Depending on the power requirements, 
variable resistors are either of carbon or wire-wound construction. The 
actual resistance element of the variable resistor is usually circular in 
shape, and the sliding tap T or "arm," which makes contact with it is 
provided with a knob and a shaft, by means of which the resistance 
can be varied smoothly. Jf birth ends of the resistance element are 
provided with connection terminals (in addition to the sliding arm} the 


Table III. Resin Lor Color Code* 



S: : '!= 





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Ctftor Number 

■OlU«ll »■ + ***■ ■ -r ■ + + * r ■ -h ■ ■ i ■ + ■ ■■ ■ t ■ ■■ ■ r ■ ■ # h 1 i J- 

Red 2 

Orange .,,...,..*................**...*,... 3 

Yellow 4 

Green ..*.....■,■■;-■ +. ............ ♦ ....... . 5 

Blue . 6 

T J.UXQ v ■*■*■■■■■■¥■¥■ ■»»*■ ■■p¥"#pi"" ■ ■ i » ■ ■ i p» f 

IT ^UJr ■■■■■■ n-riitriavl 1 ■ I V * ■ ■ Itb-hri ■-*■ ■ * I r h * | ■ %■ ■ p U 

WHU 9 

Gold *»*..,.-..*.,*...,....*.<+. 5 percent accuracy 

Silver ......,, 10 percent accuracy 

Note. If no gold or silver marking appears (to indicate tolerance) accuracy is 
110 percent {standard tolerance). 

Example: A 50,00Q-ohtn resistor, of standard tolerance, would be indicated by a 
green ring (E),a black ring (0), and an orange ring (000) , aa shown in the new 
system of marking above* In the old system of marking, shown above on the right 
hand side of the page, the resistor would be painted green (6), with a black end (5), 
uqd an orange dot or ring in. the center ^000). 

variable resistor is called a potentiometer. Figure 10® shows a poten- 
tiometer used as a volume control for a radio receiver; figure 10® 
shows a potentiometer wound of heavier wire for use in a power supply 
circuit. If only one end of the resistance element and the sliding- arm 
are brought out to connection terminals, the variable resistor is called a 
rheostat. The symbol for adjustable resistors h> the same as that for 
variable resistors. 


9. Resit+ance Calculations 

a. In repairing radio sets it is sometimes found that the exact re- 
placement parts are not at hand. It then becomes necessary to use 
whatever parts are available to make the repair. This is particularly 
1 rue in the case of resistors, since many different resistors of different 
values and sizes are required in transmitters and receivers. A repair 
depot would have to carry thousands of resistors in stock at all times, 
to have on hand the exact replacements required for the repair of radio 
equipment in the field. Obviously this is not possible, and the competent 
radio repairman must know how to calculate the resistance values of 
combinations of resistors (in series and parallel) so that he can use 
available resistors to make emergency repairs. 

b. The total resistance of several resistors connected in series ts iqitul 
to the sum of the resistances of the individual resistors. 

B t (total) = r x + r a + r a . 

Example: The total resistance of three resistors connected in series, 
the values of which are 50,000 ohms, 100,000 ohms, and 250,000 ohms 
respectively, will equal — 

B t = 50,000 + 100,000 + 250,000 — 400,000 ohms, 

c. If several resistors of equal value are connected in parallel , the 

741233*"— 47 2 13 

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total resistance unit equal the valve of one resistor divided by the num- 
ber of resistors. 

Q ,, M r (of one resistor) 

Hi (total) = — j= — - — ' 

n (No- of resistors) 

Example: If five 50,000-ohm resistors are connected in parallel, the 

effective resistance of the combination will equal — 

B t = ^^- = 10,000 ohms. " 

If several resistors of unequal values are connected in parallel, the 
reciprocal of the total resistance (one divided by the total resistance) 
wiU he equal to the sum of the reciprocals of the individual resistors, 

_i = J_ + ±+. ] .. 

Rt fi r+ T"a 

Example: The total resistance of three resistors connected in parallel, 
the resistances of which are 40,000 ohms, 20,000 ohms, and 8,000 ohms, 
respectively, will equal — 

1 1 +-.*--+ - 

on aaa a i\ 

Rt 40,000 ■ 20,000 ■ 8,000 

40,000 ' 40,000 ' 40,000 
8 1 

Hi "^40,000 5,000 
E t — 5,000 ohms. 

d. When current flows through a resistance, part of the 'electrical 
euergy is changed iuto heat; thus it is said that a resistance consumes 
power. A resistor in a circuit consumes power according to the voltage 
applied to it and the current which flows through it. This is a power 
loss (since heat produced by a resistor in a radio circuit is of no use), 
and is known as the dissipation of the resistor. It is very important to 
know how much power a given resistor dissipates in a given circuit in 
order to make any repairs to the circuit. If a replacement resistor 
cannot safely dissipate the required power, it will overheat and possibly 
burn out; and the high heat it radiates may damage other parts. For 
this reason resistors are rated in watts dissipation, so that the maximum 
power a resistor will dissipate is known. Thus, a 2-watt resistor can 
safely dissipate up to 2 watts of power, and a 5- watt resistor can safely 
dissipate up to 5 watts. It is advisable wheu replacing defective re- 
sistors to use resistors capable of dissipating more than the known 
power of the circuit; a safe rule is to use resistors rated at least 1% 
times the required power. 



(~* I,, Original from 


e. To determine the power dissipation in watts when the voltage and 
current are known, multiply the voltage by the current. 

F (watts) = E (volte) X I (amperes). 

Example: If 5Q volts applied to a given resistor cause a current of 
0.5 ampere to flow through it, the power dissipation of the resistor will 
be equal to — 

P = 50 X 0.5 = 25 watts. 

When the value of the resistance and the current through the resistor 
are known, multiply the current squared (the current times itself) btf 
the resistance to obtain the power dissipation in waits, 

P = P X # . 

Example: If a current of 2 amperes flows through a resistance of 10 
ohms, the power dissipation in watts will be equal to — 

P = 2 2 X 10 — 2 X 2 X 10 = 40 watts. 

/. Resistance offered to the flow of current by a resistor is the same 
for both alternating current and direct current. In the ease of alter- 
nating current, the resistance remains the same regardless of frequency. 

10. Reactance 

«. Two other circuit elements, inductors and capacitors, are also used 
to oppose the flow of current in circuits containing both alternating 
current and direct current. However, this opposition, unlike the re- 
sistor just studied, is not the same for both alternating current and di- 
rect current. The inductor or capacitor reacts m a different way to 
various a-c frequencies; in other words, the opposition to the flow of 
current does not remain constant as the a-c frequency is varied, 

&. In the case of the inductor, the opposition offered to the flow of 
alternating current will become greater if the frequency is increased. 
In the case of the capacitor, the effect is just the opposite, and the oppo- 
sition will decrease as the frequency is increased. This opposition that 
a capacitor or inductor offers to the flow of alternating current is 
known as its reactance . The reactance of an inductor is called induc- 
tive reactance; the reactance of a capacitor is called capacitive react-* 
a nee. Both inductive reactance and capacitive reactance are measured 
in ohms. 

1 1* Inductors 

a. An inductor is a circuit element designed to introduce a certain 
amount of inductive reactance into a circuit. An inductor may take 
any number of physical forms or shapes, hut basically it is nothing 
more nor less than a coil of wire. The unit of inductance measurement 


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is ibe henry, and the teruiH miUihntry and murohenri/ are also us#L 
One henry is e<|iial tn t t Ol)U millihrnn s, which in turn are equal tu 
l T 000,fXH) microhenrys. The inductance of an air-core coil increases 
as the size of the coil or the number of turns of wire is increased. The 
use of magnetic metal (such as iron) for the core of the coil will in- 
crease its inductance ; a nonmagnetic metal (such as brass ur copper 
will decrease the inductance. The inductive reactance of any eoil is 
increased as its inductance is increased. There are three general types 
of inductors: fixed, adjustable, or variable, 

h, Fixrd inductors have a constant value of inductance in a circuit. 
Most of the coils used in radio work are of the fixed type. The coils 
used in the tuned circuits of radio transmitters and receivers usuallv 
have air cores. The number of turns of wire depends on the frequency 
range to be covered. The only difference between transmitting and re- 
ceiving inductors is in their size, since transmitting coils must stand 
considerably more current and voltage than those used in receivers. 
A typical transmitting coil is shown iti figure 11®, and consists of a 
single winding of heavy wire. 








Single -winding taiik inductor for high-power transmitters. 
(g) Flug-in type r-f transformer for medium-power transmitters* 
(a) and © Small r-f transformers used in hf receivers and transmitters* 
® Small r-f inductor or choke aril used in receivers or low-power transmitters. 
Figure 1 J» Typical r-f induct or st and transformers 


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t=, Adjustable inductors found in modern radio equipment are of 
two main types. The first, and simplest, consists of a coil which is pro- 
vided with several taps and a switch, or clip, so that the induct a nee 
may be adjusted in several steps. This type is found mainly in the 
antenna circuit of radio transmitters, where it is desirable to adjust 
the inductance of the coil to suit the varying requirements of different 
antenna lengths. Jn the set-ond iy^r, the coil ia provided with a mag- 
netic core, which may be moved in or out by means of an adjustable 





Fig are 12* Permeability -tuned induct or, 

setserew. This type of adjustable inductor, known as a permeability- 
tuned induct or ; is sometimes used in transmitters and receivers In tuned 
circuits intended to operate at only one frequency. Figure 12 shows 
the use of a permeability- tuned inductor in a tuned-circuit assembly; 
and gives the symbol by which this type of adjustable inductor is rep- 
resented on circuit diagrams, 

d. Variable inductors are found principally in the antenna circuits of 
radio transmitters. They usually consist of two coils connected in se- 
ries, .and are so constructed that one coil may be rotated within the 
,fjther *paud the inductance; consequently varied, Such inductors are 
called variometers, 


PiA/Aiiln Original from 




Symbol : 

Figure tJ. Variometer. 


Figure i:j show* <\ typical variometer and gives the symbol I'ur rep* 
resenting variable inductors on circuit diagrams. 

<\ A vhvke coil is a fixed in due tor possessing tlie desirable property 
nJ" showing a high readmit? to tlie flor of alternating current, while 
showing a very low resistance to the flow of direct current. Thus, a 
choke toil will easily pass direct current but will try to block or 
''choke" oif tlie passage of alternating current* Very small airfare 
choke coils are used to prevent r-f alternating current from flowing 
in d-e circuits, Large iron-core choke coils are used in a-f circuits, and 
as filter chokes in power supply circuits. Figure 14® shows two smnll 
r-f ckoke colls and their symbol. An iron-core filter choke is also shown, 
with its appropriate symbol, in figure 14{T). 


12, Transformers 

a. Tf two coils are placed near to each other so that the field created 
by one coil will pass through the windings of the other, a transforma- 
tion effect will result, since one coil transfers energy from itself to tb« 


- .. ._| ,, Original from 




A-f filter vhvke* 
@) E-f chokes. 
Figure 14. Choke coils* 

other coil. For example, if one coil has an a-e generator connected to it, 
the varying lines of magnetic force from the one coil will cut through 
the second winding, causing a voltage to be induced (or originated) in 
the second coil, even though there is no metallic connection between the 
windings. The coil producing the original magnetic field (or lines of 
force) is called the primary % and the coil in which the voltage is in- 
duced is the secondary; the two coils in inductive relations to each 
other are called a transformer. In radio there are three general group- 
ings of transformers according to application: power transformers, a^f 
transformers, and r~f transformers Tile power and a-f transformers 
have cores of magnetic materials, usually some form of iron. The r-f 
transformers are generally of air?eore design. However, very small 
magnetic cores , usually consisting of powdered iron, are used in sin in ■ 
low-frequency r-f transformer known as mtermedwte-freqitenry (if) 
transformers Several different types of transformers with their corre- 
sponding circuit diagram symbols are shown in figures 11 and lfi. 

b. Power transformers used in radio receivers and transmit I ers 
transform the line voltage (usually 110-120 volts) to either higher or 
lower A^olta^es. When the voltage is raised the transformer is called a 
step-up transformer; when the voltage is reduced the transformer is 
called a step-down transformer. Power transformers having both 


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(D *•" 




I 0000 J 


pjwr} pnrgpffiT) 





either winding may be primary 
or secondary depending on 
direction in which diagram is 



(1) Mufti- winding power transformer, f'Leailn from thr. varum* windinttx protrude 
through hofes in bottom of cane*) 

&j i-f 1r<wnf'irmvr r with atftn'fmf mid // it rariabh' air enptwilors for tvmnff the 
firijntirjf tint! seermiia/rif wiiuiittf/ft. f Thix ttxs*mblu fits iiisitle the nqntire- aluminum 
raft *jj 

@ A'-/ transformer. (This assembly is mounted in the round aluminum rvui ff r ) 

(?) A-f transformer of push-putt output type* 

Figure 75, Typical transformers. 

step-up and step-down windings tin the same cure are widely used; 
such a transformer is shown in figure 15(T). 

e. Audio- frequency transformers are used to transfer voltages of 
wide a-f raiiije, ralher ihun voltages of a single frequency, as in the 
i' use of a power transformer. A-f transformers have iron cores, ami 
must be able to carry a limited amount of direct current in the pri- 
mary windings without effecting a-c audio frequency. A typical a-f 
1 ransf orruer is shown in figure 15®. 


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d. Radw-frequency transformers are used to transfer r-f voltages, 
and are usually designed to operate on one particular frequency* Re- 
ceiver transformers are quite small in size, and generally have air cores 
(figs. 11®, ®, and ©, and 15®). 

6. When r-f transformers are used for transferring radio frequen- 
cies which are low compared to the signal frequency; {as in the case of 
a superheterodyne receiver, which will be discussed later), the device 
is known aa an intermediate-frequency (i-f) transformer (fig. 15®), 
I-f transformers operate on a single frequency, and may have powdered 
iron cores, 

/. Autotransformers consist of only a single coil. It is possible to 
obtain transformer action with such a coil if a connection is made 
somewhere along the winding between the extreme ends. If a step-up 
voltage effect is desired, the winding between the tap and one end is 
considered the primary, and the entire winding acts as the seeondarj\ 
If a step-down effect is desired, the entire winding is considered the 
primary, and the section between the tap and one end acts as the 
secondary. Autotransformers are used in power circuits. 

13. Capacitors 

a. A capacitor is a circuit element designed to introduce capaeitive 
reactance in a circuit. Iu radio work the units of capacitance are the 
microfarad (abbreviated /if or mf ) and the mieromicrofarad (abbre- 
viated ppl or mmf ). One microfarad is equal to 1,000,000 micromicro- 
f arads. A capacitor is formed by two or more metallic plates separated 
by an insulating material called a dielectric. The capacitance of a ca- 
pacitor is increased as the area of the plates is increased - t the capaci- 
tance is decreased, however, as the distance between the plates is in- 
creased. The capaeitive reactance becomes smaller as the capacitance 
is increased. This is just the opposite of what happens in the case of 
the inductor, where the inductive reactance increases as the inductance 
is increased. If an ordinary battery is connected to the two terminals 
of a capacitor, the capacitor will become charged and will hold the 
charge for a length of time depending on the insulating material used 
for the dielectric. If the dielectric is an excellent insulator, the capaci- 
tor will hold the charge for a long time, and is then said to have low 
leakage. There are three general types of capacitors: fixed, adjustable, 
and variable. 

h* Fixed capacitors have a fried value of capacitance in a circuit, and 
the majority of the capacitors used in radio are of this type. Many 
types of construction are found, depending chiefly on the voltage rat- 
ing desired and the amount of leakage permissible in the dielectric. 
Fixed capacitors are generally named after the type of dielectric used 
in the construction. The main types of fixed capacitors are s mica capaci- 


(~* I,, Original from 


torx, paper capacitors, and rlfrtralytir capacitors. These different typtt 
i»f fixed capacitors arc shown iti fipure Ifi. 




@ Paptr dielectric, ott-impre-ifnatrtt. 

@ .Elect rv£y£ic P 
F^rc 16. Typical fixed capaciUtf*. 

c. Mica capacitors are used mainly in the r-f circuits of transmitters 
and receivers. Low leakage is an important requirement of such cir- 
cuits. Therefore, mica is used as the dielectric, because it is one of the 
best known insulating materials. Mica capacitors are seldom found 
with capacitance values greater than 0.05 microfarad, and they gen- 
erally have high voltage ratings. Mica capacitors, like fixed resistors, 
are often color-coded to indicate their value of capacitance. (A com- 
plete explanation of the mica capacitor color code is given in table IV.) 

Table IV, Mica Capacitor Color Code. 






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Gotw Number 

!_■ I lll.lL ■■■ ■ ■■ ■■■ ■■■ ■■ ■ ■ ■ ■ ■ ■ ■ !■*■-■■-■- + ■ % ■ 1 * ■ ■■•.-« V 

JBrown »#,.»■».« TT , *.,,.. 1 

Orange . * . * ♦ . . . . * - ,•»*.,*, ,- ;*£»*<•-• * 3 

J- % llUlff ■ ■ m. m ■■■ + ■ ■ ■ ■ lb ■ 1 i « ■ ■ * ■ 4 ■ ■ * ■ k 4 ■ + ■ ■ + I % 1 % 

\_M XK tlL I 1 I t d Jj f ■ t I ■ 4. I I (It I t 4 f'4 ■ I " ' ■ < ■ ■ ■ ■ *'* 

Blue .« * •, t **•**»•.• * ■■*'••■ i ^ 

Violet , 7 

Ol^T ...... r T . r . . T . . , . r , ... + ,*■■ + ■*. O 

White 9 

Gold •:•«*,:»-,-•*...,.....,........,. -1 percent accuracy 

Silver . ♦ , . .10 percent, accuracy 

NOTE, All values of e:ip;ns f it:ini'e are given ill rmcromicrufaradn, Alii voltage ratings 
are expressed in hundred ft of volts, 

if. Paper capacitors consist, of tinfoil and paper rolled togelhcr and 
impregnated with wax to exclude moisture. They are widely used in 
circuits where extremely low leakage is not important, such as a-f 
amplifier circuits, power supply circuits, and some r-f amplifier circuits. 

c* Electrolytic capacitors depend on a chemical action within them 
to produce a very thin film of oxide as the dielectric. Consequently, 
these capacitors are polarised; that is, they have a positive and a nega- 
tive terminal which must be properly connected in a circuit. Improper 
connections will damage the oxide film and short the capacitor. Since 
these capacitors depend on a chemical action which takes place when 
current flows through them to produce their dielectric, electrolytic 
capacitors have much higher leakage than either mica or paper capaci- 
tors. The principal advantage of electrolytic capacitors is that, for 
their size, they have a much larger capacitance than the other forms of 
capacitors. They are used chiefly in power supplies where leakage is 
not important. 

/. A dpi $1 able capacitors are used wherever it is necessary to adjust 
the capacitance of a circuit from time to time. These adjustable capaci- 
tors are sometimes known as trimmer*, and are widely used for very 
fine adjustments of the tuning; of a radio receiving set (known as 



© Air*tuned. 
@ Mica- tuned. 
Figure f7. Trimmer capaeUor^ 


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aligning). They are also often used for tuning circuits which operale 
tin only one frequency. Adjustable capacitors, or trimmers, are of two 
types: mica-tuned or air-tuned, according to the dielectric employed. 
Figure 17 illustrates both types of trimmers, 

cj. Variable capacitors are used in a circuit wherever the capacitance 
of a circuit must be continuously variable. They are used as tuning con- 
trols in practically all radio receivers and transmitters. Most variable 
capacitors used in communication circuits are of the air dielectric type, 
A single variable capacitor consists, of two sets of metal plates insulated 
from each other and so arranged that one set of plates can be moved 
in relation to the other set. The stationary plates hit the stateir; the 
movable plates, the rotor, If several variable capacitors are connected 
on a common shaft so that all may be controlled at the same time, the 
result is known as a ganged capacitor. The capacitance ran ire of vari- 
able air capacitors is from a few micromicrofarads to several hundred. 
A typical group of variable capacitors is shown in figure 18, with the 
appropriate symbols for this circuit element. 





i. " 



® Four-gang receiving t$p$* 

@ H-f transmitting type. 

® Trimmer f or padder type, 

© Sigh- power transmitting type. 

Figure IS. Typical variable capacitors. 

h. The principle of bypass and blocking capacitors is important for 
an understanding of the action of a capacitor in any circuit. Although tf 


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capacitor due to the insulating properties of its dielectric, will not 
allow direct current to flow in a circuit, it will pass alternating current, 
since the capacitor charges and discharges in accordance with, the fre- 
quency of the applied a-c voltage- The higher the frequency, the lower 
the reactance, and therefore the greater the current flow through the 
capacitor. This effect is just the opposite of that of the choke coil, 
which passes direct current, but presents a high reactance to the flow 
of alternating current. In some circuits, alternating current should not 
flow through a particular circuit element. By connecting a capacitor 
across (in parallel with) that element, a path of low opposition for the 
alternating current is provided; this bypasses the alternating current 
around the element while either the direct current or the low-frequency 
(1-f ) alternating current flows through the element. In still other cases, 
no direct current should flow through a particular part of the circuit, 
A capacitor is therefore connected in series with the circuit* thus block- 
ing the flow of direct current while allowing the comparatively free 
passage of the alternating current. 

t\ The voltage ratings of capacitors are of much the same importance 
as the power ratings for resistors. In addition to their capacitance, 
capacitors are rated as to their d-c working voltage, which is the maxi- 
mum safe operating voltage for the capacitor. Under no circum- 
stances should a capacitor be used in a circuit in which the voltage may 
exceed the rated working voltage. The safest rule to follow when re- 
placing a defective capacitor in a radio set is to use a capacitor the 
working voltage of which is at least 1*£ times as great as the highest 
voltage expected in the circuit, 

14, Capacitance Calculations 

a. To make replacement repairs in the field, it # is necessary to know 
how to determine the capacitance of capacitors when connected in sc- 
ries and in parallel, since, as in the case of resistors, a capacitor of 
exactly the right value may not be available, 

b. For capacitors in series, the total amount of capacitance is found 
in exactly the same way as for resistors in parallel. 

1 _l + tj_l 

€ (total) ci cs c 3 

Example; Determine the total capacitance of the following three 
capacitors connected in series t 200 pf , 100 j*f t and 400 fd . 

4=^ + ^,4 1 

200 ' 100 ' 400 

200 400 ' 400 400 

C = ~f- = 57 microfarads, 


(~* I,, Original from 


The d-c working-voltage rating for capacitors in series is equal, to the 
mm of the ratings of the individual capacitors. 

c- For capacitors in parallel » the total amount of capacitance is found 
by adding the values of each of the capacitors. This is the same rule as 
for resistors in series. 

C (total) =ci + C3 + c* 

Example: Determine the total capacitance of the following capacitors 
connected in parallel: 0.0005, 0.001, 0.0001, and 0.01 microfarad, 

C = 0.0005 + 0.001 + 0.0001 + 0.01 = 0.0116 microfarad. 

The d-e working-voltage rating of a combination of capacitors, in paral- 
lel is equal to that of the capacitor with the lowest working-voltage 

15* Operation of Circuit Ekmeirht 

a. Following the study of the individual properties and character- 
istics of the three circuit elements, resistance, inductance, and capaci- 
tance, it will now be shown how these circuit elements operate in an 
actual circuit. Figure 19 shows a circuit containing all three circuit 
elements, so arranged that if switch S-l is closed, direct current will 
be applied to the circuit* and if switch S-2 is closed, alternating cur- 
rent will be applied to the circuit. The ground symbol shown on the 
diagram indicates that all points in the circuit so marked with this 
symbol are connected to a metal chassis, or base, on which the circuit 
is constructed ; thus, all points bearing the ground symbol are actual I y 
connected together {-via the metal in the chassis). This chassis ground 
symbol is used quite frequently in circuit diagrams to indicate that 
a part or a circuit element is connected to the chassis. The symbol does 
nut necessarily mean that the part is actually connected to an earth 
tjrouitd, although it is sometimes used in this way in transmitter and 
receiver circuits, as will be shown later. 

b. in studying the circuit of figure 19, it will be seen that there are 
three possible paths through which current may flow. The first is 
through resistor Bl and back through ground (or the chassis) to 
whichever power source is in use ; the second is through capacitor CI 
and j resistor R2 and back through ground ; the third is through inductor 
L and resistor R3 and hack through ground, It will be assumed that 
inductor L has a large inductance, and that capacitor CI has a large 
value of capacitance. Note that all three paths are connected in poraUel* 

c. The first step in the study of this circuit is to close switch £Mi 
applying direct current to the circuit. Current will flow through re- 
sistor Rl t the first path; the amount of current which flows through 


f~* I, Original from 





r a 


I ^AAA/^/ , 


i— -VWW 






« ■ 

— D-C 
*=" SOUflCE 



Figure 19. Operation of circuit elements. 

this path will depend on its resistance. No current will flow in the 
second path since the dielectric of capacitor C acts as an insulator, and 
thus the capacitor will not pass direct current. Although no current is 
flowing in the second path, voltmeter V f which is connected across all 
three paths in parallel, indicates that there is a voltage present across 
R2 and Cl* Also, if the voltmeter were placed across Cl, the same 
value of voltage would be found across it, since there is no current 
flowing in this path, and consequently, there is no voltage drop across 
R2, This example shows that it is possible for a voltage to be present in 
a circuit, even though the circuit is open (that is, there is no flow of 
current). Current will flow in the third path, since the only opposition 
to current flow in this branch of the circuit. is the d-c resistance of the 
coil windings of inductor L and the resistance of resistor Rtt. The 
amount of current flow will be determined by the total resistance in 
this path ; that is, the sum of the d-c resistance of L and the resistanm 
of R:i. 

d* The nest step in the study of this circuit is to open switch B-l 
and close switch S-2, applying alternating current to the circuit. 
When this is done, current will flow through resistor Rl in the first 
path. Since a resistor offers the same opposition to alternating current 
an to direct current, the current flowing in this path will be the same 

ctbyC glC 

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regardless of . whet her alternating current or direct current is applied 
to the circuit. In the second path, through capacitor CI (which has a 
Jar^e value of capacitance) and resistor R2 T conditions will be similar 
to those in the first path. Due to its large capacitance, Cl will present 
a small reactance to the flow of current through this branch of the 
circuit. Thus, the impedance of this second path, or its total opposition 
to the flow of alternating current , being due to both the small react- 
ance and the resistance, will be, for practical purposes, about equal to 
the resistance R2, In path three of the circuit, inductor L has such a 
large value of inductance, that it will present a high reactance to tin 1 
flow of alternating current- The impedance of this path, which is due 
to both the large reactance and the resistance, will be so high that the 
current flow through R"A and L will he extremely small. 

e. To sum up the effects of the circuit elements on both alternating 
current and direct current, both switches are closed to apply alternat- 
ing current and direct current to the circuit at. the same time. The 
important resulte then will be: In path one, both alternating current 
and direct current will flowj in palh two, only alternating current 
will flow; in path three, a relatively larjie value of direct current will 
flow, but only a very small value of alternating current will flow. 
Thus it can be seen from this study that with both alternating current 
and direct current present in a circuit, the current flow of either may 
be permitted, stopped, or restricted, by the proper choice of circuit 

16. Audio-frequency Circuit Elements 

o t The instruments and devices used to change sound waves into 
electrical (audio) frequencies, and vice versa, are important parls of 
the complete radio transmitter and receiver* 

r*-£M7A ^^^™^^^^^^^ ^^^ MICROPHONE SYMBOL 

Figure 20, Carbon microphone T-17, 

PiA/Aiiln Original from 


b. A microphone is a circuit element for converting sound (acous- 
tical) energy into electrical (audio) energy. The various types of mi- 
crophones are named in accordance with llie methods used to produce 
this conversion, or change. Thus, there are carbon, condenser, dynamic, 
velocity, and crystal microphones. Carbon microphones uae the varia- 
tion of resistance between loosely packed carbon granules (due to 
acoustical or sound pressure in a diaphragm) to vary the electrical 
current at an audio-frequency rate. An Army microphone (Micro- 
phone T-17) is shown in figure 20, Condenser microphone* operate 
on the principle of Round energy causing a variation in the spacing 
between two plates which act exactly like a capacitor; the resulting 
variation of capacitance {due to the movement in and out of the 
plates) causes a variation at audio frequencies. Dynamic microphones 
use a low-impedance coil mechanically coupled to a diaphragm; sound 
waves move the diaphragm and the coil, and the movement of the coil 
in a magnetic field causes currents in the coil at audio frequencies. 
The velocity microphone also operates on the electro- magnetic principle, 
but uses a ribbon of dnial (a metal alloy) suspended between the 
poles of a powerful magnet When the ribbon is vibrated by acoustical 
energy, it cuts the lines of force, and a current, which varies in ac- 
cordance with the sound waves, is induced in the ribbon. One type of 
crystal microphone uses \\ Koch pile salt, crystal fastened to a dia- 
phragm. When sound waves move the diaphragm, the crystals vibrate 
and produce an alternating voltage between the crystal electrodes at 
the frequencies of the sound waves. AH of the types mentioned (except 
the crystal microphone) require either some source of current, a mfig- 
netu* field, or a polarizing voltage, 

t\ Headsets and loudspeakers are circuit elements for converting 
electrical (a-f) energy into sound (acoustical) energy. In genernl t the 


TLi?4* ft 
Figure SL Htndscl T 

7412S:i 4?- 29 

_. /""" - v^Lti Original from 


Table V. Tabulation of rommoti radio trymbola. 


