If. Amplifier, 455-Kc, Detailed Analysis

29. If. Amplifier, 455-Kc, Detailed Analysis

the plate load for V8. A11 if. transformers

(fig. 18)

in tuning unit 1 are double-tuned and loaded

t o provide optimum skirt selectivity. The

The 455-kc if. amplifier consists of four

p r i m a r y winding of T31 resonates with

tuned pentode amplifier stages in tandem,

C138 and the secondary winding resonates

to provide a high degree of amplification

w i t h C141. Resistors R67 and R68, re-

a n d selectivity. B + voltage for the tube

s p e c t i v e l y , serve as loading resistors for

plates and screens is provided to the junc-

the primary and secondary tuned networks

tion of decoupling resistor R87 and capac-

of T31. The first if. output signal is cou-

i t o r C128 only when the MEGACYCLES

pled through C144 to the grid of the second

band switch S1 is rotated to bands 1 and 3.

i f . amplifier. Capacitor C144 is used to

I n the other four positions of the band

prevent the fixed-bias voltage from short-

s w i t c h , the switch open-circuits the 150-

ing to ground through the secondary of T31.

v o l t bus, which leaves the entire 455-kc

i f . amplifier inoperative.

c. If. Amplifiers V9 through V11. T h e

s e c o n d , third, and fourth if. amplifiers

a. Bias Voltage Circuit. The amount of

are similar in circuitry to the first, except

fixed negative dc voltage applied to the grid

for their biasing networks. Capacitors

of first if. amplifier V8 is determined by

C146, C153 and C159, and inductors L9 and

the setting of the front panel GAIN control

L 1 1 (fig. 99) prevent rf from circulating

R43. This potential is applied to the grid

in the common heater supply line.

by feedthrough capacitor C129 and effec-

(1) S e c o n d if. amplifier V9 receives

t i v e l y varies the gain of the stage. Re-

a fixed bias from a voltage divider

s i s t o r R63 functions as the grid resistor;

in the negative 150-volt bus which

Cl31 decouples signal variations from the

applies approximately -1.5 volts dc

b i a s power supply. A fixed-bias potential

t o i t s g r i d . C a t h o d e bias is de-

of - 1 . 5 v o l t s w i t h r e s p e c t t o c h a s s i s

veloped by R72.

ground is coupled through resistor R71 to

(2) T h i r d if. amplifier V10 operates

the grid of second if. amplifier V9. Capac-

with self-bias. The cathode current

i t o r C143 provides additional filtering

through R77 develops a dc voltage

action for this bias potential.

w h i c h is positive in reference to

b. First If. Amplifier V8. The attenuated

the grid (pin 1). The grid has a dc

s i g n a l from the if. step attenuator is fed

p a t h to ground through the low-

t h r o u g h resistor R62, tuned transformer

r e s i s t a n c e secondary winding of

T 2 9 , and coupling capacitor C133 to the

T32.

grid (pin 1) of first if. amplifier V8. Ca-

(3) F o u r t h if. amplifier V11 operates

pacitor C133 prevents the fixed-bias volt-

with self-bias. The cathode current

a g e from shorting to ground. Capacitor

through R83 develops a dc voltage

C 1 3 2 resonates the secondary winding of

which is positive with reference to

T 2 9 to 455 kc. Resistor R64 loads the

the grid (pin 1). The grid has a dc

secondary of T29. Resistor R65, bypassed

p a t h to ground through the low-

b y C135, develops cathode bias. Resistor

r e s i s t a n c e secondary winding of

R69, shunted by C137 and C139, forms a

T33. The output signal from V11 is

plate decoupling net work for the B+ voltage,

a p p l i e d to the if. output selector

which is fed through the primary winding

through T34.

o f T31 to the plate (pin 5) of the tube.

R e s i s t o r R66, bypassed by C136, drops

30. If. Amplifier, 1,600-Kc, General

the B+ voltage to a level required by the

Analysis

s c r e e n grid (pin 6). Capacitor C130 acts

a s a bypass for the suppressor grid (pin

T h e 1,600-kc if. amplifier is identical

7 ) , which is internally connected to the

in operation with the 455-kc if. amplifier.

c a t h o d e . Capacitor C134 and inductor L8

F o r the complete schematic diagram of

(fig. 99) prevent rf from circulating in the

t h i s amplifier, r e f e r to figure 99. The

h e a t e r line. Transformer T31 serves as

1,600-kc if. amplifier, like the 455-kc if.