If. Amplifiers K207 and V208

b. If. Amplifiers K207 and V208 (fig. 25).

p l i f i e r V208. Capacitor C275 pre-

A l t h o u g h all four if. amplifiers are es-

vents the bias voltage from short-

sentially the same in function and descrip-

ing to ground. Grid resistor R252

tion, the method of biasing V207 and V208

i s connected to a fixed source of

differs from that employed in the circuits

n e g a t i v e bias potential (R244 and

for V209 and V210 (fig. 26). A second dif-

R 2 4 5 ) through feedthrough capac-

ference exists because double-tuned trans-

itor C271. Resistors R244 and R245

formers are connected in the plate circuits

form a voltage-divider network

of V207 and V208; single-tuned impedance

f o r the -15-volt bias line and are

n e t w o r k s are sufficient to maintain the

physcially 10 c a t e d on terminal

desired selectivity in third and fourth

b o a r d TB201 (fig. 64). Except for

the difference in biasing, cor-

amplifiers, V209 and V210 (fig. 26).

r e s p o n d i n g parts in the second if.

(1) T h e s e c o n d a r y o f i f . i n t e r s t a g e

amplifier state function identically

transformer T205 is tuned to 10.7

with those in the first if. amplifier

rnc and feeds the signal to the con-

stage.

trol grid of V207 through capacitor

C266. Cathode bias is supplies by

c. If. Amplifiers V209 and V210 (fig. 26).

The output of second if. amplifier V208 is

the voltage drop across R249, which

is bypassed by C268. Grid resistor

c o u p l e d to the grid of third if. amplifier

V 2 0 9 . This stage operates with cathode

R 2 4 8 is connected through feed-

bias developed across resistor R259, which

through capacitor C267 to the var-

i a b l e arm of GAIN control R241.

i s bypassed by capacitor C282. Resistor

T h e feedthrough capacitor func-

R 2 6 1 , bypassed by C283, decreases the

screen grid and plate voltage. Tuned coil

tions as an rf bypass and couples

t h e bias voltage from the panel-

L 2 2 7 completes the dc path to the plate

mounted GAIN control to the grid of

(pin 5) of amplifier V209. Resistor R263

V207. The amplification of the sig-

a n d capacitor C286 provide decoupling.

nal through V207 is dependent on

A d d i t i o n a l decoupling in the +150-volt

the bias voltage applied to the grid

supply line is provided by low-pass filter

(pin 1) through GAIN control R241.

F L 2 0 1 . This filter permits the 150-volt

Resistor R251, bypassed by C273,

B + output from the power supply to be

decreases the screen grid and

c o u p l e d through feedthrough capacitors

p l a t e voltage. Resistor R254 and

C297 and C296 to the 150-volt bus in the

capacitor C269 provide decoupling

i f . amplifier subassembly; simultaneously,

f o r the plate and screen circuits.

t h e filter coil presents a high-impedance

Bypass capacitors C299 and C302,

i n p u t to the 10.7-mc frequencies present

together with chokes L231 and

i n the plate and screen circuits. Bypass

L232, prevent rf currents from

capacitors C303, C304, and C305, together

c i r c u l a t i n g in the heater circuit,

w i t h chokes L233 and L234, prevent rf

The output of V207 is applied to the

c u r r e n t s from circulating in the heater

circuit. A second low-pass filter network,

primary of T206. The primary

winding and the parallel capacitor,

F L 2 0 2 , provides additional protection in

C272, resonate together at the

a s s u r i n g that the heater power source is

i n t e r m e d i a t e frequency. The out-

f r e e of residual rf and if. currents. The

output of V209 is applied to tuned imped-

put of T206, developed across the

ance network Z201. This network consists

secondary winding and parallel

capacitor C274, is coupled to sec-

o f tuned coil L227, which resonates with

o n d if. amplifier V208. Resistor

c a p a c i t o r C284 to the desired center fre-

R 2 5 3 lowers the Q of the trans-

quency of 10.7 mc. Capacitor C285 is in-

c l u d e d within the shielded network, and

f o r m e r tuned circuit.

(2) The signal from tuned transformer

c o u p l e s the output to fourth if. amplifier

T206 is coupled through capacitor

V 2 1 0 . Resistor R262, also housed within

C275 to the grid of second if. am-

t h e shield, is the grid return for V210.