Conductor or Wire 

Craned wir** — 
lap, connection; 

bfiNam no (on- 




Ani*mx Loop 



Wife, Shielded 


Wire, Twilled Pair 

Cable, Coaxibl 

Wir* in coble 

Capochlor, Fiicd, 


Cape crier, 

Variable, moving 

piglet *ho*n 


Variable,. Ganged 

Dual Section 

Cail or Inductor 

Coil or Indue lor. 


4 4 




Cail or Inductor, 

Coil ar Inductor, 
Iron Core 

Cod Of Inductor, 

Iron Core 

Iron Cor* 

Air Cor* 

Variable Covpfing, 
moving coil shown 

Iron Core 

Air Core, Toned 

Link Coupled 


Switch, Single Pol*. 
Double Thtow 

Switch, Votary 



Ti-&eg 7 f 

headset or the loudspeaker performs the opposite function of a micro- 
phone. When vary in**; (a-f) currents now through the windings on the 
permanent magnet of a. headset, the diaphragm vibrates in accordance 
with these currents and thus produces audible sound waves propor- 
tional to the variations of current* A typical headset is shown in 
figure 21, with the circuit diagram symbol. One type of loudspeaker 
works on much the same principle as the headset; but instead of a 
metal diaphragm, the loudspeaker uses a paper cone, moved by a small 
armature, for setting up audible sound waves. Figure 22 shows a 
loudspeaker of this type removed from its cabinet. 


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Tabic V. Tabulatum of com mo n radio itymbol* — CoatinanL 


Switch, Doufale 

Pair. Double 

S*l*<ror Switch 

Switch. Power 




Relay (typicd con* 
toil orTong*mtrtH 

Plyfl, Mirraphane, 
HttnJi«t or Speaker 

Plvg for powtr 


Power Receptacle 
at Duller 









Pafarrzed, 2- Wire, 


* * 

— &*- 



4 typical I 

I typical } 


Dry Cell or Battery 

V w 

loud Speaker 




CoU Di^thnpge 



Plate or Anodp 

Hr-am Forming 
E(r ctrnclei 


Envrtop*. Gen 

Heart* Tetrudr 

Vaiu-um Tube 

Vacuum Tub*. 
Voltage Regulator 

Vacuum Tube. 
Triadc. Octal &aie 

Urjtuum Tube, 

Tngdnr OcKil BuiC 


f.i y.lijl 

OtUc RrMificf 


Lamp Or Pilot Light 




1 7, Iniula+on 

In addition to the metal material* which conduct electricity very 
ivadily {sucn as copper and iron J, it is often necewaary to have oilier 
materials which offer a very high resistance to the flow of current, in 
order to prevent the electricity from "straying away" at points where 
physical support is essential. Such materials are known as insidators. 
White a perfect insulator does not exist, there are some materia Is, such 
as porcelain, glass, and ceramic materials, which effectively prevent 
any leakage. It is important to note that insulators which are satis- 
factory for power purposes may not be suitable for radio work. In 
radio circuits which operate with microwatts of energy, any minute 


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Figure St. Permanent mag net loudspeaker and cabinet. 

leakage of current is of definite concern. The dielectric bars which 
insulate the stator plates from the frame of a variable air capacitor 
must be kept clean to prevent any stray leakage. Any slight Jeakape 
currents on insulator surfaces, such as tube bases and sockets, are also 
important, In general, it is well to keep radio insulators away from 
strong electric fields, and to maintain all insulators dry and clean. 

IB. Symbols 

a. Tt is not practical to show radio circuit diagrams in the form of 
photographs or drawings of the actual parts or components, since only 
the outer appearance of the parts would be shown T leaving the inner 
workings obscure. Therefore, in radio circuit diagrams (also known 
as schematic diagrams) special symbols are used to repre&eat the various 
circuit elements and parts, in order to simplify the drawings. Symbols 
for the various types of resistors, inductors, and capacitors have already 
been introduced, and a complete list of all commonly used symbols 
is given in table V. The student should refer to this list whenever in 
doubt about the identification of any part of a circuit diagram. 

b. The more common symbols explain themselves by their own 
appearance, but some may cause confusion. An arrow point, for 
example, may have varied meanings. At the end of a line which seems 
to be continuing out from the schematic diagram, the arrow point 
signifies that there is more of the circuit than is shown. Arrows along 
circuit lines may indicate the direction of the signal current through 
the apparatus. If the arrow point rests against a piece of equipment 
it probably means that there is a contact which is capable of movement 
or adjustment. Finally, an arrow drawn diagonally through any other 
symbol means that the device is adjustable smoothly and continuously, 
as, for example, a variable resistor or a variable inductor. 


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19. General 

a. Tuned circuits are combinations of circuit elements so arranged 
that they produce a desired effect in the radio circuit. Both transmitters 
and receivers depend on tuned circuits for their operation on the desired 
frequency. And if it were not for tuned circuits operating in conjunc- 
tion with Tacuum tubes, modern- radio would not be possible. 

6. In radio receivers tuned circuits" are necessary not only for the 
selection of desired signals, but also for the rejection of undesired 
signals. The ability of a receiver to select the desired frequency while 
rejecting the undesired frequencies is called selectivity. The selectivity 
of a receiving set is entirely dependent on the proper operation of its 
tuned circuits. If the tuned circuits are not functioning properly, if 
they are improperly tuned, or if any of the parts of which they are 
constructed are defective f then the sensitivity of the set (ability to 
receive weak signals) will either be considerably reduced or the receiver 
will not work at all. 

c. In radio transmitters, not only are tuned circuits depended on for 
operation on the desired frequency, but the entire process of r-f power 
generation and amplification is dependent on the proper functioning of 
tuned circuits. If the tuned circuits of a radio transmitter are not 
operating properly due to a defective part or if they are incorrectly 
tuned, the power output of the transmitter (and consequently the 
transmission range) will either be considerably reduced or the trans- 
mitter will become entirely inoperative. 


20. Curv« and Graph* 

In radio work, curves and graphs are widely used to show the 
operation of parts and circuits, because a single curve or graph will 
explain the operation of the part or circuit more simply than a long 
description in words* A curve or graph gives a picture of what is 
happening to one value in a circuit as another value is changed. Curves 
and graphs used in newspapers and magazines, showing business trends 
or changes in the population over a period of time, are all familiar. The 
carves and graphs used in radio work are constructed and jread in 


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1 1 



1 ^L 



_ J_ 

exactly the same manner. They can show the voltage in a circuit in 

relation to frequency, the reactance of a circuit element in relation to 
frequency, or the voltage in a circuit in relation to current. For example, 
in section II it was shown that the reactance of a capacitor decreases 
as the operating frequency is increased. This relationship can be shown 



41 7000 

. I 6400 h 

- 5000 

% 4000 

S 30OO 


£ 2000 


50 100 200 3Q0 400 500 000 TOO 800 BOO IOOO 

Figure Sit. Graph showing reactance of 0*5 of eapafiitor /rom 

30 to 1,000 eyelet per second^ 

on a graph, illustrated in figure 23. Each point on this graph shows the 
value of reactance of the capacitor for a different frequency. Point A 
shows that the reactance of the capacitor is approximately 6 T 400 ohms 
at a frequency of 50 cycles per second. Graphs will be extensively used 
in this section to indicate what happens in tuned circuits. 

21. Refinance, Reactance* and Impedance 

a. A resistor presents the same resistance to the flow of alternating 
current as it does to direct current. The opposition offered to the flow 
of alternating current by inductors and capacitors is called reactance. 
If a circut contains both resistance and reactance, the total opposition 
offered to the flow of alternating current is called the impedance of the 
circuit. The impedance of a circuit is the combined effect of resistance 
and reactance in opposing the flow of alternating current. Impedance 
is measured in ohms. 

h. The effect of inductive and eapacitive reactance on current and 
voltage is of important concern in radio work* Inductive reactance, in 
addition to increasing as the frequency is increased, has another effect 
which plays an important part in tuned circuits: it *not only opposes 
the flow of alternating current, but also causes it to lag a fraction of a 
cycle behind the applied voltage, as shown in figure 24. If a circuit 

contains only inductive reactance, the current will lag behind the 



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voltage by exactly one-quarter of a cycle, or 90 & , Capacitive reactance 
has just the opposite effect : it causes the current to lead the voltage by 


I 'V- OR 360* 


Figure £4* Effect of inductive reactance* 

a fraction of a cyele, as shown in figure 25, If a circuit contains only 
capacitive reactance, the current will lead the voltage by 90°. 









1 r 


p ae 


' } 

w L-ZStO 

Figure 85. Effect of capacUvvc reactance* 

c. Instead of referring to fractions of a cycle as one-half of a cycle, 
or one-quarter of a cycle, in radio work parts of a cycle are expressed 
in degrees: one full cycle equals 360°, one-half cycle equals 180°, or one- 
quarter cycle equals 90° , etc. If two voltages, or a voltage and a current, 
do not reach their maximum and minimum values at the same time iu a 
circuity the difference between the two is expressed in degrees. This 
effect is called the phase shift , or the phase difference. For example, if 
the current in a circuit either lags or leads the voltage by one -quarter 
of a cycle, or 90° it is said that the two are 90° out of phase, or that 
there is a phase shift of 90°. If the current and the voltage in a circuit 
reach their maximum and minimum values at exactly the same time, 
it is said that they are in phase. 

d. Since inductive reactance causes the current to lag 90° behind the 
voltage, and capacitive reactance causes it to lead the voltage by 90°, it 


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can be seen that the difference in the two effects is 180° (or one-half of 

a cycle). Since one half of a cycle is positive and the other half is 
negative, a change of half of a cycle, or 180°, will represent a change 
in polarity. Therefore, the effect of inductive reactance can be con- 
sidered as positive reactance, and capacitive reactance can be considered 
as negative reactance. 

22. FUactanca Calculation! 

< * 



2k fV 


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a. Since inductive reactance is proportional to inductance ami 
frequency, a simple formula can be used to determine the inductive 
reactance of a coil. 

X L - 2k{L ■ 

Where X L = the amount of inductive reactance in ohms, 

L z= the inductance of a coil in henry s, 
and / = the frequency in cycles per second< 

2ji is a mathematical constant used a tf reut deal in radio work ; it is 
equal to about 6.28. 

Example: Find the react at ice of a coil of 5 henry a at a frequency of 
GO cycles per second, 

X\ = 6.28 X 60 X & 

= 1,864 ohms of inductive reactance. 

Example: Find tbe reactance of an inductance of G mil ti Henrys at 
a frequency of 1 T 000,000 cycles per second, 

X L — 6.28 X 1,000,000 X O.OQtf 

== 37,700 ohms of inductive reactance. 

It should be observed that inductances expressed in subdivisions of I he 


henry must be converted into henry s before substituting in the formula 
for reactance. 

h* Since the amount of energy stored in a capacitor (fer a given 
voltage) is fixed by the actual capacity , the total amount of energy 
stored (and subsequently restored to the circuit) in 1 second will be 
greater when the capacitor is charged many times per second tlian 
when it is charged only a few times per second. Therefore, the current 
flow will be proportional to the frequency and to the capacitance of 
of the capacitor, and tbe reactance will be inversely proportional to 
the frequency and the capacitance. The formula for capacitive react* 
ance is — 

_ J 

where X„ — the amount of capaeitive reactance in ohms, 

= the capacitance of a capacitor in "farads, 

f = the frequency in cycles per second, 
and 2k = about 6,28. 

Example: Find the reactance of a 2-microfarad capacitor at 60 
cycles per second. 

6.28 X GO X 0,000002 
= 1 T 330 ohms of capaeitive reactance 

It should be observed that capacitance in the above formula must bt* 
represented in farads. 

23, Serie* Re to nance 

a. If a coil and a capacitor are connected in series with a variable^ 
frequency source of alternating current (fig. 26), the combination of 
parts is called a series-Utned circuit, or a Reries-resouani circuit. Since 
the windings of the coil in such a circuit will produce a certain amount 
of resistance, the effect of this resistance must be considered in the 
operation of the circuit. This resistance is indicated in figure 26 as 
a resistor R* If the a-e source is set at a low frequency, it is found 
that the greatest opposition to the flow of current in the circuit is the 
reactance of capacitor C {since capaeitive reactance increases as the 
frequency is decreased). If the a-c source is set at. a high frequency, 
it is found that the greatest opposition to the flow of current is the 
reactance of inductor L (since inductive Teactanee increases as the 
frequency is increased). In other words, at low frequencies the react- 
ance of the circuit is mainly capaeitive t while at high frequencies the 
reactance is mainly inductive. 

Figure SC. Senas-resonant circuit* 

b* At some frequency between the high and low extremes, the induc- 
tive reactance will be equal to the capaeitive reactance. This frequency 
is known as the resonant frequency of the circuit, and it is said that 
the series circuit is tuned to this frequency. Since the inductive 
reactance in the circuit produces a positive effect, and the capaeitive 


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n -iu-XHwi' product-* a negative elYert, when they become equal in amount 
at the resonant frequency they cancel each other, so that the only 
opposition to current flow in the circuit is that offered by the resistor R. 
c. The current flowing in the series circuit of figure 26 can be 
measured by means of meter -4. If the source frequency is increased 










h B 

j> U 

TVfwiy .^ 




Fif/urr w. Cufrcnt flow in m rifx-rrKwuttit circuit. 

gradually from a luw to a high value, the current will rapidly increase 
until it reaches a maximum value at the resonant frequency, and then 
rapidly decrease, as shown by the graph in figure 27 « 

d> Since the current flow in a circuit is determined by the impedance 
of the circuit, the Impedance of a series-timed circuit is at its lowest. 









_ J* 









Figurr :„'#. Impt'dtMce cUTVe of jierifs-tmud circuit. 

or minimum value at the resonant frequency, and becomes greater on 
cither side of the resonant frequency, (See fig. 28.) 

e* Since the voltage drop across each element of a circuit will he 
proportional to the current flowing in t lie circuit and to the opposition 
offered by each element to the current flow, and since the current 
flowing in a scries circuit is maximum at the resunanl frequency, t lit* 
voltage appearing across each of the elements in the circuit will alstf 
be greatest at resonance, Although the voltages across the coil and 
capacitor of the series circuit in figure 26 are equal in amount and 
opposite in polarity at the resonant frequency (and* so cancel each 
other as far as the total circuit voltage is concerned), each of these 
voltages is very high. Either one of them can be used to operate other 

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radio circuits (such as vacuum tube circuits), since a very strong 
signal (amplification) can be obtained at the resonant frequency, This 
voltage amplification of radio signals at the particular frequency to 
which the circuit is resonant is one of the most important effects of 
tuned circuits. 

/, A circuit is at resonance when the inductive reactance is of the 
same value as the capacitive reactance. If the value of either the coil or 
the capacitor is changed, the resonant frequency of the circuit is changed. 
If either the capacitance or the inductance is increased, or both of 
them are increased at the same time, the resonant frequency of the 
circuit is decreased. Conversely, if either the capacitance or inductance 
is decreased, or both of them are decreased at the same time, the resonant 
frequency is increased. Thus, by maJking either the inductor or capaci- 
tor in the circuit variable, the circuit can be tuned (or resonated) over 
a wide range of frequencies. The limits of the frequency range over 
which the circuit can be tuned will depend on the value of the fixed 
element, and the maximum and minimum values of the variable clement. 
It is usually more convenient and more efficient to make the capacitor 
the variable element in a tuned circuit. For this reason variable 
capacitors, together with fixed inductance' coils, make up the tuned 
circuits of practically all modern radio transmitters and receivers. 

g* The resistance present in a resonant-tuned circuit determines the 
amount of selectivity of which the circuit is capable. Resonance curves 
for three different values of resistance (R in fig. 26) are shown in 
figure 29. These are the same type of curve as that shown in figure 27, 
where current is plotted against frequency at resonance. The resonance 
curves of figure 29 demonstrate the practicability of a tuned circuit 
as a selective device. The current flowing in a tuned circuit, when 
equal voltages of many different frequencies are applied to its terminals, 
is composed principally of frequencies equal to, or nearly equal to, 
the resonant frequency of the circuit, As resistance is. added to the 





ft FBEQUfHCV Jl _ 2 ^ o 

Figure £9* Jte&oncinpp curves showing broadening effect of Aeries resistance. 


(~* I,, Original from 


circuit, the current is attenuated in such a manner that a more nearly 
uniform but reduced resonance curve (or response) is obtained. Thus, 
resistance in the circuit acts to reduce the selectivity. It may also be 
shown that the effect of shunt resistance across either the inductor or 
the capacitor will likewise reduce the selectivity. Occasionally resist- 
ance is deliberately introduced into a radio circuit for the purpose 
of broadening the range of frequencies to which the circuit responds, 
although generally the inherent resistance of the circuit is more than 
enough for this purpose. 

A. Series- tuned circuits are often used in the antenna systems of 
transmitters and receivers. They are particularly well suited to the 
antenna circuit requirements of transmitters, since maximum current 
flows in them at the resonant frequency. This means that maximum 
current will flow in the antenna at the desired operating frequency, 
and consequently there will be a maximum radiation of power at this 
frequency. Series- tuned circuits are also used as wave traps, or filters 
(see par. 26). 

24. Parallel Resonance 

a. If a coil and a capacitor are connected in parallel (fig. 30), the 
combination of parts is called a parallel-tuned circuit t or a paraUel- 
rexojiant circuit. As in the series-tuned circuit of figure 26, whatever 
resistance may be present in the circuit because of the circuit elements 
is indicated on the diagram by the resistor B. Since the coil and 
capacitor of the parallel-tuned circuit are both connected across the 
line from the variable-frequency source of alternating current t there 
are two paths through which the current may flow : one path through 
the coil, and one path through the capacitor. If the a-c source is 

Figure SO* Paraltcl-rcsGTiant circuit. 

?&i at a low frequency, most of the current will flow through the coilj 
wince the reactance of the coil will be small for low-frequency alternating 
i urreut, and the reactance of the capacitor will be high. If the a-c 
source is set at a high frequency, most of the current will flow 
through the capacitor, since its reactance will be small for high 
frequencies, while the reactance of the coil will be high. 


(~* I,, Original from 


b* At the resonant frequency, just as in the ease of the series-tuned 
circuit, the reactance of capacitor C will be equal to the reactance of 
inductor L. However, unlike the series circuit, since the two circuit 
elements are in parallel, the current flowing through the inductive 
reactance (coil L) will be opposite iti polarity to the current flowing 
through the capaeitive reactance (capacitor 0). Since the inductive 
reactance is equal to the capaeitive reactance at the resonant frequency, 
the currents flowing through the two reactances will he equal in value 
as well as opposite in polarity, and consequently they will cancel 
each other. 

c* The current flowing in the parallel circuit of figure 30 can be 
measured by the meter A* Tf the source frequency is varied from a 
low frequency through the resonant frequency to a high frequency, 





Figure JJ* Current flute in parailel^rexonGnt circuit. 




" Cli 



" C~ 







the current will rapidly decrease from its highest value at the low 
frequency to a minimum at the resonant frequency, and will then rise 
again to a high value at the high frequency, as shown by the graph 
of figure 31* 

d. The line current is the difference between the currents flowing 
through the inductive and capaeitive branches of the circuit, as 





Figure SS, Flow r>f currents through branches of parallel- re sonant circuit. 

shown by the graph of figure 32. Because of the presence of some 
resistance, the two hranch currents can never cancel eacli other com- 
pletely. The lower the resistance, the lower is the line current. Although 


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tilt? line current may be very small, the current circulating between the 
coil and the capacitor may be very large. 

0. Since the total current, or line current, in a parallel-resonant 
circuit is minimum at the Tesonant frequency, the impedance of the 
circuit (or the total opposition to current flow) must be at a maximum 
at resonance and decrease on either Hide of the resonant frequency, 
as shown by the graph in figure 33. 







CTY 1 


f i 








Figure S3. Impedance curve of poralltrl-Tetonant cirtwit. 

/. The selectivity of a parallel-tuned circuit is inversely related to 
the resistance in either branch of the circuit ; that is, increased resistance 
in either branch of the parallel circuit acts to decrease the selectivity. 

g r For a fixed frequency of the a-c generator in a circuit such, as k 
shown in figure 30, a variation of the capacitor € is accompanied 
by a variation of the ammeter (line current) reading as the impedance 
of the circuit changes, Minimum current in the line indicates that 
there is a maximum circulating current within the parallel- tuned 
circuit, A parallel-resonant circuit in a radio transmitter is tuned in 
this manner, by watching for a dip in the ammeter reading, 

h. The impedance of parallel -tuned circuits is very high, at the 
resonant frequency and low at all other frequencies. For this reason, 
they are used with vacuum tubes to generate, detect, or amplify signals 
of a given frequency. Vacuum tubes are comparatively high-impedance 
devices, and for proper operation must be connected to high- impedance 
circuits, sucb as parallel-tuned circuits. Parallel-resonant circuits are 
also used as filters (par. 26). A third important use of the parallel- 
tuned circuit is in the principle of the tank circuit employed in 
radio transmitters, 

25. Tank Circuit Principle 

a. If the capacitor in a parallel-tuned circuit is charged by means 
of a battery (direct current) and the battery is then disconnected, 
an alternating current of very short duration will be generated at the 
resonant frequency of the circuit. 

5* This current is produced in the following manner: 


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(1) The capacitor 'will discharge into the inductor, causing current 
to flow through it. This current flow builds up a magnetic field 
around the inductor. 

(2) As the capacitor becomes discharged, the current flow stops 
and the field collapses. 

(3) A voltage, of such polarity that it causes the current to con- 
tinue to flow in the same direction, is induced in the coil by the 
collapse of the field. 

(4) This current flowing into the capacitor charges it with a voltage 
of opposite polarity to the original charge from the battery. The 
capacitor now discharges in the opposite direction through the inductor, 
and the process is repeated, 

(5) To summarize, then, the energy in the circuit which originally 
came from the battery is first stored in the capacitor as a charge 
and then is transferred to the magnetic field around the inductor by 
the current flowing in the circuit. This current is alternating, since 
it reverses its direction at the resonant frequency of the tuned circuit. 

c* This process would repeat itself indefinitely if the circuit con- 
tained no resistance. But since all circuits contain at least some 
resistance, the process will continue only until the energy which has 
been applied to the circuit has been dissipated, or used np, by the 
circuit resistance. 

d* In order to produce a sustained alternating current, it is only 
necessary to supply suflicient power to such a paTallel-tuned circuit 
to overcome the losses due to its resistance. It is possible to do this 
in certain vacuum tube circuits used in transmitters as will he explained 
later. Alternating currents generated in such parallel-tuned tank 
circuits are called oscillatory currents. It is because such a par all el - 
tuned circuit can store power for a time that it is called a tank circuit, 

26. Filters 

a. Filters are necessary for selecting energy at certain desired 
frequencies and for rejecting energy at undesired frequencies. Individual 
capacitors and inductors have properties in a circuit which make 
them suitable either singly or in combination with each other, for 
use as wide-frequency-range fitters; low-pass filters and high-pass 
filters are two examples of this type, Heson ant-tuned circuits are 
also employed as filters foT the passage or rejection of specific fre- 
quencies; band-pass filters and band-re jeetion filters are examples 
of this type. 

6. Individual capacitors and inductors have a characteristic frequency 
range discrimination. Inductors tend to pass low a-c frequencies and 
retard high frequencies; capacitors tend to pass high a-c frequencies 
and retard low frequencies. This retarding effect is know as attenuation. 
Figure 34 presents a pictorial concept of currents which flow in series 


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U J*Jt 

Figure $4* Filter action, of individual m:ries capacitors and inductors. 

circuits corresponding to various applied potentials. The characteristic 
frequency discrimination of large and small capacitors and of large 
and small inductors is shown for four different types of input signals: 
a-f, r-f, a-f aud r-f, and a-f and r-f with d-c component* The attenua- 
tion of certain of these input frequencies should be noted. Resistances 
do not provide any filtering action in themselves, for they impede 
all currents which pass through them, regardless of Frequency. The 
less the resistance in a filter circuit, however, the sharper will be the 
dividing line between the frequencies which pass and those which 
are blocked or attenuated. 

C. A low-pass filter is designed to pass all frequencies below a pre* 
determined critical frequency, or cut-off frequency, and substantially 


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reduce, or attenuate, currents of all frequencies above this cut-off 
frequency. Such a filter is shown in figure 35 with a graph of a 
typical cut-off characteristic. The low-pass filter will also pass direct 








Figure ££. Low-pa** filter &nd ittt frtquency-cwnerU characteristic. 

current and extremely low alternating current without opposition, . 
and is therefore widely used to filter, or smooth, the output of radio 
power supplies. This smoothing action is explained more fully in 
paragraph 35c. 

d. A high-pass filter is designed to pass currents of all frequencies 
above the predetermined cut-off frequency, and retard, or attenuate, 
the currents of all frequencies below this cut-off frequency. The 

Figure SB* Sigh-pass filter and its frequency- current charaoierittic 

inductor and capacitor of the low-pass filter have merely been inter- 
changed to make the high-pass filter (fig, 36), Since all frequencies 
below the cut-off frequency are greatly attenuated a filter of this 
type will stop the flow of direct current in most cases. 

e. Resonant (tuned) circuits have certain characteristics which make 
them ideal for a certain type of filter, where high selectivity is desired, 
A series- resonant circuit offers a low impedance to currents of the 
particular frequency to which it is tuned, and a relatively high 
impedance to currents of all other frequencies, A parallel-resonant 
circuit, on the other hand, offers a very high impedance to currents 
of its natural, or resonant, frequency t and a relatively low impedance 
to others. 

/. A band-passi filter is designed to pass currents of frequencies 
within a continuous band, limited by an upper and lower cut-off 

7411233*— 47- 



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frequency, and substantially to reduce, or attenuate, all frequencies 
above and below that band. A typical band-pass filter is shown in 
figure 37 T with a graph illustrating 1 the band of frequencies which it 
will pass. The series- and parallel -resonant circuits are all tuned 
to the frequency band desired. The parallel-tuned circuits offer a 
high impedance to the frequencies within this band, while the series- 
tuned circuit offers very little impedance* Thus, these desired frequencies 




t h ---j 






Figure 37. Band-pa** filter and it* frequency-current characttrUtle* 


within the band will travel on to the rest of the circuit without being 
affected; but the currents of unwanted frequencies, that is, frequencies 
outside the band, will meet with a high impedance and be stopped. 
Band-pass filters are used in the tuned circuits of tuned r-f receivers. 
They are also used in certain sections of a superheterodyne radio receiver. 
g. A band-elimination filter r or band~r eject um filter, is designed to 
suppress currents of all frequencies within a continuous band, limited 
by an upper and lower cut-off frequency and to pass all frequencies 
above * and below that band. Such a band-rejection filter is shown in 
figure 38, with a graph of its frequency characteristic/ This type of 
filter is just the opposite of the band-pass filter ; currents of frequencies 



Fiffwe 38* Band-refection fitter and its frequency-current chamcteri&tie. 

within the band are opposed, or stopped. The two series -tuned circuits 
and the parallel- resonant circuit are all tuned to the frequency band 
desired. The parallel-tuned circuit offers a high impedance to this band of 
frequencies only, and the series-tuned circuits offer very little imped- 


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ance; therefore, the signals within the frequency band are stopped. 
All other frequencies, that is, all frequencies outside the hand, pass 
through the parallel circuit which offers little impedance, 

A. A wave trap, sometimes used in the antenna circuits of radio 
receivers, is a form of band-elimination filter. There are two types 
of these wave traps: the parallel-tuned niter and the series-tuned 


O — Q 

Figure 39* parallel- tutted vfcve trap. 

filter, A parallel -resonant circuit, connected as shown in figure 39, 
is tuned to resonance at the frequency of the undesired signal; the 
wave trap then presents a high impedance to currents of this unwanted 
frequency t and allows currents of sll other frequencies to enter the 
receiver, A series-resonant circuit, connected as shown in figure 40, 

o J o 


Fiffwe 40. Series-tuned wave trap. 

can be tuned to resonance at the frequency of the undesired signal, 
and these unwanted currents will be effectively bypassed, generally 
to ground, without affecting currents of all other frequencies. 

27. Coupled Circuit*: Transformer 

a. Since every radio receiver and transmitter is composed of a 
number of circuits, or stages, methods must be devised for connecting, 
or coupling the output of each stage to the input of the next circuit. 
One of the most widely used methods for transferring power from 
one stage to another is the transformer* Two important properties 
of the transformer are the turns and voltage ratio and the turns and 
impedance ratio. (See TM 1—455.) 

b. The voltage ratio of a transformer is proportional to its turns 
ratio. In other words, if a transformer has twice as many turns of 
wire on its secondary as on its primary side, the secondary voltage 
will be twice the primary voltage. Conversely, if a transformer has 


(~* I,, Original from 


only half as many turns on its secondary winding as on it a primary 
winding, the secondary voltage will be half the primary voltage, Thns, 
by using a transformer, it is possible either to step up or step down 
the a-c voltage appearing in a circuit. This property is widely used 
in radio circuits where it is necessary to step up the signal voltage 
from one stage to the next. By using a step-up transformer it is 
possible to obtain an actual voltage gain, or voltage amplification. 

c. The impedance ratio of a transformer is equal to the square 
of the turns ratio. Thus if a transformer has a turns ratio of 3 to 1 
(or three times as many turns on one winding as on the other), its 
impedance ratio will be 9 to I, and the winding having three times 
as many turns will have nine times the impedance of the other winding. 
By choosing a transformer with the proper turns ratio, it is therefore 
possible to match the impedances of two circuits. Among the require- 
ments placed on any system for transferring power from one circuit 
to another, impedance matching is one of the most important, since 
it is an electrical rule that in order to transfer the maximum power 
from one circuit to another, the impedances of the two circuits 
must be equal. 

d. For a practical example of impedance matching with a transformer, 
assume that a loudspeaker with an input impedance of 500 ohms is 
to be connected to an a*f amplifier stage with an output impedance of 
3,000 ohms. In order to transfer the maximum a-f power from the 
a-f amplifier to the loudspeaker, the output impedance of the amplifier 
must match the input impedance of the speaker. By applying the 
impedance-turns ratio rule, the impedance ratio of the amplifier to 
the speaker will be: 

8,000 _^6 
500 1* 

e. Since the impedance ratio of a transformer equals the square of 
the turns ratio, the turns ratio equals the square root of the impedance 
ratio. In the above problem, the impedance ratio is 16 to 1, and since 
the square root of 16 equals 4 t the transformer must have a turns ratio 
of 4 to 1 in order to match the amplifier to the speaker, 

28. Coupled Circuit*: r-f Tranrformeir 

a. The properties of the transformer just discussed hold true for all 
types including r-f transformers, provided that all of the magnetic 
lines of force which cut the primary coil also cut the secondary. How- 
ever, r-f transformers serve two purposes at the same time: they are 
used to couple the output of one stage to that of another stage, and, 
together with variable capacitors, they form the tuned circuits of radio 
seta. If an r-f transformer has one of its windings tuned by a variable 
capacitor in a circuit, it is called a single-tuned transformer ; if both 

(~* I,, Original from 


of the windings arc* Limed by capacitators, it is known as a doublc-titned 
i raws former. 

bj Single-titled (rattu formers me used in t hi* majority of r-f amplifier 
circuits in radio receivers. Such transformers usually have untuned 
primary coils and tuned secondaries. The number of turns on the 
secondary will depend on the frequency range to be covered by the 
tuned circuit ; but the number of turns on the primary will depend on 
the desired voltage step-up in the transformer, and the output im- 
pedance of the circuit in which it is to be connected. The transference 
of energy from the primary to the secondary of a transformer is due 
to the field of one coil passing 1 through the windings of the other. In 
the untuned transformer, the power transferred from one winding to 

r » 1 1 ■ 'S^ p 

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ii ■ 

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1 1 1 

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Figure JL Coupled coils. 

the other will depend on how close one coil is placed to the other, and 
consequently how many lines of force of the field of one coil pass 
through the windings of the other. (8ee fig* 41.) If the two coils are 
placed close together, they arc closely coupled; if the coils are placed 
some distance apart, they are loosely coupled. From this discussion it 
would seem desirable to couple the windings of an r-f transformer as 
closely as possible, in order to obtain the greatest possible power 
transfer. However, in the case of the tuned transformer, there is 
greater concern about the selectivity of the tuned circuit (formed by 
the tuned secondary winding of the transformer) than there is about 
the maximum power transfer. In other words, a reasonable power 
transfer is wanted at the resonant frequency, and minimum power 
transfer at all other frequencies. IF the coils of the single-tuned r-f 
transformer are coupled too closely, the power transfer over all fre- 
quencies may be at a maximum, but the ratio between the power trans- 
t erred at the desired frequency and the power transferred at th? 
undesired frequencies will be luw, and i-onsequently the selectivity will 


d by Google 

Original from 

be poor. On the other hand, if the coils arc coupled too loosely, the 
power transfer even at the resonant frequency will be unsatisfactory, 
although the resulting selectivity may be excellent. Between these twu 
extremes there is a certain degree of coupling which will give both 
satisfactory selectivity and good power transfer at the resonant fre- 


Figure 4$. Selectivity curves of a typical singU-tuned r-f transformer, shoving 
variation* in transfer of pv^er with change* of frequency. 

quency. This degree of coupling is known as optimum coupling. 
Figure 42 shows the selectivity curves of a typical single-tuned r-f 
transformer for three different degrees of coupling between its primary 
and secondary coils* 

c* Double -tuned transformers have both primary and secondary 
windings tuned by capacitors, and are widely used in the intermediate- 
frequency amplifier stages of superheterodyne receivers. The double- 
tuned transformers used in such circuits are called $-| transformers 
(fig. 15®), and must he carefully tuned to allow the passage of a very 
narrow band of radio frequency known as the intermediate frequency 
of superheterodyne receiver. The effect of the degree of coupling- on 



Figure 43, Selectivity curves of a typical double-tvned r-f transformer 
showing variation* in transfer of power with change* in frequency* 

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the selectivity of double-tuned transformers is more pronounced than 
in the case of the single-tuned transformer, since two circuits, both 
tuned to the same frequency, are coupled together* The double*tuned 
transformer has greater selectivity than the single-tuned r-f trans- 
former. The selectivity curve will be more sharply peaked and will 
have steeper sides, indicating better rejection of signals on either side 
of the resonant frequency. Figure 43 shows the selectivity curves of a 
double- tuned transformer for three different degrees of coupling. Com- 
pare these curves Vith those for the single-tuned transformer shown 
in figure 42 ■ note the flat top on the curve for optimum coupling, indi- 
cating that a band of frequencies on either side of the resonant fre- 
quency will be passed by a double-tuned transformer with the proper 
degree of coupling, This band-pass effect is very important in the 
reception of radiotelephone signals, as will be seen later. Since 
double-tuned transformers will pass a narrow band of frequencies 
while rejecting all other frequencies, they are sometimes called band- 
pass filters. Note the double hump on the curve for overcoupling, in- 
dicating that a doublet tined transformer will have two resonant 
frequencies equidistant from the proper resonant frequency if the 
coupling is increased past the optimum point, 

d. The importance of maintaining the proper coupling between the 
coils of an r-f transformer cannot be over stressed. Overcoupling will 
reduce the selectivity of a set; loose coupling will reduce the sensi- 
tivity of the set. 

29, Coupled Circuit*: Retiitance Coupling 

a. Resistors are often used to couple the output of one circuit to the 
input of another, particularly in a-f amplifiers. BesFstance coupling 
may be used to step down the voltage from one stage to another. (See 





"i r~ 


Figure 44. Resistance coupling wed to sttp down voltage, 

fig. 44.} In this arrangement, if the tap on the resistor is placed half- 
way between the ends of the resistor, the voltage applied to circuit two 
will be half the output voltage of circuit one. Other step-down voltage 
ratios may be obtained by moving the tap up or down the resistor. 

6, To resistance-couple two stages, and pass only alternating current 
from one to the other, as is the case in most radio circuits, a blocking 
capacitor is used (tig. 45). This form of resistance coupling, sometimes 


cfbyC *le 

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T A M»f 

Figure 45. Resistance coupling with blocking capacitor. 

1 4- 

known as resistance-capacitance coupling, has a wide use in the a~f 
amplifiers of radio receivers. 

30. Coupled Circufa: Inductance Coupling 

a. Inductance coupling is used mainly to couple the r-f amplifier 
circuits of radio transmitters, although it finds some application in the 
a-f circuits of receivers. Inductance coupling may be used to step 
down the voltage from one circuit to another in exactly the same way 
that resistance coupling is used in figure 44, except that a tapped in- 
ductor is substituted for the resistor shown. The step-down voltage 
ratio will be equal to the turns ratio of the total winding to the tapped 
portion. That is, if the section of the winding applied to circuit two 
has only one-third of the turns of the total winding, the voltage appear- 
ing across this portion of the winding will be one-thiTd of the voltage 
across the whole coil r 

&♦ In like manner, inductance coupling may be used to step up the 
voltage from one circuit to another (fig. 46). The step-up voltage ratio 
also will be equal to the turns ratio of the total winding to the tapped 




n *m 

Figure 4G* Inductance ttntpHnfl unfit to fltrp up v&Jtag*. 

portion. Thus, if circuit one is connected across one-third of the turns 
of the coil t the voltage appearing in circuit two will he three times as 
great as the voltage output of circuit one. Since the tapped inductor 
operates in much the same fashion as does the transformer, the tapped 
inductor is often called an autotramformer. 

c. Impedance matching can he accomplished with tapped inductors, 
in much the same way as with transformers* The rule is as follows: 
The impedance ratio of the whole coil to the tapped section equals tht 
square of the turns ration of the whole coil to the tapped section- 


cfbyC glC 

Original from 

d. In inductance coupling, as in resistance coupling, to prevent the 
flow of direct current from *one circuit to the other, while allowing 




r - «ii 

Figure 41. Impedance-capaciitmoe coupling used to step down voltage. 

the a-c signal to pass, a blocking capacitor is employed. This method 
of coupling is shown in figure 47 , and is often called impedance-capaci- 
tance coupling. 

31. Distributed Inductance and Capacitance 

cl In addition to the inductance and capacitance included in induc- 
tors and capacitors* there are distributed, or stray, inductance and 
capacitance effects present in miscellaneous components of radio instru- 
ments, as in connecting wires, switches, and sockets. These become of 
considerable concern at radio frequencies. 

b. Capacitive reactance is inversely proportional to the frequency 

( X c = «— fft J ♦ This means that as the frequency of an applied volt- 
age is increased, the capacitance of the circuit offers less opposition to 
the flow of current, At high frequencies undesirably large currents 
may appear where negligible currents would flow at low frequencies* 
The capacitance which occurs between elements of a vacuum tube and 
between adjacent turns of a coil present a large capacitive reactance at 
the lower frequencies. However, at radio frequencies, the reactance may 
be reduced to such a point that the increased magnitude of the current 
flowing across it determines the upper frequency limit for the useful- 
ness of the associated circuit. 

c. Inductive reactance increases in direct proportion to frequency 
(X& = 2^ f L) f or, as the frequency of an applied voltage. is increased, 
the inductance of the circuit offers more opposition to the flow of cur- 
rent. A simple connecting wire, the inductive reactance of which may 
be insignificant at low frequencies, may have a sufficiently large induc- 
tive reactance at higher frequencies to render an instrument inoperative. 

32. Effective a-c Roilitance 

Fundamentally, a measure of the resistance of a circuit is given by 
the power dissipated as heat, when unit current is flowing in the circuit. 
In its broadest sense, the term "resistance" is taken to mean all effects 


dbyC gk 

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leading to dissipation of energy in such form that the energy is nr. 
recoverable for any useful purpose within the immediate system. Thns 
a radio antenna for transmitting is said to have a radiation resistant 
associated with radiative losses, that is, with the energy which is radi- 
ated into space; and a particular transmitter or receiver circuit mar 
be said to exhibit certain reflected resistance because of the power con- 
sumed by other circuits which it directly or indirectly supplies. With 
alternating current, for a given current magnitude, considerably more 
electrical power may be consumed than is required by the same circuit 
with direct current. The resistance which is indicated by a-c power 
consumption is called effective a-c resistance. Part of this additional 
power is required to maintain the heat losses accompanying parasitic 
circulating currents {eddy currents) which are induced in conductors 
of the circuit (in particular, in transformer cores) by the varying 
magnetic field. Another source of a-c electrical power dissipation is 
represented by dielectric and other losses, A further factor which 
makes for more required power for a given magnitude of alternating 
current is the skin effect: the tendency of alternating currents to 
travel with greater density near the surface of the conductor than at 
the center. This tendency increases with frequency. The magnetic field 
about a eurrent^carryiug conductor is more intense at the center of the 
conductor than it is near the surface of the conductor. Thus the back 
voltage set up by the rising and falling magnetic field 1b greater at the 
center than near the surface, and practically all of the current through 
a wire at high frequencies is confined to the outer surface of the conduc- 
tor. The result is increased heating for the same current, that K 
higher resistance. The nonuniform distribution of current throughout 
the cross section of a conductor at high frequencies is more pronounced 
if the conductor is wound into the form of a coil than it is if it is used 
us a Htraight wire. At radio frequencies, the effective a-c resistance 
of a coil may be 10 or 100 times its true d-c resistance. Wherever al- 
ternating currents are studied, it is generally understood, if not spe- 
cifically stated, that resistance means effective a^c resistance. 


(~* I, Original from 




33. Electron 

a. The whole foundation of electricity is based upon the electron , a 
minute negatively charged particle. Atoms, of which all matter is com- 
posed, consist of a positively charged nucleus around which are grouped 
a number of electrons. The physical properties of any material depend 
upon the number of electrons and the size of the nucleus. In all matter 
there are a certain number of free electrons. The movement of these 
free electrons is known as a current of electricity. If the movement 
of electrons is in one direction only, the current is direct. If, however, 
the source of voltage is alternated between positive and negative, the 
flow of electrons will likewise alternate; this is known as alternating 

o. If certain metal s^ or metallic substances snch as metallic oxides, 
are heated to a high temperature either hy means of a flame or by pass- 
ing current through them, they have the property of throwing off, or 
emitting^ electrons. The element in a vacuum tube which is heated to 
emit electrons is called the cathode. 

c. If the cathode is heated to a high temperature in the open air, it 
will burn up because of the presence of oxygen in the air. For this 
reason the cathode is placed in a glass or metal bulb from which all air 
has been removed. Such a space is known as a vacuum. Since it is 
difficult to heat an element in a vacuum tube by means of fire or flame, 
the cathode, which is in the form of a filament, is directly heated by 
passing a current through it, 

d. Any isolated positively charged body in the vicinity of the electron 
emitter will attract the negatively charged electrons. The positive 
charge on the body will soon be canceled by the electrons attracted to it 
unless some means is employed to remove the electrons as fast as they 
arrive. This can be done by connecting a source of constant voltage 
between the positively charged body and the electron emitter (fig. 48), 
This is the general arrangement in a two-element tube, or diode . It is 
also the basis of operation of all types of vacuum tubes, 

e. The emitter, or cathode, of a vacuum tube may resemble the 
familiar incandescent lamp filament which is heated by passing a 
current through it. The positively charged body usually surrounds the 


(~* I,, Original from 


« p r .® ■&-, 

\ /'-'C--© — * - x 


Figure 48. Emitted electron* attracted &y fl poritivety charged haay* 

emitter and is called the plate, or anode. It should be noted that elec- 
tions travel from negative to positive, 

f. Two types of cathodes, or emitters, are used in radio tubes. In one, 
known as the filament or directly heated type, the heating current is 
passed through the cathode itself. In the other, known as the indirectly 
heated type, the current is passed through a heating element, which in 
turn heats the cathode to a temperature sufficiently high for electron 
emission. In the indirectly heated type, the cathode is an oxide-coated 
metal sleeve which is placed over the heater element* 

g t The higher the temperature of the cathode, the more electrons it 
will emit. However, if too much voltage is applied to a cathode, the 
heavy eurrent flow will cause the filament or heater to burn out. The 
.safe filament or heater voltage is determined by the manufacturer, and 
this voltage rating must be observed for satisfactory operation. The 
cathode of a tube will not continue to emit electrons indefinitely. After 
several thousand hours of operation, the number of electrons emitted 
will gradually decrease, until finally an insufficient number is emitted 
for proper operation. The decrease in emission capacity is due to the 
chemical change which takes place in the cathode. This is one of the 
reasons why tubes wear out. 

34. Operation of Diode 

a. The diode is the simplest type of vacuum tube, and consists of only 
two elements: a cathode and v. plate. The operation of the diode depends 
jn the fact that if a positive voltage is applied to the plate with respect 
to the heated cathode, current will flow through the tube; if a negative 
voltage is applied to the plate with respect to the cathode, current will 
not flow through the tube. 

b* When the positive terminal of a battery is connected to the plate 
of a diode and the negative terminal is connected to the cathode, ths 
plate will be positive with respect to the cathode. Since the electrous 
emitted by the cathode are negative particles of electricity, and there 
is a positive charge on the plate, the electrons emitted by the cathode 
will be drawn to the plate {fig. 49). In other words, there is an electron 
flow through the tube, which Tesaits in a current flow in the circuit. If 


(~* I,, Original from 


Figure 49. Electron flow in a diode when plate is punitive. 

the flow of current in the circuit is measured by meter A (fig, 49) while 
the voltage applied to the plate (known as battery voltage or plate 
voltage) is increased* it will be seen that the current flow through the 
lube, known as the plate current, increases. This is illustrated by the 
plate- voltage plate-current curve of figure 50. , 



Figure Jfl, Ffate current flow in a diode, 

c r When the negative terminal of a battery is connected to the plate 
of the diode and the positive terminal is connected to the cathode (fig. 
51), the plate will be negative with, respect to the cathode, and therefore 
no electrons will be attracted to the plate. Since no electrons arc 
traveling across to the plate, no current will flow through the tube. 

d. The diode is a conductor when the plate voltage is positive, and is 
a nonconductor when the plate voltage is negative. This property of 
the diode permits the use of this tube for two very useful functions : 
rectification and detection. 


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Original from 

Figure £l w Diode action when plate « negative* 

35k Diode as Rectifier 

a. The ability of a diode to conduct, or pass, current in only one 
direction makes possible its use as a rectifier to convert alternating 
current into direct current, A diagram of a simple diode rectifier 
circuit is shown in figure 52, If an a-c source is connected between the 
plate and the cathode of such a circuit, one half of each a-c cycle will be 
positive and the other half will be negative. Therefore, the plate of tae 
diode will be made alternately positive and negative with respect to the 
cathode. Since the diode conducts only when the plate is positive, 




Figure $4. Diode u&cd an a half -wave rectifier* 

eurrent flows through the tube only on the positive half -cycles of the 
a-c voltage, as shown in figure 03. Since the current through the diode 
flows in one direction only, it is direct current. This type of diode 
rectifier circuit is called a half-wave rectifier, since it rectifies only 
daring one-half of the a-c cycle. 

0, It can be seen from figure 53 that this direct current is quite dif- 
ferent from pure direct current, since it rises from zero to a maximum 
and returns to zero during the positive half-cycle of the alternating 
cur rent j and does not flow at all during the negative half -cycle. To 


' .-■! 

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Fiffure £ w 3* Output of a half -wave recti fiet* 

distinguish this type of current from pure direct current, it is referred 
to as pulsating direct current > or rectified alternating current. 

c. To convert this rectified alternating current into pure direct cur- 
rent, the fluctuations must be removed. In other words, it is necessary 
to cut off the humps at the tops of the hal£*cycles of current flow, and 
to flU in the gaps due to the half -cycles of no current flow. This process 
is called filtering. In the circuit of figure 52, the d-c voltage output 
will appear across the load resistor B, because of the current flowing 
through it during the positive half -cycles* The capacitor C, having a 
small reactance at the a-c frequency, is connected across this resistor. 
This capacitor will become charged during the positive half-cycles, 
when voltage appears across resistor R, and will discharge into resistor 
R during the negative half-cycles, when no voltage appears across the 
resistor, thus tending to smooth out, or filter, the fluctuating direct 
current. Such a capacitor is known as a filter capacitor. It stores up 
voltage when it is present, and releases the voltage into the circuit 




Without capacitor. 
@ With caparitor. 
Figure 54. Effect of filter eapaeitor. 

when it is needed, Figure 54 shows the voltage appearing across re- 
sistor R t both with and without a filter capacitor in the circuit. It will 
be seen that the addition of a filter capacitor alone is not enough to 


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Original from 

remove completely the fluctuations or ripple; in fact, no amount of 
capacitance, however large, would completely eliminate this ripple. 
However, if a filter circuit is added to the half -wave rectifier, as shown 
by the complete circuit (fig. 55), a satisfactory degree of filtering can 
be obtained. In this circuit, capacitors Ci and 0% are both filter capac- 
itors,, and fulfill the function described above. Inductor L m a filter 
choke having high reactance at the a-c frequency and a low value of d^ 
resistance. It will oppose any current fluctuations, but will allow 
direct current to flow unhindered through the circuit. The two filter 

Figure S3. ^Uter rtrcvit tuhlrd to half-wtwe rectifier. 

capacitors C t and C 2 bypass the ripple voltage around the load resistor 
B r while choke coil L tends to oppose the flow of any ripple current 
through the resistor, 

d. The disadvantage of the half -wave rectifier is that no current flows 
during the negative half -cycle. Therefore, some of the voltage produced 
during the positive half-cycle must be used to filter out the ripple. 
This reduces the average voltage output of the circuit. Since the circuit 
is conducting only half the time, it is not very efficient. Consequently 
the fulUwave rectifier, so called because it rectifies on both half -cycles, 
has been developed for use in the power supply circuits of modem 



Figure 5G* Full-wave rectifier circuit. 

receivers and transmitters. In the full -wave 'rectifier circuit shown in 
figure 56, two diodes are used, one conducting during the first hall- 
cycle and the other during the second half -cycle. 

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Original from 

e. In the circuit of figure 56, the transformer has a center-tapped 
secondary winding, so that diode D t is connected to one half of this 
winding, while diode D t is connected to the other half, Eesistor E is 
the load resistor common to both diodes. Capacitors C x and C 3 and 
inductor L form the filter circuit. During one half -cycle, the plate of 
diode J?i will be positive with respect to the center tap of the trans- 
former secondary winding, while the plate of diode D 3 will he negative ; 
consequently , diode D x will conduct while diode D 2 will be nonconduct- 
ing. During the other half -cycle, D L will be negative and nonconducting 
while D a will be positive and conducting. Therefore , since the two 
diodes take turns in their operation, and one of them is always con- 
ducting, current Bows through the load resistor during both halves of 
the cycle. This is full-wave rectification, 

/. If no filter circuit were used in the full- wave rectifier circuit of 
figure 56, the d-c output voltage across the load resistor R would appear 


Figure 57, Output of a full-wave rectifier. 

as in figure 57. Obviously, this voltage waveform is much easier to 
filter than the half -wave rectifier output, and the action of the capacitors 
and inductors in smoothing out this waveform is the same as for the 
half -wave rectifier voltage, 

g. The circuit shown in figure 56 is the basis for all a-c operated 
power supplies used to furnish the d-c voltages required by trans- 
mitters and receivers. Note that the heater voltage for each of the two 
diodes is taken from a special secondary winding on the transformer. 


36. Oiod« Characteristic Curvet 

a. The plate-current plate- voltage curve shown in figure 50 is an 
important characteristic of the diode vacuum tube, because it shows 
the amount of current that a diode will pass for any given plate 
voltage. Different types of diodes may have slightly different char- 
acteristic curves. All of these curves, however, indicate one important 
fact : the load, or plate, current is not proportional to the applied, or 
plate, voltage. For this reason Ohm's law is strictly applicable only to 
small increments, or changes, of currents and voltages. In general, 
current- voltage relations in vacuum-tube circuits are studied by means 
of experimentally obtained characteristic curves, 

b. The curved portions, or bends, in the graph of figure 50 are the 
result of certain variations in the action of the diode, When the plate 
voltage is low, the electrons nearest the cathode are repelled back to the 

741253* — i7- — o 6| 

(~* I,, Original from 


cathode by the accumulated emitted electrons which are a little farther 
from the cathode, and only those electrons which are nearest the plate 
are attracted to the plate. This repelling effect around the cathode is 
known as the space charge. For intermediate values of the plate poten- 
tial, the space charge in the vicinity of the cathode is reduced by the 
attraction of more electrons to the positively charged plate T and any 
increase in plate potential produces an appreciable increase in current, 
as shown by the curve of figure 50. For large values of plate potential, 
when the space charge is completely removed, the number of electrons 
reaching the plate per second is limited by the number emitted per 
second by the cathode, and is independent of plate potential. This 
latter condition is referred to as saturation^ and a place along the curve 
(point S in fig. 50) is called the saturation point. 

37. Operation of Triode 

a. The triode differs in construction from the diode only in the addi- 
tion of another element, called the grid. The grid is a cylindrical 
structure made of fine wire mesh, which is placed between the cathode 
and the plate of the tube so that all the electrons leaving the cathode 
must pass through it in order to reach the plate. Figure 58 is a drawing 






Figure &8. Typical triode. 

which shows the arrangement of the grid, cathode, and plate in a typical 
triode. The grid is placed considerably closer to the cathode than is the 
plate, and consequently will have a very great effect on the electrons 
which pass through it* 

h. If a triode is connected in a simple circuit, as shown in figure 59, 

the action of the grid can be studied. When a small negative voltage 

.(with respect to the cathode) is put on the grid, there is a resultant 

change in the flow of electrons within the vacuum tube. Since the 

electrons are negative particles of electricity, and like charges repel one 


cfbyC *le 

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Figure 53* Triads with a tmall negative voltage on the arid* 

another, the negative voltage on the grid will tend to repel the electrons 
emitted by the cathode, and thus tend to prevent them from passing 
through the grid on their way to the plate. However, since the plate is 
considerably positive with respect to the cathode, its attraction for the 
electrons is sufficiently strong to enable some of them to pass through 
the grid and reach the plate in spite of the opposition offered them by 
the negative voltage on the grid. Thus, a small negative voltage on the 
grid of the tube will reduce the electron flow from the cathode to the 

Figwre 60* Effect of negative grid an plate-vurrent flaw. 

plate {fig. 60), and consequently will reduce the, value of plate-current 
flow between the cathode and the plate of the tube. 

c. If the plate current in the circuit of figure 59 is measured by means 
of meter A, while holding the plate voltage constant and making the 
grid of the tube gradually more negative with respect to the cathode, 
the plate current will vary as shown in the grid-voltage plate-current 
curve of figure 61 . Such a curve is also known as an E& — I P character- 
istic curve. Prom this curve, it can be seen that as the grid of the tube 
is made more negative, less plate current will flow, since the more 
negative the grid the fewer electrons it permits to pass on to the plate*" 
In the case of this particular tube (type 6C5) , it will be noted from the 
characteristic curve that if the grid is made sufficiently negative ( — 10 


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10 -G -6 -4 -z 

Figure 61. Grid-voltage plate- current curve* 

volts), the plate current drops to zero. Thus, this value of negative 
grid voltage has cut off the flow of elect rons within the tube* A negative 
voltage which is applied to the grid of a tube to hold its plate current 
flow at a given value is known as the grid-bias voltage, or more simply , 
the hias; that value of grid bias which will cut on* the flow of plate 
current is called the cut-off hias for that tube; Since the plate current 
in a tube increases as the plate voltage is increased, the bias required to 
cut off plate current flow will increase as the plate voltage applied to 
the tube is increased. 

d. The triode fa now connected in a circuit (fig. 62) where an a-c 
(signal) voltage is applied to the triode! in addition to the grid-bias 
voltage. The a-c signal source is adjusted so that it applies 1 volt of a-c 
voltage to the circuit. Since the signal source and the 3 volts of negative 
bias are in series, on the positive half -cycle of the a-c signal there will 
be — 2 volts applied to the grid with respect to the cathode (+ 1 — 3 = 
— 2) ; on the negative half-cycle there will be — 4 volts on the grid 
of the tube { — 1 — 3 = — 4). From the grid-voltage plate-current 
curve shown in figure 61, it can be seen that when there is no a-c signal 


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Figure 62. Triode with an a-c signal on the grid, 

applied to the tube, the plate current will be fixed at 8 milliamperes 
by the 3 volts of bias supplied by the bias battery. When the a-e signal 
is applied to the tube, on the positive half -cycles there will he — 2 







Ep- i5GV t 





10 f 




\ _ * 


\ 1 










10 VdLTi 

I i 


-e -6 







Ffffvre €8. Plate twrrent waveform resulting from an a-c grid voltage. 


(~* I,, Original from 


volts on the grid of the tube and the plate current will increase to 10 
milliamperes ; but on the negative half-cycles there will be — 4 volts 
on the grid and the plate current will decrease to 6 milliamp^ree. 
Thus, a 1-volt a-c signal will cause a plate current change of 4 milli- 
amperes in this tube. This can be demonstrated graphically by showing 
the a-c voltage waveform on the grid- voltage scale of the Eq — 1? 
characteristic curve, and plotting the plate-current waveform on the 
plate-current scale of the graph (fig, 63 )♦ 

e. An examination of figure 63 will show that the waveform of the 
plate current variation is an exact reproduction of the waveform of the 
a-c voltage applied to the tube. By carrying this process further, it 
can be shown that if the negative bias is increased to 5 volts, so that 
the grid voltage varies from — 4 to — 6 volts ovst the a-c cycle, the plate 
current change will vary from 3 to 6 milliamperes, showing a total 
change of only 3 m ill i amperes. If the negative bias voltage is increased 
to 9 volts, so that the grid voltage varies from — 8 to — 1G volts 
over the a-c cycle, then the plate current change will be only 1 milli- 
ampere. From this it can be seen thst if the negative bias is increased, 
there is a resultant decrease in the plate current change for a given 
signal input. This method of controlling the output of a tube by 
varying the bias voltage is often used as a means of volume control, as 
will be shown later in the study of radio receiver. It should be noted, 
however, that if the grid voltage is increased to too high a negative 
value (fig. 64(T)), there is noticeable distortion of the output plate 
current wave. Distortion also results if the cathode temperature is 
lowered to such a degree that the emission is insufficient (fig. 64®). 
A distorted output is generally, but not always, objectionable. 

38. Triode Circuih; Plate Loads 

a. In order to make use of the variations in the plate current of a 

4 ' 

o + 





Figure €4, Distortion due to high grid bias and low cathode temperature* 


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triode due to variations in grid voltage, some sort of a device must be 
present in the plate circuit of the tube to act as a load. This plate load 
can be a resistor, an inductor, or a tuned circuit. 

b, A typical triode circuit with a resistor used as a plate load ia 
shown in figure 65. If the tube in this circuit ia biased at — 3 volts 
and the applied a-<s signal voltage to the grid is 1 volt the plate current 
variation of 4 milli amperes will produce a voltage variation of 40 
volts across the 1 0,000 -ohm resistor. On the positive half -cycles, the 
negative voltage of 2 volts applied to the grid causes a current flow of 




Figure 65* Triode using a resistor as a plate load. 

10 milliamperea through the plate load resistor, thus producing a volt- 
age drop of 100 volts (by Ohm's law). On the negative half -cycles, the 
negative voltage of 4 volts applied to the grid causes a current flow o£ 6 
milliamperes through the plate-load resistor, and a corresponding volt- 
age drop of 60 volts. The difference between these two voltage drops, or 
40 volts, is the voltage variation in the plate circuit produced by the a-e 
voltage applied to the grid. Thus it can be seen that a signal voltage 
change from — 1 to + 1 (or a total change of 2 volts) can produce a 
voltage change of 40 volts in the plate circuit; in other words, the 
original (grid) signal voltage has been amplified 20 times. This 
process is the basis for all vacuum-tube amplification. 

c. The use of a resistor as the plate load of a vacuum tube has one 
disadvantage : its resistance will reduce the actual d-c voltage applied 


5070 100 



£5,000 A 
(AT 400 CTCLE3> 


c H IN!:! i t ■ 


Figure 66* Triode using an inductor as a plate load. 


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to the plate of the tube, and bo reduce the amplification of the tube. 
To overcome this loss in plate voltage, inductors are often used as plat* 
loada of vacuum-tube circuits (fig, 66). By choosing an inductor which 
has a high value of reactance at the frequency of the alternating current, 
a large voltage will be built up across the reactance, because of the plate- 
current changes in the tube. However, the d-c plate voltage applied to 
the plate of the tube will be quite high, since the d-c resistance of an 
inductor may be very small, and consequently the amplification of the 
tube will be increased. 

d* If it is desired to amplify a signal of a given frequency, a tuned 
circuit which resonates at this frequency may be used for a plate load 
(fig. 67) . Since the impedance of such a circuit will be very high at the 
resonant frequency, the signal voltage appearing across the tuned 


Figure 67, Triode using a tuned circuit at a plate load. 

circuit will also be high. By using a tuned circuit as the plate load for a 
vacuum tube, it is possible to obtain the amplification only at the 
resonant frequency of the tuned circuit. The circuit of figure 67 is 
typical of the r-f amplifier circuits used in radio transmitters, 

39, Triode CircuiH; Biating Methods 

a. There are several different methods of obtaining a negative grid- 
bias voltage for a triode. The simplest of these is the fixed bias, where a 
suitable negative voltage is obtained from a fixed source, such as bat- 
teries or a rectifier power supply. Examples of this type of bias are 
shown in figures 59, 62, and 65. 

?>. A vacuum-tube circuit can be arranged to produce its own bias, 
and such a method is known as self -bias. One type of self-biasing, 
called the cathode-returner esi&tor bias, is shown in a triode-amplifler 
circuit in figure 68. In this circuit, the plate current from the battery 
flows through the cathode resistor on its way through the tube and back 
to the battery through the plate-load resistor. Since the current is 
flowing through the cathode resistor toward the cathode, there will be a 
voltage drop across this resistor which will make the grid negative with 
respect to the cathode. This is the proper condition for biasing. The 


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"Hi 1 



TU M «,e <I'I'M|I 

Figure 6&> Trfode amplifier circuit with Mlf-biat. 

convenience of this type of bias is obvious, since it eliminates the need 
for a separate source of bias voltage. For this reason, eathode-resistor 
bias is widely used in both transmitters and receivers. Omission of the 
shunt capacitor, or too small a value of this capacitor, produces degen- 
eration {par* 102c) as a result of the variations of grid bias which then 
accompany the a-c pulsations of the plate current. This capacitor should 
have a low reactance at the signal frequency, thus keeping the cathode 
resistor from dropping the a-c signal voltage as well as the d-e plate 

c. Another form of self -bias is called the grid-Uak bim 7 and is used 
under conditions where grid current flows. Two examples of this type 
of bias are shown in figure 69. The bias results from the drop in poten- 
tial across the resistor when grid current flows on positive a~c signal 
swings. This resistor is called a grid-leak. The capacitor across the 
leak offers a low impedance to alternating current, so that the bias is 



J& 1 


Shunt arranffement. (g) Series arranffement. 
Figure £3. Grid-leak bias circuits. 


essentially steady in character and is a function of only the magnitude, 
or size, of the grid current. A disadvantage of grid-leak bias is that if 
for any reason the excitation is removed, the bias is removed also, and 
the plate current may assume dangerous proportions, causing damage 
to the vacuum tube, 

d* To combine the advantage of grid -leak and battery (or fixed) bias t 
transmitter amplifiers often use a combination of both types in series. 


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Some types of amplifier tubes are conveniently designed, as regards bias 
supply, to operate with the grid at cathode potential; these are known 
aa sera-bias tubes. 

40. Triod# Characteristic Curve* 

a« There are two general types of characteristic curves for triodes. 
One is for the case of no load in the plate circuit, and is called the 
static characteristic curve; the other is for the case of a load in the 
plate circuit, and is known aa the dynamic characteristic curve. Use 
has already been made of the static curve in figures 61, 63, and 64, where 
the tube was operating without a plate load, In practice, however, the 
output of a tube feeds into some sort of load which can he represented 
by a resistance value (assumed to be the equivalent of the load). This 
results in dynamic characteristic curves that reflect more accurately 
the operating conditions of the tube, A comparison of the static and 
dynamic curves, with the two circuits that are used to obtain each, is 
shown in figure 70®. The difference in the slope of the two curves is 
dlue to the fact tha,t the plate-to-cathode potential for no load is constant 
regardless of the plate current, whereas with a load in the plate circuit 
the potential across the load (and consequently the plate-to-cathode 
potential) varies with the current. Assume that the normal operating 
point is the same for the tube with or without external load? that is, 
regard the operating point as the point of intersection of the two curves 
of figure 70®. Without an external load (fig. 70®) on a positive 

® * 


Without external load, (t) With external load 

(D Corresponding characterietiea. 

Figure 70. Triode characteristic curve*. 

swing of signal potential A (fig. 70®), the plate current rises by an 
amount B r With an external load (fig. 70®), the increase in current 
which follows a positive grid swing is in turn accompanied by a 
potential drop (I X -K) across the load resistor (as read by voltmeter 
V 2 ) r Thus the potential available across plate to cathode within the 
tube (aa read by voltmeter YJ is reduced? and the consequent increase 
In current C is less than it was under the no load condition. On the 
negative half -cycle of the signal voltage, the plate current is reduced, 


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and the potential drop across the load is less than it is when no signal 
is applied. Thus the voltage across the tube rises, so that the available 
plate-to-cathode potential exceeds the corresponding' value under the 
no load condition, A typical set of static plate-current grid-voltage 
curves for various plate potentials is shown in figure 71. Many hand- 
books on vacuum tubes confine the characteristics illustrated to families 
of curves of the static type. 

Figure 71 Flote-current v*. grid-potential curves for triode* 

b. Observe fTom the set of static characteristic curves of figure 72, 
that of the three quantities, grid potential, plate potential, and plate 
current, aaay two will determine the third. Thus, corresponding to a 
plate current of 10 milliamperes and a plate potential of 50 volts, the 

n*TE P6T£NTI*L ■» VOLT* 


Figure 7£. Plate-current vs. plate. -vult age curves for a triode. 

required grid potential is — 8 volts. Suppose it is desired to obtain 
these same relations — plate current, 10 milliamperes \ plate potential, 50 
volts ; and grid potential — 3 volts — -with a load resistance of 4,000 
oh ins. This requires a total plate-supply potential of 50 + [4,000 X 
(10/1,000)] volts = 90 volts, 50 across the tube and 40 across the 
load resistance. The current in the load resistance follows Ohm's law, 
that is, the current through the resistance is proportional to the 
potential across it. This proportionality can he represented b^ a 
straight line on the current- voltage graph of figure 73. The line is 
determined by any two points on it, two convenient points being P and 


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50 »0 IO 10 »o 40 SO » M «o » 


(J) 1X**M 

fi^Kffl 7J„ X^od Hw for a triode r 

Q t as in figure 730. P is for a current of 10 milliamperes and a 
voltage drop across the resistance of 40 volts (50 volts across the tube) ; 
Q is for zero current and zero drop across the resistance (90 volts 
across the tube). If P is taken as the normal operating point, the grid 
swing due to an impressed signal voltage will cause variations along 
this load line in both directions from P. Corresponding to an in- 
stantaneous grid potential of 10 volts, the plate current, plate voltage, 
and -voltage across the load can be found by following the 10- volt 
characteristic to where it intersects the load line. From the curves of 
figure 73®, this yields 16 mill iam per es plate current, 25 volts plate 
potential, and 90 — 25 = 65 volts drop across the load. The family 
of plate-current plate-potential curves is thus useful in determining 
the limitations of a particular tube under various operating conditions. 
A particular tube can be selected to fit certain circuit constants, or 
vice versa, with the aid of the information contained in the vacuum- 
tube characteristics. 

41. Special Characteristic* of Vacuum Tuba* 

a. Since many different types of vacuum tubes are used in modem 
radio circuits, it is important to have different means of classifying 
these tubes according to the performance which may be expected of 
them. Among these characteristics, as they are called, are the amplifica- 
tion factor, the mutwU conductance , and the plate resistance of the tube. 

b. The amplification factor p, or mil, of a tube is the ratio of the 
plate- voltage change and the grid-voltage change required to produce 
the same plate-current change in the tube. For example, if the plate 
voltage of a tube must be increased by 20 volts in order to increase the 
plate current as much as would a 1-volt change of grid voltage, then 
the tube has an amplification factor of 20. The amplification factor of a 
tube is stated for a given set of operating conditions, such as grid-bias 
voltage, plate voltage, etc., since the amplification factor will change if 


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these conditions are changed. The amplification factor of a tube gives 
a theoretical approximation of the maximum voltage amplification 
which can be expected from the tube under given operating' conditions. 

c r The mutual conductance, or transconductance, of a tube ia a char- 
acteristic from which the power sensitivity can be estimated, since it 
determines what plate-current change may be expected from a given 
grid- voltage change under a given set of operating conditions. Mutual 
conductance, or transconductance, is the ratio of a small change in 
plate current to the change in grid voltage producing it. It is measured 
in mhos, which is simply the word ohm spelled backwards and with an 
"s" added. For example, if a grid voltage change of 1 volt produces a 
plate-current change of 1 ampere in a given tube under certain operat- 
ing conditions, the tube will have a mutual conductance of 1 mho. 
But since very few tubes will stand a plate current flow of 1 ampere 
(receiving tubes draw only a few milli amperes of plate current), it is 
more convenient to rate mutual conductance in micromhos (or mil- 
iionths of a mho). Thus, if a tube has a mutual conductance of 5,000 
micromhos, a 1-volt change in grid voltage will produce a 5 milliampere 
change in plate current. 

d. The plate resistance of a tube is simply the resistance between the 
cathode and plate of the tube to the flow of alternating current, It is 
the ratio between a small change in plate voltage and the correspond- 
ing change in plate current. For example, if a 10- volt change in plate 
voltage produces a 1 -milliampere change in plate current t the plate 
resistance of the tube is 10,000 ohms. 

42. Intorelecfrode Capacitance 

The inherent capacitance between grid and plate elements of a triode 
ia of sufficient importance at high frequencies to require special consid- 
eration in radio circuits. "Where this capacitance is undesirable, it can 
be counteracted by introducing a neutralizing eircuit which presents 
r-f potentials equal in magnitude but opposite in phase to those occur- 
ring across the interelectrode capacitance, with the result that the 
effects of the interelectrode capacitance are nullified. The extra circuit 
complications can generally be avoided by the use of tetrodes or pen- 
todes, 4- and S^element tubes t respectively, which are particularly de- 
signed to have low interelectrode capacitance. The gTid-plate capaci- 
tance of an ordinary receiving triode runs about 3 micromicrofarads. 
This represents a capacitive reactance of 53,000 ohms at 1 megacycle 
and only 530 ohms at 100 megacycles. Tetrodes and pentodes offer 
corresponding reactances of about 16,000,000 ohms at 1 megacycle and 
160,000 ohms at 100 megacycles. 

43. Tetrode 

a In an effort to reduce the grid -plate capacitance within the tube 
(par. 42), a fourth element was added to the conventional triode. This 


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fourth element is called a screen grid t and is placed between the grid 
and the plate of the tube. A typical screen grid, or tetrode (4-element) 
tube connected in a circuit ia shown in figure 74. Observe the changes 
in this circuit due to the addition of the screen grid. Notice that the 
screen grid is operated at a positive voltage somewhat lower than that 

Flow /* 


Scr&a/r Gr/d 





Figure 74. Tetrode amplifier circuit* 

applied to the plate. Since it is operated at a positive voltage, the 
screen assists the plate in attracting electrons from the cathode. Some 
of these electrons will be attracted to this grid by the positive voltage 
on it, thus causing screen current to flow in the circuit. However, since 
the construction of the screen grid is similar to that of the control 
grid, most of the electrons will pass through the spaces between its 
wires on to the plate , because of the attraction of the higher positive 
voltage on the plate. Since the screen grid is bypassed to the negative 
side of the circuit (bypassed to ground) by a screen bypass capacitor 
having a small reactance at the signal frequency, it acts as a shield or 
screen between the grid and the plate, and thus effectively reduces the 
capacitance between these two electrodes* 

b. If the screen grid in this circuit is not operated at a positive 
voltage, but is connected to the cathode, it will have a controlling effect 
on the electron flow, similar to that of the control grid of the tube, 
thus reducing the plate-current flow to a value too small for satisfactory 
operation. The value of a positive voltage on the screen grid of a tetrode 
will determine to a large extent the maximum value of current which 
will flow in the plate circuit. Thus, improper screen voltages can cause 
faulty operation in tetrode amplifier circuits. 

o. The tetrode has several advantages over the triode, in addition to 
its greatly reduced grid-plate capacitance. Among these are a" higher 
amplification factor, and greater power sensitivity. In general, tetrodes 
can be used for the same purposes as triodes. Since they were deveJ- 


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oped to overcome the need far neutralization in r-f amplifier circuits, 
tetrodes have been widely used in the r-f amplifier stages of radio re- 
ceivers and transmitters, 

44, Paittoda 

a. Although the tetrode would seem to be an ideal tube, since it over- 
comes the disadvantage of the higher grid-plate capacitance of the 
triode and, at the same time, is capable of providing higher amplifica- 
tion in a circuit than is the triode, the effect known as secondary emis- 
sion limits its application to a great extent. The pentode, or 5-element 
tube, was developed to overcome the effect of secondary emission. If a 
tetrode is operated at fairly high plate and screen voltages, and large 
values of signal voltage are applied to its control grid, the electrons 
strike the plate with sufficient force to knock loose other electrons al- 
ready on the surface of the plate. These other electrons, known as 
secondary electronSj are attracted by the positive voltage on the screen 
grid. When secondary emission occurs, the screen gets more than its 
share of the available electrons, while the number reaching the plate 
is greatly reduced. Thus, the screen current will increase while the 
plate current will decrease, causing a reduction in the amplification of 
the tube and distortion in its output. 

b* If a third grid is placed between the screen grid and the plate of 
the tetrode, and is connected to the cathode so that it will have the 
same charge as the electrons, it will force any secondary electrons back 
to the plate, since like charges repel one another. This third grid is called 
the suppressor grid, since it suppresses the effects of secondary emis- 



Figure 75* Pentode amplifier circuit. 

si on by preventing the flow of secondary electrons to the screen. The 
suppressor grid will not reduce the electron flow to the plate, even 
though it is operated at a negative potential. This is because it is 
placed SO close to the plate that the attraction of the positive voltage 
on the plate is much greater than any tendency on the part of the 
suppressor grid to repel the electrons. 


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c. A pentode used with a typical amplifier circuit is shown in figure 
75. Note that the only difference between this circuit and the tetrode 
amplifier circuit of figure 74 is the addition of the suppressor grid. 
Both the cathode and the suppressor grid are at the same potential. 


ruealuitd to 
001 Inch 


O.QO? inch lhicli 


Of atmospheric p*et r 
sure it s* j fev^l 


trnfHAd under 

fnUrifftt |igti( 

for ilriini 


Figure ?S. Typical Pentode. 

(L The construction of a typical pentode power amplifier tube is 
shown in figure 76. Such a tube is suitable for use in the power-output 
stages of radio receivers. 

45, Variable-mu Tube 

a. The amplification of a tube is controlled by varying the bias volt- 
age applied to the grid, but normally the range of this control is In* 1 * 
ited by the value of cut-off bias for the tube. It is most desirable in the 
r~f amplifiers of receivers, the gain of which is controlled by automatic 
volume control f to be able to vary the amplification over a much wider 
range, so that large values of signal voltage (strong signals) may ^ 
handled. To permit this increased range of gain control, the variable- 
mu tube has been developed. This type of tube is also known by several 
other names, two of which are supercontrol and remote cut-off. The 
only difference in construction between variable-mu tubes and norni^ 
or sharp cut-off, types, is the spacing of the turns of the grid. In sharp 
cut-off tubes, the turns of the grid wire are equally spaced, while in 
remote eut-off types the grid turns are closely spaced on both ends 



.. s 

Original from 

and widely spaced in the center* When small negative voltages are 
applied to the grid of a variable-mu tube, the electrons will flow through 
all the spaces in the grid. As the negative voltage is in creased t how- 
ever, the electrons will no longer be able to pass through the narrow 
spaces on the ends of the grid structure, though they will still be able 
to pass through the relatively greater spaces at the center of the grid. 
A much greater value of negative voltage will thus be required to cut 
off the plate- current flow in this type of tube. This remote cut-off tube 
is so named because the cut-off bias value is greater than (remote 
from) the value required to cut off plate-eurrent flow in tube of evenly 
spaced grid turns. 

& # Figure 77 shows the E G —I P curves for a typical sharp cut-off pen- 
tode and a typical remote cut-off pentode on the same graph. Note that 
the cutoff bias for the tube with the uniformly spaced gr id is — 6 volts. 

VT-91 (flJT) AND A VT-flfl fflR7^ 


YT-86 (6M7J super -control gr?d 

VT$t (&J?) ttfiiferTfty ty&ee, 

-24 ■& ^0 -16 -W 

Grid Bias to 

Figure ??. Comparison between a sharp cut-off pentode and 

a remote cut-off" pentode. 

Thus the range* of gain control which can be effected by grid-bias 
variation, and the maximum value of signal voltage which can be ap- 
plied to the grid, are both limited. But the curve for the supercontroi 
pentode shows that plate current still flows even at a grid bias of — 24 
volts. Thus, by the use of a variable-mu tube, both the range of gain 
control by grid bias variation and the value of signal voltage which 
can be handled by the grid have been extended several times. 

c~ Variabie-mu pentodes are used in the r-f amplifier stages oE prac- 
tically all modern radio receivers. They are not generally used in a-f 




.. s 

Original from 

amplifiers, however, because of extreme curvature, or non linearity, of 
their E G — 1? curves, which would result in distortion of the output 
voltage when large signal voltages were applied to their grkU. 

46. Beam-power Tube 

a. In recent years a new type of power-amplifier tube has been de- 
veloped. Compared with other tetrode and pentode power-amplifier 
tubes, this tube has the advantages of higher power output, higher 
power sensitivity, and higher efficiency. This type of tube is called the 
beam-power tube, since by its construction the electrons are caused to 
flow in a concentrated beam from the cathode, through the grids, to the 
plate. The only difference in construction between the beam-power tube 
and normal tetrodes and pentodes is that the spaces between the turns 







Figure 7S r Internal structure of a be&m-powcr tube. 

of the several gTids are lined up and two beam-forming plates are 
provided. Figure 7$ shows the internal const ruction of a beam -power 
tetrode. Since the spaces between the turns of the gTids are lined up, 
fewer electrons will strike the screen grid. The screen current will 
therefore be decreased f while the plate current w T ill be increased. Since 
the power output of a circuit is proportional to the value of plate cur- 
rent flowing through the load, the power output ivill thus be increased. 
The two beam -forming plates are usually connected to the cathode &ut\ t 
having the same charge as the electrons, cause them to flow in a beam 
from the cathode, through the grids, to the plate. The placement of the 
beam-forming plates is such that it forces the electrons to flow through 
the desired portions of the grids, and prevents them from striking 



.. s 

Original from 

the wires which support the grids. Thus, by causing the electrons to 
flow in a beam, the number of electrons reaching the plate can be in- 
creased , thereby greatly increasing the operating efficiency of the tube, 
b. Figure 79 illustrates an a-f power-amplifier circuit using a beam- 
power tetrode* Notice that in this case the beam-forming plates are 
connected to the cathode inside the tube. 

scan -FOftruNt 


TO PfftCTCtfH^ 




Figure 7&. Beam^power tetrode, e\~f power amplifier. 

c. A beam-power tube operated at the same voltages as a normal 
tetrode or pentode type will provide more power output for a given 
value of signal (input) voltage than the latter, and have a much 
higher plate-circuit efficiency. Both beam tetrodes and beam pentodes 
are used in radio receivers and transmitters. In beam tetrodes, the 
effect of secondary emission is reduced to a minimum by the action 
of the beam, and the replacement of the beam-forming plates, Beam- 
power tubes are widely used as r-f and a-f amplifiers in radio trans- 
mitters, and as output a-f amplifiers in radio receivers* 


47, Multielement Tube* 

a. In addition to the diodes, triodes, tetrodes, and pentodes which 
have been studied, there are many special types of vacuum tubes used 
in radio circuits; a large number of types are used which combine 
the electrodes of two or more tubes in one envelope. These complex 
tubes are usually named according to the equivalent single-tube types 
of which they are composed. Thus a twin triode contains the electrodes 
for two triodesjn one envelope. Other complex tubes are diode triode^ 
diode pentodes, triode pentodes. One complex type has recently been 
introduced which combines the functions of three tubes within one 
envelope, namely, a diode, a triode, and a power-output pentode. All 
of these tubes however complex follow the basic rules for tube operation. 


Original from 

To understand the operation of any one of them in a circuit it is only 
necessary to consider the effect of the various electrodes on the flow 
of electrons within the tube. 

b* The pentagrid'Converter tube is a special type which has five 
grids, and is used in a certain stage of the superheterodyne receiver 
to take the place of two separate vacuum tubes. The pen tagr id -converter 
tube is used for frequency conversion. (See sec. VIII,) 

c. The duplex-diode tri&de and the duplex-diode pentode are two 
popular types of receiver tubes. In receiver circuits, one of the diodes 
is used together with the cathode as a diode-detector circuit* while 
the other diode is used together with the cathode to rectify the signal 
voltage in order to produce a source of automatic volume control- The 
triode or pentode section of such tubes is used as an a-f amplifier. 

48* Directfy and Indirectly Heated Cathode* 

a. A cathode which is in the form of a filament directly heated by 
passing a current through it has the disadvantage of introducing a 
ripple in the plate current when alternating current is used for heating. 
The ripple is most objectionable if the plate and grid returns are 
made to one end of the filament. In figure 80 the resistor AB repre- 

3 VOiT* fcC 





Figure SO. Directly heated cathode. 

sents a filament which is heated by applying 5 volts of alternating 
current across it* When no current flows through the tube, the plate 
is maintained at a potential of 100 volts above that of point B* For 
a 5-milliampere steady plate current, the potential across the tube 


from B to the plate is always 100 — [2,000 X 



90 volts; 

whereas the potential from A to the plate varies from 85 to 95 volts, 
depending upon the potential of point A relative to point B. The 
total plate current rises and falls at the frequency of the filament 


C Io< 

Original from 

current. This condition is remedied to a large extent by connecting 
the grid and plate returns to the electrical center of the filament* 
as in figure 81© or ®. But even with a center-return arrangement, 
with a 60-cycle filament current, there is still present a 120-cycle 
modulation of the plate current. This double- frequency ripple arises 
from the effects on the plate current provided by the intermittent 
rise and fall of the filament temperature, the voltage drop in the 
filament, and the alternating magnetic field set up by the filament 
current. Temperature fluctuations in the filament are ordinarily neg- 
ligible. The magnetic field about the filament serves to deflect the 
electrons from their normal paths j and, in effect, serves to reduce 


h- - 










Figure 32. Methods of utilising a-o filament supply* 

the plate current. The resulting plate current is largest when the 
heating current is zero, that is, at intervals which occur at double 
the heating current frequency. With a voltage drop in the filament, 
the space current from the negative half of the filament exceeds that 
from the positive half, because of the manner in ■ which space current 
varies with the electrostatic field across the tube. (Space current 
varies as the three-halves power of the plate potential.) The result is 
that each time the current is at a maximum in either direction in the 
filament, that is, at a frequency which is double the heating- current 
frequency, the space current is slightly greater than its value during 
those instants when the current through the filament is zero and the 
potential of the filament is uniform, 


Original from 

b. In transmitting tubes and in the power stages of a receiver the 
signal currents are large, and the double-frequency ripple current 
is negligible in comparison. However, in all other receiver tubes, 
indirectly heated cathodes (fig. 81©) are necessary wherever a-c fila- 
ment operation is desired. An indirectly heated cathode is formed 
by a metallic sleeve closely surrounding a heated filament and electri- 
cally insulated from the filament. The cathode is heated by radiation 
from the filament. Such an emitter is sometimes referred to as an 
equipotential cathode, since all parts of it are at the same potential 
For purposes of simplicity, tube-heater elements and heater-power 
circuits are not shown in circuit diagrams throughout this manual. 


C ti 

Original from 



49. Detection 

a. There are two general kinds of radio- frequency (r-f) signals that 
can be received by a radio receiver : modulated r-f signals which carry 
speech, music, or other audio sounds, and continuous wave (c-w) 
signals which are "bursts" of r-f energy conveying code. These types 
of r-f signals are described in more detail in sections XI and XII. 
The process whereby the intelligence carried by a r-f signal is ex- 
tracted as an a-f (audio-frequency) signal is called detection, or 

b. The modulated r-f signal can be detected by any one of several 
types of vacuum-tube detectors: the simple diode detector, the grid- 
leak detector, the plate- detector, or the regenerative detector. The c-w 
signal is generally detected by the heterodyne detector. 




Figure 8$. Formation of a modulated wave farm* 



.. s 

Original from 

SOL Phon* Detection 

In paragraph 5 it wag shown that a radiotelephone or a modulated 
signal is produced by controlling the r-f output of a transmitter at an 
a-f rate. The chart in figure 82 shows an r-f voltage, an a-f voltage, 
and the two of them combined to form a modnlated-signal voltage. Ttw 
modulated signal is the waveform of the voltage which will appear in 
the antenna circuit of a radio receiver when a modulated wave is being 
received. The detector, then, must separate the a-f voltage from the r-f 
voltage, so that the a-f voltage can be converted into sound by means 
of a headset or loudspeaker. The detector must demodulate the signal. 

SI. Diode at Detector 

a. In the study of the diode as a rectifier (par. 35) it was shown 
that the diode is a conductor when the plate voltage is positive, and 
that it is a nonconductor when the plate voltage is negative. This 
property of the diode makes the tube useful for the detection of 
r-f signals, 

b. The action of the diode as a detector can best be explained by an 
examination of a simple diode radio receiver (fig. S3). In this receiver 
the modulated r-f signal voltage will appear across the parallel -tuned 



Fiffttre 8$. Simple diode radio receiver, 

circuit formed by the coil L and the variable capacitor C± when this 
antenna circuit is tuned to resonance with the incoming r-f signal. 
Since the diode is connected to this antenna circuit, it will rectify the 
signal voltage, and the rectified-signal current will flow through the 
headset, thereby producing sound. Obviously, the a-f part, or com- 
ponent, of the voltage which appears across the headset must not be 
filtered out, as this voltage produces the sound. But the headset will 
have an extremely high reactance at the frequency of the incoming 
signal, which would reduce the amount of r-f current flowing in the 


Original from 

circuit. For this reason capacitor C z is placed across the headset 
(fig, 83). The size of this capacitor is chosen so that it will have a low 
reactance at the radio frequencies, and a relatively high reactance at 
the audio frequencies, thus providing minimum opposition to r-f current 



Figure 84. Effect of a bypass capacitor. 

flow in the entire circuit* while providing maximum opposition to a-f 
current flow* Consequently, the maximum a-f voltage appears across 
the headset* Figure 84 shows this rectified voltage appearing across the 
headset, both with and without bypass capacitor C 2 connected. 

c. The action of the diode as a detector is essentially the same as its 
action as a rectifier, since the diode actually detects the r-f signal by 
rectifying it, The circuit shown in figure 83 is the basic detector circuit 
for many of the radio receivers now in use. However, since the diode 
does not amplify the signal it is detecting^ its use as a detector requires 
several preceding stages of r-f amplification to bring the level of the 
signal rap to a point of satisfactory output. This is done in modern 
radio receivers with a large number of tubes. If, however, a radio set 
is to use a smaller number of tubes, and consequently have fewer 
stages of amplification, it must have a detector winch is more sensitive 
than the diode; in other words, the detector must amplify the signal as 
well as detect it. The triode as a detector fulfills this requirement. 

52. Grid-leak Detector 

a. The grid-leak det&ctor functions like a diode detector followed by 
a stage of triode amplification. Figure 85 shows only the grid and the 
cathode of a triode connected as a diode detector; the triode grid acts 



triode: grid 


Zfiffure 35. Diode action in a triode* 


Original from 

aa the plate of the diode. It can be seen that the grid-leak resistor 
forms the load for the diode circuit , while the grid capacitor is th« r-l 
bypass^ or filter capacitor, in the circuit. When a modulated a-c signal 
voltage is applied to the circuit of figure 85, current will flow through 
the tube only on the positive half -cycles, and consequently the signal 
will be rectified, or detected. Since electrons flow only from the cathode 
to the "plate" of the diode, the voltage drop across the grid-leak 
resistor, caused by the current flow on the positive half-cycles, will 
make the diode *'plate ,T (the triode grid) negative with respect to the 
cathode. This rectified-signai voltage thus acts as bias for the triode grid, 
&. Consider nest the complete grid-leak detector circuit shown in 
figure 86. Since the bias for the triode is produced by rectifying the 
modulated-signal voltage, the bias will increase and decrease in value 
in proportion to the modulation on the r~f signal (at an a-f rate). In 
other words, the grid voltage will vary in just the same manner as it 
did in figure 62, where an a-c voltage was applied to the grid (of a 
triode) m series with a source of fixed negative grid bias. Since the 
triode plate current is determined by the grid voltage, the plate current 
in the circuit shown in figure 86 will vary in proportion to the voltage 
appearing across the grid-leak resistor* The plate current in this 
circuit flows through the headset as a load. The voltage drop across 






Figure 8€. Grid4eak detector circuit. 

the headset, produced by the variations in plate-current flow, will 
therefore be an amplified reproduction of the voltage appearing across 
the grid-leak resistor. The capacitor connected across the headset in 
figure 86 bypasses any r-f voltage {amplified hy the tube) around the 
headset. Since the plate current in a circuit decreases as the grid is 
made more negative, the average plate current of the grid-leak detector 
circuit will decrease as the applied signal voltage becomes greater. The 
maximum plate-current flow will occur in this circuit when no signal is 
being received, because at that time there is no bias voltage developed 
by the grid leak. Since the actual detection of the signal in the grid-leak 
detector takes place in the grid circuit, this type of detector is also 
known as a grid detector* 


Original from 

c. The chief disadvantage of the grid-leak detector circuit is that it is 
easily overloaded by strong r-f signals with consequent distortion of 
output. When grid-leak detectors are used to handle large r-f signal 
voltages, they are called power detectors, and are sometimes used in 
radio receivers which have several stages of r-f amplification preceding 
the detector stage. 

53. Plate Detection 

a. When the triode*detector circuit is arranged so that rectification 
of the r-f signal takes place in the plate circuit of the tube, such a 
circuit is called plate detection. If a sufficient negative grid bias is 
applied to a triode circuit so that the plate-current now is cut off when 
no signal is applied, the proper conditions have been established for 
plate detection. This cut-off bias may he supplied either by means of & 
cathode resistor; or by means of a fixed source of bias {fig, 87). If a 
modulated r^f signal is applied to the circuit of figure 87 1 plate current 




1 1 III III 


Figure 87. Plate detection. 

will flow during the positive half -cycles of the r-f voltage, since the 
positive voltage will cancel part of the negative bias voltage, thereby 
reducing the grid voltage below the cut-off point* Plate current will 
not flow during the negative half -cycles of the r-f voltage, since the 
negative voltage merely adds to the bias voltage, making the grid more 
negative. Thus, the tube acts as a plate detector , since plate current 
flows only during the positive half-cycles of the r-f voltage. 

b. The action of the plate detector can be further demonstrated by 
means of the Eq — I P curve shown in figure 88* The modulated r-f is 
applied to the grid-voltage scale of the graph, and the resultant plate- 
current waveform is developed on the plate-current scale. Since cut-off 
bias is applied to the plate detector, no plate current will flow when no 
signal is applied to the circuit* The average value of the plate current 
will increase as the strength of the applied signal is increased \ this 
effect is opposite to that of the grid-leak detector. In general, the plate 
detector is less sensitive than the grid-leak detector, but it has the 
advantage of being less easily overloaded. 


Original from 


Ip-£e Cfiervcfortsftic 

Ptefa Current 

Aver&ge Vatite of 



App/sea J& TAeGrtit 

Figure 88. Operating condition* of a plate detector. 

54. Regenarativ* Detection 

a. The process of feeding some of the output voltage of c vacuum- 
tube circuit back into the input circuit, so that it adds to, or reinforces 
(is in phase with) the input voltage, is known as regeneration. The use 
of regeneration in a circuit greatly increases the amplification of the 
circuit, since the output voltage fed back into the input circuit adds to 
the original input voltage, thus increasing the total voltage to be 
amplified by the tube. 

b. Regeneration, sometimes called positive feedback, can be applied 
to a grid-leak detector circuit by connecting a coil in series with the 


Figure 89, Regenerative detector. 

plate circuit and magnetically coupling it to the grid coil (fig. 89). 
When an r-f signal is applied to the circuit, voltage will be built np 
across this feedback, or tickler coil {Zj 8 in fig. 89), because of the 
plate-current variations and the reactance of the coil. Since this tickler 
coil is magnetically coupled to the grid coil (L 2 on the diagram), trans- 
former action takes place between the two windings and a voltage is 
set up in the grid coil. Since the tickler coil has been so placed that the 
voltage it induces into the grid coil will be in phase with the incoming- 

Qriginal from 

signal voltage, the voltage feedback will add to the incoming r-f signal 
voltage and increase the total voltage to be amplified by the tube, thus 
increasing the amplification of the circuit. It is important that the 
position of the tickler coil with respect to the grid coil be correct, for if 
it is not and the feedback voltage is out-of -phase with the input voltage, 
it will cancel some of the input voltage, and thereby reduce the amplifi- 
cation of the circuit. In the circuit diagram of figure 89, the antenna 
coil 2/i, and the grid coil, Z 2j form an r-f transformer. Since there are 
more turns on L 2 than on L lt the voltage appearing 1 in the antenna 
circuit will be stepped up by the use of this transformer, thus producing 
additional gain in the circuit. The secondary of the transformer, -L 3 , 
and variable capacitor, C, form the parallel-tuned circuit of the set. 
C 2 bypasses any r-f currents in the plate circuit around the headset 
and the plate battery E B , As is very often the case in Army sets, the 
filament is heated by means of a battery* The regenerative-detector 
circuit of figure 89 is the most sensitive triode- detector circuit possible, 
< and when used as a receiver it is capable -of receiving signals over 
extremely long distances under good conditions, 

55. C-w Defection 

All detector circuits previously discussed are used to detect modulated 
signals, since they separate the audio frequencies from the radio fre- 
quencies. All of these detector circuits will also rectify unmodulated, 
or continuous-wave (e-w) signals, but no a-f voltage will appear in 
their output circuits, since there is no a-f voltage component present 
in an unmodulated signal. In order to receive c-w signals from a 
radiotelegraph transmitter, it is necessary to have some method of 
producing an a-f voltage in the detector circuit when an unmodulated 
r-f signal is being received. 

56« Heterodyne Detector 

a. If two a-c signals of different frequencies are combined, or mixed, 
in a circuit, a third signal, called a beat frequency, will be produced. 
The frequency of this beat is equal to the difference between the fre- 
quencies which are mixed to produce it- Thus, if two a-f voltages are 
combined, the frequencies of which are 500 and 600 cycles per second 
respectively, a beat frequency of 100 cycles will be produced. 

&. If two r-f signals are combined, the frequencies of which differ 
by an audio frequency, a beat frequency of an a*f voltage will be 
produced. For example, if a 1,000-kilocycle signal is mixed with a 
1,001 -kilocycle signal, a beat, with a frequency of 1 kilocycle (1,000 
cycles, or an audio frequency), will be produced. If some way can be 
found of generating a signal in a detector circuit, the frequency of 
which differs from the frequency of the incoming signal by an audio- 
frequency amount, then an a*f voltage will be produced in the circuit. 


("nn'ilc Original from 


This can be done by making the regenerative-detector circuit oscillate, 
If the regeneration, or positive feedback, in a regenerative-detector 
circuit is increased beyond a certain critical point, the circuit will 
oscillate, or produce an alternating current, the frequency of which 
is equal to the resonant frequency of its tuned circuits. Thus, by making 
the regenerative detector an oscillating detector, and tuning it bo that 
the frequency it generates will differ from the incoming r-f signal 
frequency by an audible amount, it is possible to detect unmodulated 
r-f signals. This process is known as "heterodyning," and an oscillating 
detector is called a heterodyne detector. The heterodyne principle is 
used in radio receivers whenever c-w reception is desired, It is also the 
basis for most of the oscillator circuits used in transmitters and receivers. 

57. Vacuum-tube Voftmefor 

o. The plate-detector circuit, discussed in paragraph 53, is used as 
the basis for a very important measuring device in radio : the vacuum- 
tube voltmeter ; A circuit diagram of the vacuum-tube voltmeter is 
shown in figure 90, and its similarity to the plate- detector circuit will 

ftR ID 














Figure 90. Vacuum-tube voltmeter.. 

be obvious. When no voltage is applied to the grid of this .circuit, no 
plate current will now, since the grid is biased to cut-off. If an a-c 
voltage is applied to the grid, however, plate current proportional to the 
peak (or highest value of the applied voltage) will flow, and operate the 
milliammeter (which replaces the headset of the plate-detector circuit). 
If a d-c voltage is applied, the plate current indicated by the milli- 
ammeter will be proportional to the applied voltagej provided that the 
positive terminal of the voltage being measured is connected to the 
grid, and the negative terminal is connected to the bias battery. 

&♦ By calibrating the milliammeter so that it reads either a-c volts or 
d-c volts, or both, the circuit becomes an effective voltage-measuring 
device. The advantage of the vacuum-tube voltmeter is that it draws 
little or no current from the source of voltage being measured. This 
is in contrast to conventional meters, and thus gives far more accurate 
results when critical measurements are being made. 


Original from 



53, Voltago and Power Amplifier* 

a. The basic manner in which a signal can be amplified by a vacuum 
tube (par. 37} can be applied to vacuum-tube amplifiers which fulfill 
various special requirements of transmitters and receivers. Tbe impor- 
tance of amplifier circuits can be seen from their wide variety of 
uses in radio work. 

(1) In transmitters, the r-f power generated by the oscillator is 
too small for satisfactory long-distance transmission; therefore, r-f 
power-amplifier stages are used to increase this power to the desiretl 
level before transmitting. 

(2) The a-f voltage output of a microphone is too small to operate 
tbe modulator stage of a radiotelephone transmitter; therefore, Orf 
voltage-amplifier stages are used to increase tbe output of the micro- 
phone to the amount required for proper operation of the modulator, 

(3) R-f voltage-amplifier circuits are used in receivers to increase 
the strength of weak signals, so that satisfactory detector operation 
may be realized. 

(4) A-f voltage-amplifier stages are also used in receiver to amplify 
the a-f output of the detector stage for greater headset volume. 

(5) If loudspeaker operation is required in a set, the output a-f 
amplifier stage will be an a~f power amplifier. 

b. From this discussion of amplifier circuits, it may be concluded 
that a vacuum-tube amplifier stage , either r-f or a-f, can be classified 
as a voltage amplifier or a power amplifier, according to the purpose 
for which it is to be used. 

c Voltage amplifiers are amplifier stages designed to produce a large 
value of amplified-signal voltage across a load in the plate circuit. 
In order to produce the largest possible value of amplified-signal 
voltage across the load of such a circuit, the opposition of the load 
to plate-current change (that is, its resistance, reactance, or impedance) 
must be as high as is practically possible. 

d> Power amplifiers are amplifier stages designed to deliver a large 
amount of power to the load in the plate circuit. In a power amplifier, 
not only must there be a large output voltage across the load, but 
theTC must also be current flowing through the load, since power 
equals voltage times current* 


Original from 

_| t Original mom 


e. Voltage and power amplifiers can be recognized by the charac- 
teristics of their plate-circuit elements. Thus, an amplifier stage 
designed to produce a large amp lined-signal voltage across a high 
impedance is a voltage amplifier, while one designed to deliver a 
relatively large plate-current flow through a load of lower impedance 
is a power amplifier. Although any vacuum tube may be operated as 
either a voltage or power amplifier, certain tubes hav* been developed 
which serve best as voltage amplifiers, while others have been designed 
for use as power amplifiers. These are referred to as voltage-amplifier 
tubes and pouter-amplifier tubes, respectively, 

/. In addition to the two general types of amplifiers just discussed, 
there is a further classification of both voltage and power amplifier*. 
The operation of all vacuum-tube amplifiers may be classified according 
to the bias voltage applied to their grids, and according to that portion 
of the a-c signal -voltage cycle during which plate current flows. These 
types of amplification are designated as class A f class AB r class B, 
and class C. 

59* Claw A Amplification 

a. If the grid of an amplifier tube is biased so that plate current will 
flow during the entire cycle of the applied a-c signal voltage, the 
circuit is called a class A amplifier. The class A operation of a tube is 
illustrated graphically by the grid-voltage plate-current curve of 

£*-(p curve 




tl titer 

Figure 91. Clots A operation. 

figure 91. An examination of this graph will show that plate current 
flows during both the positive and negative half -cycles of the a-c signal 
voltage applied to the grid. Notice that the Eq — t F curve of figure 91 


Original from 

is not linear over its entire length, that is, it is not a straight line. 
In order to produce a plate*current waveform which, as nearly as 
possible, is an exact reproduction of the signal -voltage waveform, the 
tube must be biased so that it will operate on that portion of its E G — I P 
curve (fig. 91) which is a straight line. 

ft. If the grid of the tube is biased incorrectly, so that the grid 
voltage varies over a nonlinear portion of the curve, a distorted plate* 
current waveform will result (fig. 92). Since the plate current varia- 

e* i F curve 


1 \ SlfrH*L VOLTAGE 

Figure 9£* Distortion in a cla*& A amplifier due to improper bio** 

tions flowing through the load produce the output voltage in an 
amplifier circuit, a distorted plate-current waveform will produce a 
distorted output voltage, It is important, therefore, that the bias 
voltage be kept at the proper value in class A amplifier stages, in 
erder to avoid distortion. 

c. Distortion will also occur in a class A amplifier if too great a 
value of a-c signal voltage is applied to the grid of the tube, and the 
total grid voltage {the bias voltage plus or minus the signal voltage) 
will vary over both linear and nonlinear portions of the E a — ii> curve 

(fig- W). ■ 

d. The maximum power output which can be obtained from any 
amplifier stage will depend on the efficiency of the circuit and the per- 
missible plate-dissipation rating of the particular plate used. The 
efficiency of an amplifier stage is the ratio of the power output (the 
power of the signal frequency available at the load) to the plate-power 
input (the d-c plate voltage times plate current), expressed in percent. 
For example, if the plate-power input to an amplifier stage is 40 



Original from 

Ztftp CURVE 



7L fc*M 

Figure 9$* Distortion fo a efrus jJ ampler due to 
excessive signal voltage. 

watts, and the power output of the stage is 10 watts, the efficient 
of the amplifier stage is 25 percent. The plate dissipation or the 
power consumed by the plate circuit, of an amplifier stage is the 
difference between the power input and the power output. Thus, in 
the example above, the plate dissipation would be the difference between 
40 watts and 10 watts, or 30 watts. Each type of tube is rated by the 
manufacturer as to its maximum safe plate dissipation; this value 
cannot be exceeded without damaging the tube. The efficiency of class A 
amplifier stages generally is about 20 to 25 percent. 

e. Practically all the amplifier stages of radio receivers, both r-f 
and a-f are class A operated. Also, the speech-amplifier stages of 
radiotelephone transmitters (audio stages used to amplify the a-f output 
of the microphone to the proper signal- in put level for the modulator) 
are class A amplifiers. 

60. Claw 8 Amplification 

a. If the grid of an amplifier tube is biased at cut-off, so that plate 
current will flow only during the positive half -cycles of the applied 
a-e signal voltage, the circuit is called a doss B amplifier. The Eq — h 
curve in figure 94 demonstrates the relation between grid voltage and 
plate current in a tube, operated class B, From figure 94, it can be 
seen that plate current flows only during the positive half -cycles of the 
a-c signal voltage applied to the grid, and consequently the plate-current 
waveform is not a replica of the signal- voltage waveform. 

Original from 

Cfr'lp £HR*E 

Pi Alt 

6Nt> V0L7«C 

JPt^urfi iM. Class B operations 

h. The signal voltage applied to the grid of a class B amplifier is 
usually much greater in value than that applied to the grid of a class A 
stage. In fact, the applied signal voltage may be so large that, during 
part of the positive half -cycles, the grid is actually operated at a 
positive voltage with respect to the cathode (fig. 94}, Since the grid 
is positive with respect to the cathode during the positive peaks of 
the applied signal voltage, some of the electrons will be attracted to 
the grid, and therefore grid current will flow, 

c. In order to avoid the large amount of distortion present in the 
output of a single-tube, or single-ended, class B amplifier stage, two 
tubes can be arranged in a push-pull amplifier circuit. (See fig. 95.) 
One tube will operate during the first half -cycle of the a-e signal 
voltage, and the other tube will operate during the second half -cycle. 
The action of the push-pull grid circuit in figure 95 is similar to that 
of the full-wave rectifier circuit. Since plate current flows during one 
half -cycle in one tube, and during the next half -cycle in the other, 




Figure &$. Pn&h-pull amplifier circuit. 


Original from 

the plate current waveforms of the two tubes can be combined in the 
load circuit. The load circuit in figure 95 is the center-tapped primary 
of a push-pull output transformer. Since the plate currents of these 
two tubes flow in opposite directions through their respective halves of 
the transformer winding, one tube will generate a voltage across the 
transformer primary during one half -cycle. During the next half- 
cycle, the other tube will generate a voltage of opposite polarity acTOsa 
the winding. Figure 9$ shows the voltage developed across the trans- 

r\ r\ r\ ■ 

rnOM tubi ONf 



Figure 36. Output of a push-pull clot* B amplifier. 

former primary winding by each tube and the resultant voltage across 
the transformer secondary, due to combining the voltages over the 
complete signal voltage cycle. Thus, by using two tubes in push-pull^ 
it is possible to obtain a reasonable un distorted output voltage from 
a class B amplifier, 

d* Class B amplifiers have an efficiency of about 50 to 60 percent, 
which means a reduced value of plate dissipation and an increased 
power output for a given power input. They are generally used where 
it is desired to develop a relatively large power output in the load 
circuit. Single-tube class B amplifiers are never used for a-f amplifica- 
tion} because of the distorted output of a single tube. Push-pull, class B, 
a*f amplifier circuits are, however, widely used iu the modulator stages 
of radiotelephone transmitters. They are also occasionally used in the 
power-output stages of radio receivers, 

e. Although the single-ended, or single-tube, class B amplifier is 
never used in a-f amplifier circuits, it can be used successfully in r-f 
amplifier stages having a parallel-tuned circuit as the plate load 
The parallel -tuned circuit is sometimes called a tank cirmit 7 because 
it has the ability to store power. When it is used as the plate load of 
a single-ended, class B amplifier stage, the capacitor in the parallel- 
tuned circuit will be charged by the output voltage produced by the 
flow of plate current through the load on the positive half -cycles, 
Although no current flows through the tube on the negative half -cycles 
of the applied signal voltage, the capacitor will discharge into the 
inductor during this period, and thus supply the missing half -cycle 
in the output voltage. This so-called flywheel effect of the tank circuit 
will occur only when the resonant frequency of the parallel-tnned 

* t\f\Cl\{> Original from 


circuit ia equal to the frequency of the applied signal voltage. Both 
single-ended and push-pull class B r-f amplifiers are used in the r-f 
stages of radio transmitters. 

61, Clan A8 Amplification 

a. It is possible to compromise between the fidelity (low distortion) 
of class A amplification and the relatively high efficiency of class B 
operation by biasing the amplifier circuit so that it will operate part 
w&y between class A and class B, This is known as class AB amplification* 

b. In biasing a tube part way between class A and class B, the 
tube will not operate over the entire linear portion of its Eg — Ip curve, 
and therefore some distortion will be present in the output. For this 
reason, push-pull amplifier circuits are generally used for class AB 
a-f amplifiers. Because class AB amplification ia less efficient than 
cither class B or class C amplification, it is seldom used in r-f amplifier 

c. If the a-c signal voltage applied to a class AB amplifier is kept 
below the point where grid current flows, the resultant operation is 
called class ABi amplification. If the applied signal voltage is great 
enough to cause grid current to flow daring the positive peaks of 
the aignal voltage cycle, the resultant operation is called class AB% 



62, Class C Amplification 

a. If the bias applied to the grid of an amplifier stage is appreciably 
greater than the cut-on* value, the amplification is called class C* The 
operation of such a tube is shown by the E a — Ip curve in figure 97. 
Note that in this case the bias voltage is 20 volts, or twice the cut-off 


Eft-lp CURVE 







<3 YVt 



TL *±« 

Figure 97. Class C operation, 

value; the use of twice the cut-off bias is common practice in many 
class C amplifiers. The curve shows that plate current will flow only 


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during that portion of the positive half -cycle of the applied a-c signal 
voltage, which is numerically greater than the cut-off bias for the tube; 
that is, plate current flows only during the positive peaks of the applied 
signal voltage, The curve also shows that the applied a-e signal 
voltage must greatly exceed the cut-off bias in order to produce a 
large value of plate-current flow. 

b T Almost all of the r-f amplifier circuits used in radio transmitters 
are operated class C« The parallel-tuued circuits used as plate loads 
for class C amplifiers exhibit the same flywheel effect as for class B 
amplifiers. The advantage of class C operation is that it has a high 
efficiency ; efficiencies as high as 75 percent are not uncommon in class C 
r-f amplifiers. Class C operation is never used in a-f amplifiers because 
of the high degree of distortion in these circuits. 

63. [nierstage Coupling 

a r Any of the coupling methods, described in paragraphs 27 to 30. 
may be used to couple the output circuit of one amplifier stage to the 
input circuit of the next stage. Three types of interstage coupling 
are shown in the circuit of figure 98 t In this circuit, the elements #i 
are the plate loads for their* respective tubes, cathode bias (H e ) is used 

Rests fa/tce /mpetfcf/tce \ Tr&t?sf&r/??er &r //?- 
Coi/pf/ftg I C&tff?///?p i rf&cffye Co&pftng 

Figure &£. Three type* of interstage coupling. 

for all four tubes, and the input a-c signal voltage is indicated by $o- 
All three of these forms of interstage coupling are widely used in the a-f 
circuits of both transmitters and receivers. The most common form of 
coupling found in the r-f amplifier circuits of receivers is transformer 
coupling, while in the r-f circuits of transmitters, both impedance 
and transformer coupling are widely used. 

o. The first stage of the circuit in figure 08 is resistance-coupled 
to the second stage, since the amplified -signal voltage is developed 
across a resistor in the plate circuit. This signal voltage is applied to 
the grid of the second stage through the blocking capacitor (sometimes 
called a coupling capacitor), indicated as capacitor C on the drawing. 
The resistor in the grid circuit of the second stage provides a d-c 
path for the bias applied to the grid. Since the blocking capacitor 
plays an important part in the operation of this circuit, this type of 
coupling is sometimes called resi&tdTice-c&pacitance coupling* As the 



.. s 

Original from 

blocking capacitor will pass alternating current, it applies the amplified- 
signal voltage developed across resistor Zi to the grid circuit of the 
next stage. At the same time it blocks the flow of direct current from the 
plate circuit of the first stage to the grid circuit of the second. If this 
capacitor should break down or develop leakage, some or all of the 
positive d-c voltage applied to the plate of the first stage would appear 
on the grid of the second, canceling some or all of the negative bias 
applied to this tube, and thus causing distortion in its output. A 
leaky blocking capacitor, therefore, can be a source of distortion in 
an amplifier circuit, 

c. The coupling between the plate circuit of the second tube and the 
frid circuit of the third tube (fig, 98) is similar in operation to the 
coupling between the first and second tubes, except that an inductor 
having a high value of reactance at the signal frequency is used as 
a plate load. The function of the blocking capacitor is the same 
for both resistance and impedance, or inductance, coupling. 

d. A transformer is used to couple the output of the third tube to 
the input of the fourth in the circuit of figure 98« The primary of this 
transformer is the plate load for the third tube, while the signal volt- 
age applied to the grid of the fourth tube is developed across the 
secondary of this transformer. If the transformer has more turns on 
the secondary than on the primary, the signal voltage applied to the 
grid of the fourth tube will be proportionately greater than the signal 
voltage developed across its primary. Thus, some voltage amplification 
is obtained by the use of transformer coupling in amplifier circuits, 

e. In general, the only difference between the coupling methods used 
in a-f amplifier circuits and those used in r-f amplifier circuits lies in the 
employment of tuned circuits. The circuit of figure 99 shows a tuned 
impedance-coupled r-f amplifier circuit; the plate loads for the two 

Figure 99. Tuned impedance- coupled r-f amplifier circuit, 

tubes are the parallel- tuned circuits formed by L\ and C\ } and L% and 
Oa, respectively, A two-stage, single-tuned, transformer-coupled, r-f 
amplifier circuit is shown in figure 100 ; this circuit is typical of those 
found in most radio receivers. Figure 101 shows the use of double- 
tuned transformer coupling between two stages of r-f amplification. 
This circuit has the advantage of providing high selectivity and high 
gain (amplification) at the frequency to which it is tuned. It should 


Original from 


Figure 100* Tuned transform?? coupled rf amplifier circuit. 

be noted in J; he circuits of figures 99, 100, and 101 that batteries com- 
mon to two stages have been used to provide both negative grid bias 
and positive plate voltage for the tubes. 

/. One of the main considerations in the transformer coupling of a-f 
power amplifiers is that very little or no distortion should occur. The 
use of the push-pull circuit greatly reduces distortion in a-f amplifier 
circuits. For this reason, push-pull is widely used in the a-f power 
amplifier circuits of radio transmitters and receivers. In class A ani- 

Figure 101* Double-tuned tranjtformer-cuupled r-f amplifier circuitt 

nlitiers, the use of the push-pull circuit permits the application of con- 
siderably higher signal voltage per tube for a given amount of distor- 
tion in the output than would he possible in single-ended amplifiers, 
The use of the push-pull circuit is required in classes AB and B a-f 
amplifiers for low distortion in the output voltage. A center-tapped 
transformer is the most convenient means of supplying two voltages 
which are equal to and out of phase with the two grids of a push-pull 
amplifier circuit from a single-ended stage. Transformer coupling is 
therefore more widely used than any other method for this purpose, 
g. In class AB 2 and class B a-f amplifier circuits, there is another 
and more important requirement, which necessitates the use of trans- 
former coupling; Most of the tubes intended for use as power ampli- 
fiers are so designed that their grids may he operated at a positive 
voltage in push-pull amplifier circuit without undue distortion. If a 
positive voltage is applied to its grid, a tube will draw a certain amount 
of grid current, since some of the electrons will be drawn to the grid. 
If the grid circuit of a tube drawing grid current contains a large 
value of d-c resistance, the grid current flowing through this resistance 
will produce a bias voltage, because of the grid-leak action of the 
resistance. Since this voltage would be applied to the grid in addition 



.. s 

Original from 

to whatever value of bias is applied in the circuit, it would change the 
operating characteristic of the circuit, reduce the power output, and 
cause distortion. Both class AB% and class B a-f amplifier circuits are 
usually operated so that they wiLl draw grid current when large signal 
voltages are applied. The d-c resistance of the grid circuit of such am- 
plifiers must, therefore, he kept small in order to prevent the develop* 
znent of undesired additional bias voltages. The tow d-c resistance of the 
windings of a transformer satisfies this requirement. Accordingly, 
transformer coupling is always used for class AB% and class B opera- 
tion in a-f power amplifier circuits. 

h. If the grids of an amplifier stage draw current, they will require 
a certain amount of power from the signal source. In order to obtain 
the maximum transfer of power from the plate circuit of the preceding 
stage, usually called the driver stage, the output impedance of this 
stage must be matched to the input impedance of the push-pull ampli- 
fier circuit. This requirement is most conveniently and efficiently met 
in this type of a-f amplifier by the use of transformer coupling. 

64. Gain Control in r-f Amplifieri 

a. It was shown earlier (par. 37e) that the gain, or amplification, of 
a triode may be conveniently controlled by varying the bias voltage 
applied to its grid. This method of gain control is used more frequently 
in r-f amplifier circuits than any other method. 

b. For manual control of the gain of an r-f amplifier, the cathode- 
bias resistance often is formed by a fixed resistor and a variable re- 
sistor connected in series. The fixed resistor is of the correct value to 
bias the tube for its maximum amplification. The variable resistor may 
be set at any value from zero to that resistance required for cut-off 
bias. This provides a convenient method of adjusting the gain of the 
circuit to any desired value. 

c* For automatic volume control, additional negative grid bias may 
be supplied to the r-f amplifier stages of a receiver from the diode-load 
resistor in the -detector circuit. The negative voltage developed across 
this resistor will be proportional to the signal voltage applied to the 
detector by the r-f amplifier. The additional negative grid bias applied 
to the r-f amplifier tubes thus will tend to keep the level of the signal 
applied to the detector, and consequently the detector output, at a 
constant value. The circuits and application of automatic volume con- 
trol are discussed in detail in the sections on radio receivers. 

65. Gain Control in a-f Amplifier* 

a. The most popular method of volume control for a-f circuits is the 
use of a potentiometer as the grid resistor of a vacuum-tube amplifier 
(&%. 102). Since the signal voltage is applied across this variable 


("nn'ilc Original from 


resistor, the position of the variable arm of the resistor will determine 
the value of the signal voltage applied to the grid, and consequently 
the output of the amplifier. 


Fiffun 102. Simple volume-control circuit. 

b. Automatic volume control is seldom applied to a-f amplifier cir- 
cuits, as it is generally more convenient to control the gain of a radio 
set in the r-f amplifier circuits which precede the detector, 

66. Distortion 

a. Distortion in an amplifier may be broadly classified under three 
headings r frequency distortion, nonlinear 'distortion, and delay (or 
phase) distortion. Frequency distortion arises because of the inability 
of an amplifier to amplify all frequencies equally. Nonlinear distortion 
is a consquence of operating over a curved (nonlinear) portion of a 
tube's characteristic, ao that harmonic or multiple frequencies are in- 
troduced. Delay distortion results from the effects of transmission of 
different frequencies at different speeds, giving a relative phase shift 
over the frequency spectrum in the output. Except at the ultra-high 
frequencies or in transmission line work, the affects of delay distortion 
are usually insignificant. Frequency distortion in r-f transmitter am- 
plifiers is ordinarily of little concern, since these amplifiers operate 
over only a relatively narrow range of frequencies at any one time. 


Figure 1Q$ W Special oirouit arrangement to an r-f amplifier to provide 
uniform response over & oand of frequencies. 

k\ In r-f receiver amplifiers, various compensating devices are some- 
times employed to provide uniform response to a band of frequencies. 


Original from 

Figure 103 illustrates one such compensating arrangement. A high- 
inductance primary winding P f loosely coupled to the secondary 8 t 
resonates (due to self -capacitance) at a lower frequency than the 
lowest for which the amplifier is to operate. This gives high gain at the 
low end of the band because of the high plate-load impedance at the 
lower frequencies. The small capacitance C, due to a loop of wire 
hooked around the top of the secondary, provides increased coupling at 
the higher frequencies to improve the response at the upper end of the 

c. Distortion, which arises from operating a vacuum tube over a 
nonlinear portion of its characteristic , consists principally of multiple 
frequencies (harmonics) and of sum-and-differenee frequencies corre- 
sponding to each frequency present in the original signal. Suppose, 
for instance j that the input signal to a non-linear r-f amplifier is com- 
posed of three frequencies: 500,000; 501,000; and 501,025 cycles. The 
output then contains, in addition to the three original frequencies, the 
following distortion frequencies: 

(1) Harmonics: 1,000,000 j 1,500,000 

1,002,000 j 1,503,000 
1,002,050; 1,503,075 

(2) Sum frequencies: 1,001,000; 1,001,025; 1,002,025. 

(3) Difference frequencies: 1,000; 1,025; 25. 

d. The filtering action of a parallel-resonant circuit in an amplifier 
plate circuit which is tuned to about 500,000 cycles minimizes the- effects 
of all these distortion components. The extent of this suppression of the 
Distortion frequency components may be controlled by proper design 
of the tuned circuit. At frequencies well off resonance, the parallel cir- 
cuit offers essentially the impedance of the lowest impedance branch. 
In a circuit tuned to 500,000 cycles, the impedance offered to currents 
of 1^000,000 cycles is practically that of the capacitor alone, and the 
impedance offered to currents of 1,000 cycles is practically that of the 
inductor alone* Thus a low L to C ratio minimizes the voltages devel- 
oped across the parallel circuit at the distortion frequencies. Link 
coupling (fig. 104) is sometimes used to transfer energy between two 
tuned circuits. This avoids incidental coupling between the two circuits 
due to the distributed capacitance of the turns and also avoids the 
transfer of harmonics from one circuit to the other. 

e. In an a-f amplifier the distortion frequencies generally overlap 
components of the desired signal frequencies, so that filtering is not 
feasible. In a-f amplifiers, the problem demands prevention rather 
than cure. Class A operation is one solution, Push-pull arrangements 
are of further assistance. 

A Of the harmonic frequencies, the second (first overtone) is usually 
the predominant one. The rest are ordinarily weak. It is the objeetion- 


/"",,. Original from 


F igurt 104. Link- coupled tuned circuits* 


able second harmonic (as well as all other even-order Harmonics) 
which is absent in the output of a push-pull amplifier. That this is the 
case may be seen from a consideration of the curves of figure 105. Here 
Q represents a fundamental signal frequency (first harmonic) ; ® and 
© are multiple-frequency curves, second and third harmonics of the 
signal, respectively. The solid curve of ® is obtained by adding the 
fundamental (T) and the second harmonic ®, The solid curve of ® is 





® ® 

© Fundamental. 
® -Second harmonic. 
(3) Third harmonic. 

g) Fundamental plus second harmonic. 
(D Fundamental plus third harmonic. 
© Fundamental plus second and third harmonics. 
Figure 1Q5. Analysis of harmonia distortion. 

obtained by adding the fundamental and the third harmonic ©. 
Fundamental, second harmonic, and third harmonic are compounded 
to yield the solid curve of ®. The resultant in ® is such that if the 
negative half -cycle of the curve is shifted along the abscissa (horizon- 
tal axis), so as to be directly below the positive half -cycle, the negative 
half-cycle then presents a mirror image of the positive half -cycle about 
the abscissa. It can be shown that any combination of odd-order har- 
monics possesses this same symmetry; further, that any resultant wave 
formed by a combination of harmonics and possessing this symmetry 



Original from 

cannot contain any even-order harmonic elements. In push-pull action 
two tubes interchange roles during alternate half-cycles, ao that if the 
dashed curve of figure 106 represents the output of one tube, the dotted 
curve of the same figure represents the output of the companion tube. 
Dissymmetry in. the output waveform of each individual tube indicates 
definite even*order harmonic content, whereas symmetry of the com- 
bined waveform shows complete absence of any even-order harmonics. 


V / V / V J TL*Z»T4 

Figure 106+ Waveforms in a push-pull amplifier. 

g, Push-pull operation serves to lessen distortion in other ways. 

(1) The direct currents present in the two halves of the output trans- 
former primary balance each other in their magnetic effects, so that 
the core cannot become saturated with direct current. (Saturation is 
a state of magnetization of the core which results from reasonably 
large currents, so that further increase in current produces only a 
an] all increase in magnetic induction.) 

(2) Alternating- current components of plate-supply potential, 
which are due to incomplete filtering, produce no effect in the output 
transformer secondary, since the potentials thus developed across the 
primary balance each other. Because of the difficulty of obtaining 
perfect balance, particularly in tubes, the full possibilities of push-pnD 
amplifiers are seldom realized in practice. However, under conditions 
of moderately good balance, the push-pull amplifier offers a definite 
improvement in quality over a comparable single-ended amplifier. 

k. For doubling the frequency at radio frequencies in a transmitter, 
with a single-ended amplifier operating into an appropriately tuned 
LG circuit, harmonic distortion within the tube is deliberately 

-I v Original Irani 



Original from 




67, Principle of t-r-f Receiver 

The tuned radio-frequency receiver, or, as it is more commonly 
called, the t-r-f receiver, consists of one or more stages of r-f amplifica- 
tion, a detector stage, and on& or more stages of a-f amplification- A 
block diagram of a typical t-r-f receiver is shown in figure 107. Radio 
energy waves from a distant transmitter cause a r-f signal current to 






/VY 7 


Figure 107. Block diagram of a t-r-f receiver, .showing the 
signal passing through the receiver. 

flow in the receiving antenna. This r-f signal is amplified by the r-f 
amplifier stages, and is then detected, or demodulated, by the detector 
The resulting a-f output from the detector stage is amplified by the 
a-f amplifier stages, and the audible sound is heard in either a loud- 
speaker or earphones. The waveforms below the block diagram of figure 
107 give a comparative indication of this process of converting t4 
signals' into intelligible a-f signals* 

68. R-f Amplifiers 

a. Tuned r-f amplifier stages increase the selectivity and the sensi- 
tivity of the t-r-f receiver. The more stages that are used the greater 
will be this increase. Important aspects of the r-f amplifier to be 
considered are the types of tubes, r-f transformers, capacitors, and 


Original from 

resistors employed, and the nature of band spread and special de- 
coupling circuits. 

b. The tubes generally used in r-f amplifiers are tetrodes and pentodes* 
Any tube suitable for voltage amplification may be used. Triodes, which 
were used at one time, are not as satisfactory because they have a strong 
tendency to cause undesirable oscillations in r-f amplifier stages. They 
also require very careful neutralization {adjustment) to prevent feed- 
back from stage to stage. 

c. The basic circuit of a pentode class A t-r-f amplifier is shown in 
figure 108. The tuned circuit L X G X is coupled to coil L t which in this 
ease is the antenna coil, but could be the plate coil of a preceding stage* 
Resistor R ± and capacitor C 2 are the cathode bias resistor and cathode 
bypass capacitor. Capacitor C 3 is the screen bypass capacitor and R t 


Figure 108. R-f stage of a t-r-f receiver. 

is the screen voltage-dropping resistor. A second tuned circuit, L a G& f 
is coupled to coil L 3 . Coils L and L^ form the primary and secondary 
windings, respectively, of an t4 transformer. Coils £ 3 and L a also 
form an r-f transformer. 

d. The r*f transformer used in most t-r-f receivers consists of a 
primary coil and a secondary coil. The secondary coil L t is designed to 
cover the desired frequency range when tuned by the tuning capacitor 
Ci connected across the secondary. Most r-f transformers in use at the 
present time are of the air-core type. A few special types may be found 
which use powdered- iron cores when the frequency of operation is not 
too high* If a receiver is required to cover a greater frequency range 
than one coil and tuning capacitor will provide, the tuning circuits of 
the receiver must be changed to tune to these additional frequency 
bands. One system is to use plug-in coils, which may be changed to 
provide the different tuning ranges required. Another system is to 
mount the various coils for the different frequencies in the receiver, 
and bring the leads out to a multi-contact rotary switch. This is called 
6 and switching, and by turning the switch, any desired band can be 
selected. In both methods the same tuning capacitors are used for all 
tuning ranges. Both systems of band changing are widely used in 
Signal Corps receivers. 

e. Most t-r-f receivers use two or three r-f stages preceding the 
detector, with each stage tuned to the same frequency. It is therefore 
more convenient to have all of the tuning capacitors mounted on a 



Original from 

common shaft, so that ail stages can be tuned simultaneously. These 
are called ganged variable capacitors. In a receiver havin g two &i 
stages, a three-gang capacitor would be used, with one of its sections 
tuning each of the three tuned circuits in the receiver. When these 
tuning circuits are ganged, the coils and the capacitors must be identical 
This is necessary in order that all the circuits will tune to the same 
frequency for any dial setting. Inaccuracies of the coils and capacitors, 
and stray circuit capacitances will prevent the circuits from tuning to 
the same frequency. Thus, there must be some method of compensating 
for these irregularities. This is provided by connecting small trimmer 
capacitors across each tuning capacitor. These trimmers are adjusted 
with a screw driver or small wrench, so that each circuit may be tuned 
exactly to the signal frequency. This process is known as alignment 
In practice, these capacitors are adjusted at the h-f end of the dial, 
where the plates of the tuning capacitors are meshed very little and 
their capacitances are small. The circuits will now be properly adjusted 
at one dial setting, but they may not tune to identical frequencies at 
other dial settings. In some sets, this is. corrected by slotting the end 
rotor plates of the tuning capacitors, so that any portion of the end 
plates may be bent closer to or farther away from the stator plates. 
When all of the stages tune to identical frequencies at all dial settings 
they are said to be tracking, and maximum gain will be obtained from 
the receiver, In receivers using band changing, the trimmers for each 
range are usually mounted on the individual coils. In receivers cover- 
ing only one band, the trimmers are usually located on the ganged 
capacitors, one for each section. 

/. Resistors used in the r-f amplifier and in the detector circuits are 
practically all of the small carbon type. The wattage rating will 
depend upon the voltage drop in the resistor and the current through it. 

g* Band spread is the process of spreading out a small section of the 
tuning range of a receiver over the entire scale of a separate tuning dial. 
The purpose of band spread is to assist in separating stations crowded 
together in a small space on the main tuning dial. There are two types 
of band spread : electrical and mechanical* 

(1) In electrical band spread, a small variable capacitor is connected 
in parallel with the main tuning capacitor in the tuned circuit. The 
tuning range of the ban&spread capacitor is only a fraction of the 







Parallel capacitor band ftpr&*d. ® Tapped-coU band spread. 
Figure 109. Two type* of electrical band spread. 


Original from 

range of the main tuning capacitor. To increase the amount of band 
spread, the small capacitor may be tapped down on the coil, ho that it 
times only a small portion of the coil. Figure 109 shows two methods 
of electrical band spread. 

(2) In mechanical band spread, the band-spread dial is geared to the 
main tuning dial, so that one complete rotation of the band-spread dial 
moves the main tuning dial and capacitor over only a fraction of 
its range, 

h. "When several amplifier stages are operated from a common plate 
supply, there is a possibility of undesirable oscillations being set up 
because the plate circuits of the various stages are coupled together by 
the common impedance of the plate supply. (See fig. 110<T)-) Note 
that the plate voltage of both tubes is obtained from a common B, or 
plate, supply. The infernal resistance of this common supply is repre- 
sented by R* Any change of plate-current flow in tube 2, such as a 
signal cur rent f will cause a change of voltage across R. This causes a 

WW \ M 

1 s 




-r -m 




^_ — ^.. 

— 1 


* fr lrf * — - 


Figure. 110. E-f amplifer T without and with decoupling circuit. 

change of the B supply voltage to the plate of tube 1 T and induces a 
voltage in L l7 which is connected to the grid circuit of tube 2. This 
tube will amplify the change and it will appear across L z as a larger 
change. Thus, it can be seen that a part of a signal from the plate of 
tube 2 is fed back to the grid circuit of the same tube. This condition 
may cause unwanted oscillations. Circuits to prevent this condition aTe 
called decoupling circuits, and shown in figure 110®, The capacitors 
G and C\ f together with resistors R t and R 2t make up the decoupling 
circuit. The resistors R x and R 2 offer a high impedance to the signal 
voltage, while the capacitors C and d bypass the signal voltage around 
the B supply, A choke coil may be used instead of the resistors R x and 
fi x . The bypass capacitors for the cathode, screen-grid, and plate 




Original from 

circuits in t-r-f receivers are usually paper capacitors, except in circuits 
intended to operate on extremely high frequencies and in receivers 
designed for special applications, such as aircraft receivers. In most 
Signal Corps receivers, the paper capacitors are inclosed in metal 
cases, two or three capacitors often being grouped together in one can. 
Where one connection to each capacitor is connected to ground in the 
circuit, the metal can itself is often the common-ground terminal. In 
some cases, a single terminal may be provided as a common ground 
for all capacitors in the can. 

OT. Detector Circuit! 

Since the voltage amplification of the r-f amplifiers of the modem 
t-r-f receiver is relatively great, the signal voltage at the input circuit 
of the detector stage is quite large. As the grid-leak detector is easily 
overloaded by such large voltages, it is rarely used in present day t-r-f 
receivers* The two most widely used detector circuits are the diode 
detector and the power detector. 

70. Volume Control 

a. Because all signals will not arrive at the receiving antenna with 
equal intensity, a gain or volume control is provided so that the volume 
of the signal received can be varied. This can be accomplished by vari- 
ous means. Those most commonly used are shown in figures 111 and 112. 

Figure 111. Grid-bias volume- control* 

In figure 111, the control is in the grid-bias circuit of a variable-mu 
pentode r-f amplifier. It will be recalled that varying the bias of 
variable-mu tubes causes the amplification factor to increase or decrease, 
thus controlling the gain of the stage. The resistor R provides the 
proper bias for maximum gain when R c is adjusted to zero resistance. 
The bias voltages of all r-f amplifier tubes in the receiver are usually 
controlled when this method is used. Another method, illustrated in 
figure 112, controls the amount of a-f voltage^applied to the grid of the 
a-f amplifier from the diode detector. 

b. Once the volume or gain control of a receiver has been set, the 
output should remain constant, regardless of the strength of the incom 


Original from 



Figure 11£. Detector output volume control. 

itkg signal. The development of the variable-mu pentode tube makes 
this possible, since the amplification of the tube may be controlled by 
the grid-bias voltage. All that is needed, then, for automatic volume 
control is a source of voltage which becomes more negative as the 
signal strength becomes greater. If this voltage is applied as bias to 
the grids of the variable-mu r-f amplifier stages, the grids will become 
more negative as the signals grow stronger. This will reduce the 
amplification, thus tending to keep the output of the receiver at a 
constant level. The load resistor of the diode detector is an excellent 
source of this voltage, as the rectified signal voltage will increase and 
decrease with the signal strength. A typical detector diode with an 



TO fiRlDt OP 


^W vJ 


Pf -L^ 



Figure US. Automatic volume-control circuit. 

automatic volume control (a-v-e) circuit is shown in figure 113. The 
signal is rectified by the diode detector, and the rectified current 
flowing through the load resistor causes a voltage drop across the 
resistor, as indicated in figure 113. The negative voltage developed is 
impressed on the grids of the variable-mu tubes in the isf stages. Any 
increase in signal strength results in a greater voltage drop and thus 
is an increase in negative bias to the amplifiers. This results in a 
decrease in signal strength to the detector. A decrease in signal strength 
to the detector reduces the amount of negative bias on the amplifier 
tubes, increases gain in those stages and the input to the detector 
increases. The filter circuit removes the a*f component of the signal, 
and only the slower variations due to fading or change in position of 
the receiver effect the gain of the amplifier stages. Automatic volume 
control is particularly desirable for mobile receivers in which the signal 
strength is changeable as the receiver is moved, 


Original from 

c. The variable-mil tube is designed to operate with a minimu m bias 
of about 3 volts. This minimum bias is usually provided by a cathode 
resistor, and the a-v-c bias U in series with it, A disadvantage of 
ordinary automatic volume control is that even the weakest signal 
reduces the amplification slightly. An adaptation which avoids this is 
shown in figure 114, and is referred to as delayed automatic volume 
control. In this particular circuit the a-v-e diode is separate from the 
detector diode, and both are housed in the same vacuum tube with a 
pentode amplifier. The tube is called a duplex-diode pentode* Part of 
the energy which is fed to the plate of the detector diode is coupled to 
the a-v-c diode section by the small capacitor C. The plate of the a-v-c 
diode is maintained at a negative voltage by means of a cathode-biasing 
resistor R> This heeps it from rectifying and producing the a-v-c voltage 

R.F oa if -> 





Figure 114. Delayed automatic volume control* 

until the peak voltage coupled to it through C counterbalances the 
negative voltage of the diode* For very weak signals, which do not 
produce enough voltage on the plate of the a-v-c diode to overcome the 
existing negative potential, no a-v-c voltage is developed. Thus, the 
sensitivity of the receiver remains the same as if automatic volume 
control were not being used. On the other hand, when normal strength 
signals are being receivedj which do not need maximum sensitivity of 
the set, enough voltage will be coupled to the a-v-e diode to overcome 
the small negative plate potential and produce an a-v-c voltage drop 
across resistor B. This voltage has- the a-f and r-f components filtered 
from it and is applied to the grids of the variable-mu tubes, as in the 
ordinary a-v-e circuit, 

d> Duplex-diode triode and duplex-diode pentode tubes are widely 
used to supply a source of a-v-c voltage. In addition, the second diode 
in these tubes is used, together with the cathode, as a diode-detector 


Original from 

circuit, and the triode or pentode section is used as a separate amplifier, 
Thus, by the use of such multi-element tubes, the functions of detection, 
a-v-c voltage rectification, and amplification, is combined within a 
single tube. 


71 • Apf Amplifier* 

Since the signal output of a detector stage in a t-r-f receiver is low, 
or weak, it is usual to have at least one stage of a-f amplification. The 
output of this first a-f amplifier may be further amplified if necessary, 
depending upon the requirements of the receiver. A headset may 
require no further amplification after the first a-f stage, while a large 
loudspeaker may require several additional stages of a-f amplification. 

72. Shielding 

In order to prevent coupling between two circuits, metal shields are 
used ; iron for a-f circuits, and copper or aluminum for r-f circuits. All 
shields should be grounded to the chassis of the receiver, which is the 
common ground for all connections in the set. Since shielding changes 
the inductance of a coil, it changes the resonance frequency to which it 
responds. It is necessary, therefore, to make many adjustments in 

radio sets with the shields in place* 


73. Circuit of a t-r-f Receiver 

a. The complete circuit diagram of a five-tube tuned r-f receiver is 
shown in figure 115. This receiver uses three pentode r-f amplifier 
stages, a diode-detector stage, and a pentode a-f amplifier stage 
energizing a loudspeaker. The A supply (heater voltage) and B supply 
(plate voltage) are furnished the vacuum tubes by means of batteries 


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Figure 116 '. T-r-f receiver with automatic volume control 


Original from 

when the double-pole single-throw switch is dosed. The dotted lines 
connecting the four tuning capacitors indicate that these capacitors 
are ganged, A small trimmer capacitor is connected in parallel with 
each section of the ganged tuning capacitor for proper alignment of 
the receiver. These small trimmers compensate for inequalities in any 
of the circuit constants. The detector stage is considered as a diode, 
since the grid and the plate are connected together. Figures 116 to 
120, inclusive, reproduce this same receiver diagram with various 
circuits emphasized to facilitate study. 

&. In figure 116, all parts of the t-r-f receiver at ground, or chassis, 
potential are denoted by heavy lines. All points on the heavy (ground) 
line will be at the same potential, which is considered to be zero volte 
with respect to the rest of the receiver circuit. All voltages in the 
receiver are compared to this ground potential. 



&fltp «K nrgtrCO TO fl»M 


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Figure 116, T-r-f receiver. Ground-potential elements 
denoted by heavy lines. 

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Figure 117, T-r-f receiver. Elements at high r-f potential 

denoted by heavy lines. 


Original from 

c. In figure 117, all of the elements of the t-r-f receiver at high r-f 
potential are denoted by heavy lines* By means of this diagram, it is 
quite simple to trace the path of the r-f signal from the antenna circuit 
to the diode detector. 

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d m In figure 118, the high voltage d-c plate supply is shown by heavy 
lines. When the switch is closed, the four pentodes receive the high 
positive plate voltage necessary for their action as amplifiers. The diode, 
operating as a detector, does not require d-c plate voltage* Note the 
decoupling resistors in the plate leads of the first three pentodes. 

pit kf suet 


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Figure 119. T-r-f receiver, (Detector circuit shown in heavy lines,} 

g, In figure 119, the complete detector circuit is shown in heavy lines. 
The tube used in this stage is considered to be a diode. The grid and the 
plate of the triode are connected, or tied together t resulting in a 
two-dement tvbe> or diode* The rectified or detected signal is taken 


Original from 

from a portion of the potentiometer B (through a capacitor) to the 
grid of the pentode a-f amplifier* 

/. In figure 120, the a-v-c circuit is shown in heavy lines. The 
rectified signal voltage necessary for the operation of an a-v-c circuit is 
taken off the negative end of the potentiometer R, and returned to the 
first two stages of the receiver. It should be noted that only the first 
and second r-f amplifiers are supplied with an a-v-c voltage in this 
receiver. A switch is provided for short-circuiting the a-v-c when it is 

2nd RFSTAff 


ftWE t*TTW fUTE *HQ 
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Figure 120* T-r-f receiver* (A-v-c circuit *h&ton in heavy lines,} 

not desired. If this is done, the potentiometer R is then used as a 
manual control of volume without affecting the normal operation of 
the t-r-f receiver. 

74* Capabilities of t-r-f Receiver 

Although the t-r-f receiver will give satisfactory results when cover- 
ing a single low- or medium-frequency band, such as the broadcast 
band, it has several disadvantages which make it impractical for use 
in high-frequency or multi-band receivers. The chief disadvantage of 
the t-r-f receiver is that its selectivity (ability to separate signals) 
does not remain constant over its tuning range. As the set is timed 
from the low-frequency end of its tuning range toward the higher fre- 
quencies, its selectivity decreases. At the high frequencies, which are 
widely used for Signal Corps communication f this lack of selectivity 
becomes extremely troublesome, Also, the amplification, or gain, of the 
fc-r-f receiver is not constant with frequency. It is Y%rf difficult to 
design r-f amplifiers which will provide sufficient amplification for 
communication requirements at extremely high frequencies. The super- 
heterodyne receiver has been developed to overcome these disadvantages. 


Original from 



75. Principles of Superheterodyne Operation 

a. The essential difference between the t-r-f receiver and the super* 
heterodyne receiver is that in the t-r-f receiver the r-£ signal is ampli- 
fied at the frequency of the signal, while in the superheterodyne receiver 
the signal is amplified at a new, lower frequency called the intermediate 

h* The deficiencies of the t-r-f receiver (par, 74) are largely over- 
come in the superheterodyne receiver by combining the received signal 
with a different frequency in the receiver to produce a lower inter- 
mediate frequency. Though much lower than the original, this new 
frequency retains all the modulation characteristics of the old signal. 
By amplifying this lower frequency, it is possible to use circuits which 
are more selective and capable of greater amplification than the circuits 





0£T£C TO* 








Figure l&l. Block diagram of superheterodyne receiver, showing 

signal passing through receiver. 

used in t-r-f receivers. The block diagram of a typical superheterodyne 
receiver shown in figure 121 indicates the manner in which the signal 
changes as it goes through the different stages. The received r-f signal 
is first passed through a r-f amplifier, A locally generated unmodulated 
r-f signal is then mixed with the carrier frequency in the mixer stage. 
This produces an intermediate frequency signal which contains all 
the modulation characteristics of the original signal, hut is much lower 


Original from 

in frequency, This intermediate frequency is amplified in one or more 
stages, called intermediate- frequency amplifiers, and is then fed into 
the second detector t where it is detected or demodulated. The detected 
signal is amplified in the a-f amplifier and then fed to a headset 
or loudspeaker. 

c. The conversion of the original r-f signal to the intermediate fre- 
quency is an important function of the superheterodyne receiver. By 
means of a vacuum tube operating as a detector, it is possible to change 
the frequency of a radio signal to another frequency, and yet retain 
everything that existed in the original signal. This process is known 
as frequency conversion. The tube is called a mixer, or converter, and 
sometimes a first detector. If a 1,000-kilocycie signal and l,465-kilocycl& 
signal are fed into a mixer, various frequencies are obtained in the 
output. One of the most prominent of these is the beat frequency, 
which is the difference between the two, or 465 kilocycles. This is the 
intermediate frequency. In the superheterodyne receiver these two 
signals come from different sources. One of them is the received signal. 
The other comes from a special stage used in all superheterodynes, 
known as the local, or heterodyne oscillator. Unlike the received signal, 
the signal from the heterodyne oscillator is unmodulated. In the 
superheterodyne receiver the intermediate frequency is set at a definite 
value. The frequency of the local oscillator must differ from that of the 
signal being received by an amount equal to this intermediate frequency* 
Thus, as the receiver is tuned to signals of various frequencies, the local 
oscillator must he tuned simultaneously so that its frequency is always 
separated from that of the signal by the same amount. For example, 
if the intermediate frequency is 465 kilocycles, a commonly used 
frequency, and the range of the receiver is from 500 to 25,000 kilocycles, 
the oscillator would have to operate over a range of either 35 to 34,535 
kilocycles or 965 to 25,465 kilocycles. Whether the oscillator frequencies 
are higher or lower than the signal, the difference is still 465 kilocycles. 
The higher range is generally used, except when receiving signals of 
rather high frequencies. The i-f amplifier stages are permanently tuned 
to 465 kilocycles. 

76, Frequency Conversion 

a. The combined circuits of the oscillator stage and mixer stage 
form the frequency converter of the superheterodyne receiver. There 
are a large number of possible combinations of tubes and circuits which 
may be employed for frequency conversion. Triodes, pentodes, and 
multi-clement tubes are used in various circuits, and several methods 
are used to mil the oscillator-output frequency with the incoming- 
signal frequency in the mixer stage. The oscillator output may be fed 
into the grid, cathode, or suppressor-grid circuit of the mixer tube; 
or the coupling may be achieved by means of a special grid built 


("nn'ilc Original from 


into the tube for that purpose. Multi -element converter tubes have 
been designed bo that the functions of oscillating and mixing may be 
combined in one tube; the pentagrtd converter tube is an example of 
this widely used type, 

h. When the frequency converter uses a separate, single tube as a 
local oscillator, the basic circuit is similar to the diagram shown in 
figure 122. A pentagrid (five-grid) mixer tube (fig. 122) combines 







Figure lH. Pentagrid mwen 

the frequency from the oscillator (usually a triode) with the incoming 
r-f carrier, 

c. A typical frequency-converter circuit using a triode oscillator 
and a triode mixer is shown in figure 123. The oscillator output is 
fed or injected into the grid of the mixer through a coupling capacitor. 
This is known as grid injection. The coil and tuning capacitor in the 
mixer-grid circuit are tuned to the frequency of the incoming r-f signal. 
The oscillator grid circuit is tuned to a frequency lower or higher than 
the signal frequency by an amount equal to the intermediate frequency. 
The i-f transformer in the plate crieuit of the mixer stage is tuned 








Figure li$> Frequency-converter circuit using triode 
oscillator and triode mixer. 

to the intermediate frequency. The oscillator uses the same circuit as 
the regenerative detector studied in paragraph 54. The feedback is 
of such a value that the circuit is oscillating at a frequency determined 


Original from 

by the values of L and C* The capacitors C and Ci are ganged so that, 
as the frequency of the signal being received is changed, the oscillator 
frequency will also be changed, 

d. Two other means of coupling the mixer and local oscillator are 
shown in figure 124, where a pen tag rid mixer and triode oscillator are 
used. Figure 124(T) shows the local oscillator coupled to the mixer 
tube by means of coil L& in the cathode circuit of the mixer tube. TJie 






(T) By means of coil L H in eatkutle circuit of miser tfuif* 
(i) By means of injection grid of mixer tube 
Figure 124. Local ohcillator-to-mixer coupling method** 

r-f voltage induced in coil L a causes the plate current of the miser 
tube to fluctuate at thi* frequency. The incoming signal induced in 
coil L2 in the grid circuit of the mixer also affects the plate current 
These two frequencies are mixed together and the beat between them, 
which is the i-f frequency, will be produced in the tuned plate circuit 
Interaction between the oscillator and mixer is reduced somewhat by 
coupling the oscillator voltage to the cathode, as shown. Figure 124® 
shows a second method of frequency-conversion coupling between a 
pentagrid mixer having two independent control grids and a separate 
local-oscillator tube. Besides a heater and a cathode, the tube has 
five concentric grids and a plate. Grid 1, which is nearest the cathode, 
and grid 3 are the control grids of the tube, while grids 2 and 4 are 
screen grids. Grid 5 is a suppressor grid. The local oscillator is coupled 
to grid 3 T and the incoming signal is applied to grid l t which is called 
tin 1 signal grid. The A r oltages applied to these grids affect the plate 
current, thus producing a beat note or intermediate frequency in the 
plate circuit of the tube. This tube provides superior performance in 



.. s 

Original from 

the high- frequency bands because of the excellent shielding between 
the oscillator and signal grids. 

e. Another type of frequency conversion employs a single tube having 
the oscillator and frequency miser combined in the same envelope. This 
type of tube also has five grids* and is called a pentagrid converter. 
The basic circuit for the pentagrid converter is shown in figure 125, 
and should he compared with the diagram in figure 122. The pentagrid 
converter depends on the electron stream from the cathode for coupling. 
It may be visualized as a device in which the plate current is modulated 
by variations in the cathode emission. The performance of a pentagrid 
converter is such that only one tube is necessary for converting the 









Fiffure 125. Pentagrid converter* 

frequency of the desired signal from its original value to an inter- 
mediate frequency. Grids 1 and 2, and the cathode are connected to 
a conventional oscillator circuit and act as a triode oscillator. Grid 1 
is used as the grid of the oscillator, and grid 2 is used as the plate. 
In this circuit, the two grids and the cathode can he considered as a 
composite cat h ode , which supplies to the rest of the tube an electron 
stream that varies at the oscillator frequency. The signal voltage is 
applied to grid 4, which further controls the electron stream so that 
the plate-current variations are a combination of the oscillator and the 
incoming-signal frequencies. The plate circuit of the pentagrid con* 
verier is tuned to the desired intermediate frequency. Grids 3 and 5 
are connected together within the tube so as to form a screen grid which 
serves to accelerate the electron stream and to shield grid 4 electro- 
statically from the electrodes, 

/. A typical pentagrid-converter circuit is shown in figure 126. The 
incoming r-f signal is fed from L x into the tuned grid circuit of L% and 
Ci. It is then applied to the control grid of the tetrode section of the 
tube at grid 4, In the oscillator section of the tube, the r-f energy is 
fed back from the plate circuit inductance L^ to the tuned grid circuit 
consisting of L^ t C 2 , and O4. C 2 is the main tuning capacitor, Grid 
bias for the tetrode section of the tube is secured by the flow of plate 
current through the cathode resistor R 2 , The incoming signal, and the 


Original from 

oscillator voltages are heterodyned in the electron stream flowing from 
cathode to plate. The output voltage is a beat frequency equal to the 
difference between the incoming signal and the oscillator frequencies. 

■, j — r j 

Figure IBB. Coupling of OBciUa&or to mixer by mean* of modulating 
electron tUeam from cathode of mixer tube* 

g. The capacitor C4, placed in series with the tuning 1 capacitor C* f 
is called a padding capacitor. This padding capacitor is necessary 
because the frequency of the oscillator tuned circuit is higher than that 
of the r-£ circuit. It is thus necessary to have a low value of inductance 
and capacitance in the oscillator circuit in order to obtain a higher 
frequency. In some superheterodyne sets, this is accomplished by 
having a smaller capacitor and coil in the oscillating circuit. In others, 
such as in figure 126, it is more convenient to use the same size 
capacitors in both circuits and reduce the value of the oscillator 
capacitor by placing a fixed or variable capacitor in series with it. 
A small trimmer capacitor may also be placed across the oscillator tuning 
capacitor to take care of any slight frequency deviations. 

77, l-f Amplifiers 

a. The intermediate-frequency amplifier is a high-gain circuit per- 
manently tuned to the frequency difference between the local oscillator 
and the incoming r-f signal. Pentode tubes are generally used in these 
amplifiers » which may consist of one, two, or three stages. Each stage 
is adjusted to the selected intermediate frequency. Since all incoming 
signals are converted to the same frequency by the frequency converter, 
this amplifier operates at only one frequency. The tuned circuits, 
therefore, may be permanently adjusted for maximum amplification and 
desired selectivity. It is in this amplifier that practically all of the 
voltage amplification and selectivity of the superheterodyne is developed* 

ft. The i-f transformers used with i-f amplifiers are tuned by adjust- 
able, or trimmer, capacitors to the desired frequency. Both mica and 
air-trimmer capacitors are used. Generally the i-f transformers are 
double tuned, that is both primary and secondary coils are tuned to 
the proper frequency. For special applications, single- tuned i-f trans- 


Original from 

formers are used, in which case the secondary winding alone is tuned. 
I-f transformers are made with both air and powdered -iron cores* 
Some iron core if transformers have fixed mica tuning capacitors. 
The tuning is accomplished by moving the iron cores in or out of the 
coil by means of an adjusting setscrew. This is known as permeability 
tuning. The i-f transformers and capacitors are mounted in small 
metal cans, which serve as shields. When adjustable capacitors and 
fixed inductors are used, the capacitors are small compared with the 
large ganged tuning capacitors used in r-f stages. Small adjusting 
shafts protrude from the top of these capacitors and can be reached 
through a small hole in the can with a hexagonal wrench or screw 
driver. Thus, adjustment of the capacitor is possible without removing 
the assembly from the shield. 

TL-4G$& V 

Figure 127. Circuit diagram of single-stage i-f amplifier 

using pentode tube. 

c. The diagram of a single-stage i-f amplifier using a pentode tube 
is shown in figure 127. Transformer T x is the input i-f transformer. 
The primary of the transformer, Xi-A, is in the plate circuit of 
the mixer and is tuned to the selected intermediate frequency. The 
secondary circuit, L 2 -0^ which is inductively coupled to the primary, 
is tuned to this same frequency and serves as the input circuit to the 
grid of the tube. Resistor R x in the cathode circuit provides the neces- 
sary grid-bias voltage, while capacitor C a bypasses r-f currents around 
this resistor. Resistor # 2 and capacitor 4 are the screen-volt age-limit- 
ing resistor and the screen bypass capacitor, respectively. Resistor R^ 
and capacitor <7„ serve as a decoupling network to prevent any of the 
signal currents from flowing back th rough the circuit ami causing inter- 
action between stages. Capacitor (7 fl furnishes a low-impedance path to 
the cathode or ground for the signal currents, while resistor /? ; > prevents 
any of the signal currents from flowing to the plate supply. These 
decoupling networks may be employed in grid, screen -grid, or plate 
circuits. Circuit £*-£« is the tuned-primary circuit of the second i-f 
transformer 3T 2 , The secondary circuit L A ~C 7 is coupled to the primary, 
and is the input circuit of the next tube, which may be another i-f 



.. s 

Original from 

amplifier or the second detector. The two resonant circuits of the 
second i-f transformer T 3 are tuned to the same frequency as the 
circuits in T\ w 

d. Since the i-f amplifier is intended to furnish most of the gain 
of the superheterodyne the number of i-f amplifier stages used will 
depend generally on the sensitivity required of the receiving set. From 
one to three i-f amplifier stages will be found in modern superhetero- 
dyne receivers, 

e v The intermediate frequency of a superheterodyne will depend, in 
general, on two factors, the first of which is the desired selectivity. 
The higher the intermediate frequency, the broader (or less selective) 
will be the tuning of the receiver. The second factor is the difference 
between the signal frequency and the intermediate frequency. It ia 
not practical for the intermediate frequency to he very much lower than 
the signal frequency. For this reason, receivers used on the extremely 
high frequencies often use a fairly high intermediate frequency. The 
most common intermediate frequency is in the neighborhood of 456 to 
465 kilocycles, although frequencies as low as 65 kilocycles, and as 
high as 12,000 kilocycles, are found in receivers designed for special 

/, If extremely sharp tuning is required of a receiver, a piezo- 
electric quartz crystal may be used as a crystal filter in the i-f amplifier. 
The crystal acts like a tuned circuit but is many times more selective 
than those made of coils and capacitors. The crystal will operate only 
on one frequency which is determined by the thickness of the crystal. 

Figure IBS, Typical crystal-filter circuit. 

A typical crystal-filter circuit used in communications receivers is 
shown as figure 128. Unless steps are taken to balance it out, the 
small capacitance between the metal plates of the crystal bolder will 
bypass some undesired signals around the crystal. This balancing is 
accomplished by taking a voltage from the center-tapped coil I** M 180° 
out of phase with the signal voltage, and applying it through the 
crystal-phasing capacitor C so that it bucks or neutralizes the undesired 
signal. The balanced-input circuit may be obtained either through the 
use of a split-stator capacitor, or by the use of a center-tapped coil 
as in figure 128. Closing the switch across the crystal shorts the 
crystal-filter circuit, leaving an ordinary i-f stage. The output of the 



("nn'ilc Original from 


crystal Biter is applied to a tap on £ 9l which is the input circuit of 
the next stage, in order to provide the proper impedance match. 

0, To keep the intermediate frequency of a superheterodyne centered 
on its band, automatic-frequency control is sometimes used. This 
arrangement is useful in compensating for any changes in frequency 
of the local oscillator. While details of its operation can be understood 
only after studying section XIII, the principle is not difficult. If the 
intermediate frequency shifts off the center of its band, that is, varies 
slightly from its correct frequency, the discriminator {a rectifier) 
turns the frequency change into a proportionate voltage change. This 
voltage is fed to a tube in the frequency-control circuit which, together 
with a capacitor and resistor across the tank circuit of the local 
oscillator, will change the reactance t but not the resistance of the 
tank circuit, and hence will change the frequency of the local oscillator. 
When properly adjusted, any shift in the i-f will be applied through 
the automatic-frequency control circuit to bring the local oscillator to 
its correct frequency, 

h. Noise limiters are employed occasionally in the i-f circuits of 
superheterodynes to suppress strong impulses of short duration, such 
as interference from spaiking motor contacts or atmospheric static. 
In one such noise limiter circuit, a part of the intermediate frequency 
is diverted along a path paralleling the regular i-f amplifier* It reaches 
a special detector tube which is so heavily biased that the i-f signal is 
stopped at this point. If a sudden sharp pulse raises the detector 
tube above cut-off, the pulse will pass through, and will be fed back out 
of phase, thus blocking the sadden pulse which will be trying to pass 
through the regular i-f amplifier. 

78. R-f Amplifier* 

a. An r-f amplifier is not absolutely necessary in a superheterodyne, 
but it is a valuable addition for the following reason. If the converter 
stage were connected directly to the antenna, unwanted signals might 
be received. These unwanted signals are called images. Since the 
mixer stage produces the intermediate frequency by heterodyning two 
signals whose frequency difference equals the intermediate frequency, 
&ny two signals whose frequencies differ hy the intermediate frequency 
will produce an i-f signal. For example, if the receiver is tuned to receive 
a signal of 2,000 kilocycles and the oscillator frequency is 1,500 kilo- 
cycles, an i-f signal of 500 kilocycles will he produced. However, a 
signal of 1,000 kilocycles finding its way into a mixer will also produce 
an i-f signal of 500 kilocycles, since the difference between its frequency 
and the oscillator frequency is 500 kilocycles. Therefore, some method 
must be found to keep these unwanted signals, or images, out of the 
mixer stage. The extra selectivity provided by an r-f amplifier is the 

("nn'ilc Original from 


solution. Since the r-f amplifier greatly amplifies the desired signal, 
siid does not amplify the image, the possibility of image interference 
i« reduced considerably. 

&. Almost all superheterodyne receivers are provided with at least 
one r-f amplifier stage. The r-f amplifiers used are of the same type 
as those discussed in section VI. When used in a superheterodyne 
receiver, r-f amplifiers are sometimes called preselectors. 

79. B«at-frequency Oscillator* 

a. In order to receive c-w code signals on a regenerative detector, 
it will be necessary to make the detector oscillate at a frequency 
slightly different from that of the incoming signal so as to produce 
(by heterodyning) an audible signal, (See par. 56.) In superhetero- 
dyne receivers, this is done by a separate oscillator, known as the 
beat-frequency oscillator^ which is tuned to a frequency that differa 
from the intermediate frequency by an audible amount. For example, 
a beat-frequency {b-f ) oscillator tuned to 501 kilocycles will produce 

Cbup&vp Cbpddkn-^ 




i t 






Figure 1£9* A o-f oscillatur coupled to second detector 

of a superheterodyne. 

a beat note of 1 kilocycle, an audible frequency, when heterodyned 
with a 500-kilocycle i-f signal. The output of this oscillator is coupled 
to the second-detector state of the receiver. 

b, A b-f oscillator circuit is shown in figure 129. A switch and a 
means of frequency control are usually located on the front panel 
of the receiver to turn on the oscillator stage and to control the fre- 
quency, or pitch, of the audible signaL 

80. Second Detectors 

The detectors used in superheterodyne receivers to detect, or de- 
modulate, the intermediate frequency are of the same general types 
as those employed for t-r-f receivers. Automatic volume control is 
widely used in superheterodyne circuits. The a-v-e voltage may be 
applied to any or all o£ the stages before the second detector except 
the local oscillator, 


Original from 

81. Audio Amplifier* 

The a-f amplifiers used in superheterodyne receivers follow the same 
general principles as those employed in t-r-f receivers. The desired 
power output is the main consideration. 

82. General Superheterodyne Circuit 

a. The circuit diagram of a six-tube battery-operated superheterodyne 
receiver is shown in figure 130. This receiver has one stage of tuned 

Figure ISO, Superheterodyne receiver. 


r-f preselection (r-f amplification) a triode acting as a local oscillator, 
a pentagrid mixer, two stages of i-f amplification, a diode supplying 
voltage for delayed automatic volume control, a diode detector, and 
a pentode a^f power-output stage feeding into a loudspeaker. The 
heater supply and B supply (plate voltage) are furnished to the 
various stages by means of batteries when the double-pole single-throw 
switch is closed. The amplifier tubes obtain their grid bias from the 
resistor and capacitor combination in the cathode circuit of each of 
the five tubes. The dotted lines connecting the three tuning capacitors 
indicate that these variable air capacitors are ganged. Small trimmer 
capacitors are connected in parallel with each of the ganged tuning 
capacitors for proper alignment of the receiver. The first i-f stage uses 
a complex tube known as a iriode-pentode. The pentode section of the 
tube functions as a straightforward i-f amplifier, and the trjode section, 
operating as an oscillator, can be switched into the circuit to provide 
a heterodyne action for the audible reception of c-w signals. The 
second i-f stage combines several functions in one tube known as a 
dupUx-diode pentode. This tube contains a pentode i-f amplifier and 
two diodes, one diode acting as straight signal detector, the other 


Original from 

supply ing A rectified a-v-c voltage. Figures 131 through 134 reproduce 
this same superheterodyne receiver diagram with various circuits 
emphasized to facilitate study* 

&, In figure 131 all parts of the superheterodyne circuit relative to 
the second detector are denoted by heavy lines. A single diode (in the 
duplei-diode triode tube) supplies an audia-frequency signal voltage 


Figure 131. Superheterodyne receiver. (Second detector circuit 

ghovsn in heavy lines.) 

across the variable resistor, or volume control. Any portion of this 
voltage can be fed to the pentode a-f power amplifier, and the level Bet 
by the volume control will be maintained by action of the delayed 
automatic volume control. 


fir tTAff 


Y"\ MlUfl 

■ * 

1* \ 


Figure l$8 r Superheterodyne receiver* (Delayed th-v-Q circuit 

»hown in heavy line**) 


Original from 

c. In figure 132 the delayed a-v-e circuit is shown in heavy lines. 
The rectified signal voltage necessary for the operation of an a-v-c 
circuit is obtained by the second diode of the duplex-diode triode. It id 
passed through isolating resistors, filtered by action of the r-f bypass 
capacitors, and applied both to the first r-f amplifier stage and the first 
i-f amplifier stage. 



Figure 133. Superheterodyne retewer. (Local oscillator 

shown in heavy lines,) 

d. In figure 133 the local oscillator circuit is shown in heavy lines. 
The tuned circuit, which determines the frequency of the local oscilla- 
tions, is composed of a fixed coil and a variable amount of capacitance, 

Figure 134, Superheterodyne receiver^ (B-f otcillator for 
o-w reception shown in heavy lines.) 


Original from 

consisting of a variable air-tuning capacitor t an adjustable trimmer 
capacitor in parallel with the tuning capacitor, and an adjustable 
padding capacitor in series with the tuning capacitor. 

e. In figure 134 the b-f oscillator circuit is shown in heavy lines. The 
pentode section of the first i-f amplifier tube, a triode- pentode, functions 
as a normal i-f amplifier when the b-f-o power switch is open, as shown 
in diagram. When this switch is closed, the pentode continues to operate 
as an i-f amplifier,- but oscillations also take place in the triode section 
of the tube at the frequency of its external tuned circuit. By means of & 
small variable capacitor, the frequency of this tuned circuit can be 
altered for different incoming signals, so that the local oscillations pro- 
duced in the regenerative eircuit are only slightly different in frequency 
from the incoming signal. When these two frequencies are mixed in the 
first i-f amplifier stage, there is a heterodyne action producing aa 
audible signal which can be used for the reception of c-w code, 

43. Typical Army Superheterodyne Receiver 

a. A complete schematic diagram of a typical Army superheterodyne 
is shown in figure 135, This receiver is operated from 110 volts alter- 
nating current and uses eight vacuum tubes. The r-f signal voltage 
from the antenna circuit is amplified by a pentode r-f amplifier stage. 
Another radio frequency generated in the local oscillator stage is 
mixed with the signal voltage in the pentagrid mixer stage, to create an 
i-f carrier. This intermediate frequency is amplified by a pentode i-f 
amplifier stage, and is then detected by the diode detector section of a 
duplex-diode triode. The resulting a-f signal is applied to the triode 
section of this complex tube which operates as an mdio-frequency 
voltage-amplifier stage. This signal is further amplified by a push-pull 
audio-frequency power-amplifier stage of two pentodes, and then is 
fed to the loudspeaker. High- voltage direct current for the plates, and 
low-voltage alternating current for the heaters of the vacuum tubes 
are obtained from the power supply stage, which uses a fuH-^ave 
rectifier circuit. It should be noted that every tube and circuit element 
in figure 135 has an identifying number. This is to facilitate a more 
thorough analysis of the set, as the signal is traced through the receiver 
from the antenna to the speaker. 

b. Assume that the receiver is tuned to a l } 000-kilocyele signal and 
that the i-f amplifier frequency is 465 kilocycles. The signal is picked 
up by the antenna and fed to the grid of the r-f amplifier tube ( VT-117) 
through the r-f coupling transformer T\. The signal is then amplified 
by the tube and fed to the r-f coupling transformer IV It is then 
applied to the control grid (grid 1) of the mixer tube (VT-87). The 
grid circuits of the r-f and mixer stages are both tuned to the 1,000- 
kilocycle signal by a single dial controlling the ganged capacitors 0\ 
and CV C 4 is a bypass capacitor for the grid-decoupling resistor R\- 


("nn'iL* Original from 


R% is the biasing resistor and C 5 the bypass capacitor for R 2 . R s is 
the screen grid voltage-dropping resistor and Cq is the bypass capacitor 
for J?a* R* and Cy constitute the plate current filter which prevents 
the r-f signal from feeding back through the power supply to ground 
and thereby producing common coupling between stages. The high -fre- 
quency oscillator (VT-65) must generate oscillations 465 kilocycles 
higher in frequency than the r-f carrier. It is therefore tuned by O3 
(which is ganged with € x and C;>) to 1,465 kilocycles, €<& is a trimmer 
for €s and C26 ^s a trimmer for C25 which is the padder capacitor used 
to make the oscillator track with the r-f amplifier. R22 i R the oscillatoT- 
biasing resistor and C^ is a blocking capacitor used to prevent the 
oscillator inductor from shorting R^- Rn is the oscillator plate voltage- 
dropping resistor and (7 27 is the bypass capacitor for R 2 \. C 2 t also 
serves as a blocking capacitor to prevent shorting the plate voltage to 


1-F AMPLIFIER ^I!£ T 25p?, F1E R 


'L 4i ■">* 

Figure 15$. Circuit diagram of a modem Army superheterodyne receiver. 

ground* The high-frequency voltage is injected into the electron stream 
of the mixer tube by grid 3, R$ is the injector grid-biasing resistor, 
and O22 13 the coupling capacitor for the oscillator. The 1,000-kiloeycle 
signal and the 1,4 65 -kilocycle signal are mixed in the electron stream 
of the mixer stage. The i-f stage functions in the same manner as the 
r-f stage, except that it always works at the intermediate frequency 
and therefore is much more efficient than the r-f stage. The i-f trans- 
formers !T 3 and T \ are permanently tuned to the 465-kilocycle inter- 
mediate frequency, and usually need only occasional checking for 
correct alignment. The lower diode section of tube 4 (VT-S8) is the 



.. s 

Original from 

detector, with Ri$ and H lfl as the detector load resistor, Ci? is the r-f 
bypass capacitor. With #i& it forma an r-f filter to prevent the r-f 
component of the signal from feeding into the a-f section through the 
blocking capacitor C« and the volume control R i9 . The audio signal 
voltage developed across Ru also appears across the volume control 
Rn* All, or a portion, of this voltage, depending on the setting of the 
variable arm, is fed to the grid of the first audio amplifier (triode section 
of tube 4), Rn is the bias resistor for the first a-f amplifier, and C» 
is its bypass capacitor. The i-f voltage from the plate of the i-f amplifier 
tube is fed through the blocking capacitor C lt . to the upper diode plate, 
which rectifies the signal voltage to develop the a-v-c voltage. R u is 
the a-v-c diode load resistor. The d-c voltage developed across this 
resistor is in series with the r-f amplifier, mixer, and i-f amplifier grid 
circuits. It is applied to the grids through the a-v-c filter resistor By*. 
Bis and C M act as a filter to eliminate any audio component from this 
voltage, and thus prevent the grid bias of these tubes from fluctuating 
at an a-f rate. The output of the first a-f amplifier is fed to the grids 
of the push-pull amplifier through the interstage-coupling transformer 
IV -Raa is the bias resistor for both of these tubes and <7 2 a is its bypass 
capacitor. The output of the power amplifier is fed to the voice coil of 
the speaker through the output (matching) transformer IV 

c. r T is the power transformer ; tube 8 the power rectifier ; I*! and L* 
the filter chokes; Cas, C a0 and C a i the filter capacitors, and R 2 * the 
bleeder resistor. The specific function of each of these parts will be 
diseussed in section IX. 

84, Alignment 

a. In order to operate one or several r-f stages of a superheterodyne 
with a single control, the tuning capacitors of the r-f stages and the 
oscillator are ganged together on a common shaft. When the control 
knob is turned, the various r-f stages must all tune to the same fre- 
quency and the local oscillator must track in such a manner that the 
frequency difference between the local oscillator and the r-f stages is 
always equal to the later mediate frequency. When the circuits are 
adjusted in this manner, they are said to be tracking. The trimmer 
(parallel) capacitors are used to assure tracking at the high-frequency 
end of the band, and the padder (series) capacitors are used to assure 
tracking at the low-frequency end of the band. In general, only the 
local oscillator is supplied with a padder capacitor. It is also necessary 
to adjust the i-f stages so that they all tune to the intermediate fre- 
quency. Misalignment in any stage of a superheterodyne will cause a 
decrease of sensitivity or selectivity , or both. 

h. A calibrated oscillator or signal generator, insulated screw drivers, 
insulated adjustment wrenches, and some form of output indicator are 
necessary to properly align a modern superheterodyne receiver. The 


("nn'ilc Original from 


signal generator is an oscillator calibrated in frequency and capable of 
delivering either a c-w or a modulated signal. Provision is made for 
controlling the output signal voltage from a few microvolts to the full 
output voltage* The insulated screw drivers and wrenches are used to 
adjust the tuned circuits. The screw drivers and wrenches may be of a 
composition material and usually have bits and heads of metal, which 
give more substantial service and at the same time place a minimum of 
metal in the field of the circuit that is being adjusted. The output 
indicator may be an output meter, loudspeaker, headset, oscillograph, 
or a tuning-indicator tube. 


-I v Original Irani 




85. Power Requirements of Radio 

a. Vacuum tubes used in various circuits of radio receivers and 
transmitters require voltages of various values for the filament, screen, 
and plate circuits. It is the purpose of the power supply to provide 
these voltages. Except for filament power t which can be alternating 
current, the output from a power supply must be as nearly pure direct 
current as possible, and the voltage must be of the correct value for the 
apparatus for which it is to be used. Radio transmitters require more 
power than receivers. Consequently, transmitter power supplies operate 
at higher voltages, with greater current flowing. 

ft. Power to heat the filaments of tubes is sometimes called the A 
supply, and normally will be a low voltage. In portable field radio sets 
the cathode, or filament, power supply is furnished by batteries, gen- 
erators, or dynamotors. Semiportable and mobile sets generally use 
storage batteries for filament-heating purposes. In permanent ground 
installations, filaments are heated from the standard a-c lighting 
circuit through a step-down transformer, 

c« The plate and screen power supply is sometimes called the B 
supply , and will usually be a high voltage. The plate supply in a 
lightweight transceiver (small combined transmitter and receiver) is 
furnished by batteries. Dynamotors driven by storage batteries or by 
hand are generally used for plate power in portable and mobile sets, 
while many large semiportable transmitters carry gasoline-engine- 
driven generator equipment. Permanent installations ordinarily use 
some sort of rectifier-filter system plate supply. 

d. When a grid-bias voltage is used, it is sometimes called the C 
supply. Grid bias for voltage amplifiers is customarily taken from a 
part of the plate supply by some means of self -bias. For large power- 
amplifier tubes a separate rectifier-filter system or d-c generator is 
frequently employed. 

e. Radio power supplies may be divided into three general classes: 
battery, a-c T and electro-mechanical ay stems. 

86. Battery Power Supply 

Small portable receivers and transmitters usually operate from dry 
batteries. The current drain from the batteries is low and the apparatus 


_l t Original rrom 


can be operated several hours from this type of supply before it must 
be replaced. Battery pacts containing the filament , plate, and grid 
batteries are provided for some sets ; separate filament, plate, and grid 
batteries are used in others. Batteries have the advantage of being 
capable of delivering a smooth, unfluctuating direct current. Where 
large voltages and currents are required, however, they become 
cumbersome and expensive, 

87. A-c Power Supply 

fl. This type of power supply is generally used whenever commercial 
power is available. The Army also uses it for field installations which 
are equipped with gasoline engine driven generators designed to supply 
a source of alternating current ♦ An a-c operated power system differs 
from other types in that no batteries or mechanical devices are used. 
It makes use of an a-c source of power, and since the usual commercial 
supply is 110- volt, 60-cycle, this voltage will be assumed in the following 
discussion of &-C power supplies- 

&. All a-c operated power supplies may be divided into four parts: 
the transformer, rectifier, filter, and bleeder, or voltage divider system. 
The transformer provides a means of increasing or decreasing the 
volUge by transformer action. (See par, 27.) The rectifier serves to 
convert the alternating current to pulsating direct current. The filter 
smooths out the pulsating direct current, and the voltage divided system 
is used to obtain various d-c voltages for the plate, screen, and control- 
grid circuits, 

68. Vacuum-tube Rectifiers 

a. The diode finds its most important use as a rectifying tube in both 
transmitter and receiver power supplies. A simple a-c rectifier con- 
sisting of a single diode is shown in figure 136. When an a-c voltage 
is applied between points A and B t electrons will fiow from the cathode 
to the plate of the diode during the positive alternation of each cycle 
(between points 1 and 2 in figure 136®). During the next alternation, 




Figure 1S€. Balf-wave rectifier. 


between points 2 and 3, the plate voltage is negative with respect to 
the cathode and no current will flow in the circuit. Thus, since the 
diode will pass current only during the positive alternation of each 

J 35 

("nn'ilc Original from 


cycle, current will flow in only one direction through load resistor i. 
Since only one-half of each cycle is used in thia type of rectifier, it is 
called a half -wave rectifier. Figure 136® shows the a-c input and 
136® the pulsating output voltage of a half -wave rectifier. It should 
be noted that the d-c pulsations have the same frequency as the applied 
a-c voltage. This makes it difficult to filter properly. If a higher 
voltage is necessary, a step-up transformer may he used, 

b. A full -wave rectifier consists of two half -wave rectifiers working 
on opposite alternations, thus utilizing the complete cycle of alternating 
current. The two rectifiers are connected in such a manner that both 
half -waves are comboned in the output, as shown in figure 137. Kefer- 

Fiffure 1S7 Full-wave rectifier. 

ring to figure 137, assume that during the first alternation the plate of 
tube A is positive with respect to the center tap of the transformer. 
Since the plate of this tube is positive, electrons will now as indicated 
by the solid arrows. During the nest alternation the voltage across the 
secondary winding of the transformer will be reversed, thus making 
the plate of tube B positive with respect to the center tap, and the 
plate of tube A negative. No current will flow through tube A because 
the plate is now negative. The plate of tube B is positive, however, 
and electrons again will flow through load resistor L. Electron flow 
during the negative alternation is represented by the dotted arrows. 
It should be noted that current through resistor L is always in the 
same direction. Observe also that there are two d-c pulsations for each 
a-c cycle, one for the positive alternation and one for the negative 
alternation. Thus it may be seen that both alternations are combined 
and that the output pulsations of a full-wave rectifier are twice the 
frequency of the input power. This results in lower filter requirements. 
For relatively low voltages, such as those required in receivers, the 
full- wave rectifier may consist of two plates and a filament or cathode 
in one envelope. For the higher voltages required in transmitters, two 
separate tubes are usually used. 


Original from 

c. Vacuum-tube rectifiers are of two general types, high-vacuum and 
mercury-vapor tubes. The former offers tbe advantage of ruggedness, 
the latter, high efficiency. Both tubes contain two elements, a plate 
and a cathode, and both operate on the principle of current flow only 
during intervals of positive plate potential. High* vacuum diodes are 
employed as rectifiers for power supplies of radio receivers and low- 
powered stages of transmitters. The voltage drop across this type of recti- 
fier is proportional to the current drawn through the tube, and ia fairly 
high in comparison to some other types. The mercury-vapor rectifier tube 
is of greatest value where high voltage and large current are to be 
handled. The voltage drop across a mercury- vapor tube is extremely 
loWj being approximately 15 volts regardless of the current drawn 
by the load. 

89. Power Supply R I tort 

a. The output of vacuum-tube rectifier systems is made up of pulsa- 
tions of current and voltage, all in the same (positive) direction. Before 
this rectified voltage can be applied to the plate or grid circuits, it 
must be smoothed out into a steady, nonfluctuating d-c flow. Such 
smoothing out of the pulsations is accomplished by means of filter 
circuits, which are electric networks consisting of series inductors and 
shunt capacitors. Filter circuits may be classified as capacitor-input 
or choke-input filters, depending on whether the filter input consists 
of a shunt capacitor or a series inductor (choke coil). Figure 138® 
shows a filter of the capacitor-input type ; figure 138® shows a choke- 


C Q 



TO * 


CD © 

Figure 138, Types of filter network** 


input type. A resistance load connected to the output of a full- wave 
rectifier is shown in figure 139®. The voltage across the load will 
follow the rectified a-c pulsations as shown in figure 139®. This fa 
the condition for a rectifier without a filter network. 






"Figure 139. Load connected across output of a full* wave rectifier, 
and current waveform through the load. 


Original from 

h. The capacitor-input filter (fig. 1400) is the simplest type of fllttr 
and consists of a single capacitor, C, connected across the rectifier 
output and in parallel with the load. During the time the rectified 
alternating current is approaching its peak vaiuej it is charging the 
capacitor and delivering a current to the load. After reaching the 


® Rectifier v>ith capacitor filter. 

Vvltage across C with small load current. 

(a) Voltage across C with large load current. 

Figure 140. A capacitor jitter and output waveforms with 

small and large loads, 

oeak voltage, the output of the rectifier begins to decrease until the 
alternation is completed. During this decrease in applied voltage, the 
capacitor has a higher voltage than the rectifier-output voltage. An the 
capacitor cannot discharge back through the rectifier tube, it must 
release its energy through the load. The values of the capacitor and 
the applied voltage determine the amount which the capacitor can 
store* If the load current is small, the capacitor will discharge slowly. 
(See fig. 140®.) A large load current will cause the capacitor to 
discharge more rapidly. (See fig, 140®.) This filter, while eliminating 
some of the ripple voltage from the output of the rectifier system, has 
several disadvantages. The amount of ripple voltage remaining in the 
output is greater than can be tolerated in the plate supplies of receivers, 
amplifiers, and radiotelephone transmitting equipment. Another dis- 
advantage of the capacitor- input type of filter is the heavy current 
drawn through the rectifier tube. While the capacitor is charging, it 
draws a current several times that drawn by the load. This charging 
current plus the load current may be great enough to cause damage to 
the rectifier tube. The capacitor-input system is not advisable when 
using the mercury-vapor rectifier tube at high voltages, because the 
heavy rush of current which charges the capacitor may damage 
the cathode. 

c. A series choke may be added to the simple capacitor filter of figure 
140® with an appreciable improvement in the filtering action. Such a 
capacitor-input filter is shown in figure 141. The inductor, or choke coil, 
has an iron core, and may be from 10 to 45 henry s in value. Care mast 
be exercised when replacing choke coils in faulty power supplies. A 
choke coil designed for use on the negative side of the filter system is 


("nn'ilc Original from 


not sufficiently insulated to withstand th« high voltages which exist 
between the positive aide and ground. Since the entire load current 
flows through this choke, it should have small resistance to direct cur- 
rent. The choke coif offers high opposition to the pulsations in the 
current. This property of coil L produces a smoothing effect upon the 
rectified output, and when combined with shunt capacitor C, an additive 
smoothing effect is produced. The action of the capacitor when used 
with the choke is similar to that of the single capacitor filter r capacitor 
C charges during the increase in voltage until the peak is reached t 
and the current begins to flow through L to the load at the same time. 




■* * 


Figure 141. A simple capacitor filter with choice coil* 

But the inductance of choke coil L prevents any rapid change in the 
current flowing to the load and thus helps capacitor C to store energy 
until the next charge. The complete action of this type of filter is shown 
in figure 142. The capacitor has become fully charged at A of figure 



<T) Rectifier output. 

Capacitor charge and discharge c$cle T 
@ Waveform of filter output. 
Figure 142. Waveform* of filter shown in figure 141. 


142<D and the input voltage is beginning to decrease. The choke, by 
its inductive action, opposes any decrease in the load current, and the 
capacitor j being charged to a higher voltage than the applied voltage, 
begins to discharge slowly through the coil. But before the capacitor 
has lost much of its charge, it begins to receive another charge from 
the next impulse, as shown at B of figure 142(2). The capacitor receives 
energy from the rectifier during the time interval from Bto<7 (fig, 142®), 
again becoming charged to approximately the peak voltage of the rectified 
wave. The action of the choke and capacitor for the second alternation 
of the wave is the same as for the first, and this is repeated for every 
half-cycle. The output voltage waveform applied to the load is shown 
in figure H2®. 


Original from 

d. The addition of & second shunt capacitor C\ across the capacitor 
input filter, as shown in figure 143, lowers the ripple output voltage 
below that of figure 142®. This network, consisting of one choke coil 



= = $ LOAD 



Figure 143. Circuit Qf a complete capacitor -input filter and 

waveform o/ the output, 

and two shunt capacitors, is considered one filter section. If a more 
elaborate system is desired, another section may be added, aa shown 



W Q a* 



f - 




Figure 144* Two-section capacitor-input filter and output waveform. 

in figure 144, considerably improving the output voltage to an almost 
steady condition. 

e. The choke-input type of filter, like the capacitor-input filter, may 
have several different forms, A simple choke -input filter, consisting 
of a single inductor, is shown in figure 145. The output voltage wave- 
form (across the load) for a given rectifier waveform, is also shown in 
figure 145. The choke coil offers a high reactance , or opposition, to any 





' it 

\ t vl \ £ 

Figure 14S, Choke input filter and output waveform. 

change in the current flowing through it. The filter-input voltage 
increases from zero at the beginning of the input alternation, but the 
current builds up more slowly than in capacitor-input systems, The 
coil smooths out some of the ripple voltage by opposing any sudden 
increase in current flowing through it, and acts to keep the current at a 
steady value when the output from the rectifier begins to decrease* The 
coil likewise delays the decrease in current until the second alternation 
from the rectifier again begins to supply energy to the circuit Ti& 
same process is repeated for each succeeding alternation* 


Original from 

/, A single capacitor added to the simple choke-input filter of figure 
145, will eliminate more of the ripple from the filter output The 
capacitor is placed across the output in parallel with the load, and is 
known as a single-section choke-input filter. The circuit of a single- 


TO* 1 * J. 



Figure I4G Singh-section and t&Q-*eeti&n. cht&f-input filters, showing 

input and output waveforms* 

section filter is shown in figure 146®, with the voltage output wave 
(across the load) for a given waveform from the rectifier, A two-section 
filter, with similar waveforms is shown in figure 146(2). 

g. The capacitor-input and inductor-input filters have been shown 
to have about the same effect upon the ripple component of the rectifier- 
output wa^e. However, they possess quite different characteristics in 
another respect. The first capacitor of the capacitor-input filter system 
is charged to approximately the peak voltage of the rectified alter- 
nating current and does not completely discharge between alternations 
or pulsations. The capacitor remains charged very near to this peak 
voltage, thereby keeping the output voltage of the filter system at a 
value comparable to its peak input voltage. For small load currents, 
the voltage output from the filter will approximate the peak voltage 
of the rectified alternating current. However, the output voltage drops 
off rapidly as the load current increases. A capacitor-input filter will 
give satisfactory service only in applications where the load conditions 
are reasonably constant, such as a class A amplifier, where the average 
value of current drawn from the power supply does not vary. The 
output voltage from a power supply using a choke-input filter will be 
approximately equal to the average value of the rectified a-c voltage. 
This type of filter finds its greatest use where constant voltage must be 
maintained under varying load conditions, as is the case with class B 

90. Bleeder* 

In most power supply units, the rectifier tube is of the filament type 
which begins to pass current immediately after it is turned on. The 

T412G8*— 47 10 


Original from 

tubes used in receivers and amplifiers, however, are usually of the 
indirectly heated type, and do not begin operating as soon a a the high 
voltage is applied. A bleeder resistor places a load on the power supply 
i named lately t thus preventing any high-voltage surge through the unit. 
In transmitter power supplies, the bleeder serves as a device to maintain 
a more constant voltage when the transmitter is keyed. The bleeder 
also serves to discharge the capacitors in the power supply after it lias 
been shut off, thus eliminating any danger of a high-voltage shock to 
the operator f should he have occasion to repair the equipment, 

91. Voltage Divider* 

tt. The various tubes and the different tube elements require different 
voltages, which may be obtained by means of a voltage divider con- 
nected across the output terminals of the filter* This voltage divider 
also serves as a bleeder resistor, with the bleeder current usually 
averaging between 10 and 15 per cent of the total current drawn from 
the power supply. The currents flowing through the resistor and the 
value of resistance between the taps determine the division of voltage 
along the voltage divider. The voltage and current requirements for 
the load must be determined before the power supply and voltage 
divider can be designed. Any change in the load current drawn from 
any particular tap on the voltage divider will affect the voltage dis- 
tribution of the entire voltage- dividing system. 

0. A voltage-divider system typical of those used in modern receivers 
is shown in figure 147. The voltage divider is connected across the 
output terminals, A and E t of the capacitor-input filter, with taps at 
B, C t and D properly located to provide the voltages shown in the 
diagram. The taps A, B, and C are at a positive voltage with respect 

-*— r^uTTi 




Figure 147. Voltage-divider etreutt for radio receiver. 


Original from 

to tap D t which is grounded. The terminal at E is negative with respect 
to D, and any tap along the resistor between D and E will be negative 
with respect to ground. In this instance the maximum plate voltage 
required is 300 volts for the power-output tubes, and the maximum 
negative voltage is 25 volts for bias to the grids of these tubes. The 
total voltage of the power supply must therefore he 325 volts. The 
total current drain for all tubes in this case is 85 milliamperes. To 
this must be added the bleeder drain 15 milliamperes, making a total 
current of 100 milliamperes required from the power supply. Figure 
147 shows this total of 100 milliamperes flowing from the Alter to 
point A of the voltage divider, where it then divides : 60 milliamperes 
go to the plate circuit of the output tubes, and the remaining 40 
milliamperes pass through the resistor R^ to point B. The voltage 
drop across R\ f which is necessary to decrease the filter-output voltage 
to 200 volts for the plates of the amplifier tubes, must be 100 volts. 
Hence, to calculate the value of R lt divide the required drop (100) by 
the current flowing through Ri, which in this instance is 40 milliamperes. 
Thus Ri establishes a voltage of 200 volts between points B and 1) 
on the voltage divider. By Ohm's law : 

fii = -yor ~jj| = 2,500 ohms 

At B the current divides, so that 20 milliamperes is delivered to the 
amplifier tubes and 20 milliamperes continue through resistor 2£ 2 to 
terminal C. The resistor B% must decrease the voltage from 200 to 
90 volts with 20 milliamperes flowing through it. This is a voltage 
drop of 110 volts and by Ohm's law it is found to be 5,500 ohms. At 
the current drain again divides, so that 5 milliamperes is delivered to 
the screen grid and the oscillator plate circuit. The remaining 15 
milliamperes, which is the bleeder current, passes from (J to D t causing 
a voltage drop of 90 volts between C and the grounded tap D r The 
resistance of the resistor between € and D f again calculated by Ohm's 
law, is 6,000 ohms. Bias for the r-f and a-f amplifier tubes may be 
obtained from taps X and Y properly located between D and E t or 
by resistors in series with the cathode circuits. Power dissipation for 
each resistor may be calculated by the formulas PR or El. The latter 
formula is preferable in this particular example, since the voltage 
across each resistor has already been established. The resistors used 
should be of proper wattage rating to carry safely whatever current 
must flow through them, without undue rise in temperature. It has 
been found that resistors maintain their values and have longer life 
if they are worked at about 50 percent of their rated power-carrying 
capacity. The power expanded in resistor Jfi is 4 watts ; therefore, 
it should have a rating of 8 watts to conform to the rule given above, 
A 10-watt resistor is the closest stock size to this value and is thus 
the logical choice. 



("nn'ilf* Original from 


92* Efoctro-mtchawul Poww Supplrn 

Electro-mechanical power equipment includes motor generators, 
gasoline engine driven generators, hand-driven generators, dynamotora, 
and vibrator systems. (All of these electro-mechanical power supplies, 
with the exception of dynamotors and vibrators, are discussed h 
TM 1-455.) Dynamotors and vibrators are used in radio power circuits 
where it is necessary to convert a low d-c voltage (such as might be 
supplied by the ignition battery of a truck, tank, or airplane) to the 
higher voltages required for receiver and transmitter operation, 

93. DyMmoiort 

a. A dynamotor is used to change a low d-c voltage to a high dk 
voltage, thereby fulfilling the requirements of radio receivers and 
transmitters. It is essentially a motor and a generator mounted, or 
wound, on a common frame. A single field winding is used to provide 
the magnetic field for both driving and generating purposes. The 
armature consists of two windings, both of which are wound on the 
same armature core, but connected to separate commutators. One 
winding serves to produce the driving force when energized by a 
low d-c voltage. The other winding generates a high voltage when 
rotated within the magnetic field. 

o. The functional characteristics of a dynamotor are shown in 
figure 148, The heavy line indicates the low-voltage motor circuit 
Current from the battery flows through the field coils and the motor 
winding of the armature, setting up a magnetic field around both. These 
magnetic fields oppose each other and cause the armature to rotate. 





Figure 148* Functional diagram of a dynamotor. 

Since the armature and field windings are in parallel this is called 
a shuTvt-wound motor. With this type of winding the speed of the 
motor remains fairly constant with changes in the load placed upon it 
by the generator. The high-voltage winding, represented hy the finer 
lines between the fields (fig, 143) is wound on the same armature so 


Original from 

that it will rotate with the motor winding. When taming, St cuts the 
lines of fore© of the common field and generates a voltage which is 
collected by the brushes at the high-voltage commutator. The greater 
the number of turns in the high- voltage armature winding the greater 
will be the voltage output, 

e. Filters are placed in the high- voltage leads to filter out high- 
frequency currents produced by sparking between the brushes and the 
commutator segments, so that it does not cause interference with 
radio reception. The filter consists of a combination of r-f chokes and 
capacitors* The purpose of the chokes is to prevent circulation of the 
r-f energy through the external wiring. The capacitors bypass this 
energy to ground. Some additional audio filtering must also be provided 
to eliminate commutator ripple* This will usually consist of a series 
inductor of comparatively high value, and a shunt capacitor. Functional 
characteristics of tbe audio filter are similar to filtering action discussed 
under a-c power supplies, 

d. The circuit diagram of a typical dynamotor power supply is shown 
in figure 149. Filter 1 is an r-f unit to eliminate any r~f energy in the 


Fa *f*t 

Figure 149. Diagram of dynamotor power supply and filter networks*. 

low-voltage circuit. M is the motor section of the dynamotor and is 
connected to the battery, which provides driving power* G is the 
generator side of the dynamotor and the output from this unit is fed 
through the choke coils 2, 3, and 4. Chokes 2 and 3 are r-f choke coils ; 
choke 4 is an iron-core coil. In combination with the capacitors across 
the line, these chokes serve to prevent radiation of r-f energy and to 
reduce commutator ripple in the output voltage. 

e. The maintenance of dynamotors is important to their efficient 

(1) If a dynamotor stops, there may be an open circuit in the motor 
armature, or the field. As a first step, fuses should be checked in the 
low-voltage supply circuit. 

(2) If the dynamotor runs but no high voltage is present, the trouble 
is in the generator section of the armature. Fuses in the high- voltage 
circuit should he checked. 

(3) Brushes and commutators may cause trouble if oil or dirt collect 
on them. Worn brushes should be replaced. 


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