If. Preamplifier, Detailed Analysis - TM-11-6625-351-350095

b y p a s s e d by capacitor C432, is the B+

capacitor C419 to output jack J406. Co-

v o l t a g e - d r o p p i n g resistor. Resistor R421

axial cable assembly W402 feeds the out-

and capacitor C433 decouple the plate from

p u t signal to the 42-mc if. preamplifier.

the B+ line. The 42-mc rf output of 2401 is

fed to first if. amplifier V404 through C431,

46. If. Preamplifier, Detailed Analysis

which is housed within the shielded tuned

(fig. 29 and 30)

net work.

A three-stage 42-mc preamplifier is

c. First and Second If. A m p l i f i e r s

u s e d to increase the signal output of the

V404 and V405 (fig. 29 and 30). Two pen-

t u n e r before application to the if. step

tode if. amplifiers, similar in circuit

attenuator. This preamplifier consists

a n a l y s i s , increase the signal amplitude

of one cascode driver-amplifier stage

d e v e l o p e d by cascode amplifier V403B.

a n d two if. voltage amplifiers. Connector

Since the two stages are similar, the fol-

J 4 0 7 on the if. preamplifier chassis cou-

lowing description applies to both stages,

ples the incoming signal from the rf tuner

with the exceptions noted below:

t o the primary of if. transformer T401.

(1) C a t h o d e bias for the first if. am-

The secondary of transformer T401 is con-

plifier, V404, is developed by the

nected to the grid (pin 2) of cascode driver

voltage drop across R419, which is

V403A. Capacitor C423 resonates with the

bypassed by C434. Cathode bias for

s e c o n d a r y of T401 at the intermediate

the second if. amplifier, V405 (fig.

triode

a. Cascode Driver V403A.

3 0 ) , is developed by the voltage

cascode driver is used because it is a low-

drop a c r o s s R427, which is by-

n o i s e stage, w h i c h is desirable for low-

passed by C441.

level input signals. Its low input impedance

(2) F i r s t i f . a m p l i f i e r g r i d r e s i s t o r

matches the low output impedance of the

R418 is connected by feedthrough

c o u p l i n g circuit, w h i c h consists of if.

c a p a c i t o r C447 to the front-panel

transformer T401. The amplified output of

GAIN control, R466. Resistor R465

t h e driver is coupled through capacitor

GAIN control R466 serve as a volt-,

C427 to the cathode of grounded-grid

age divider to develop bias voltage.

cascode amplifier V403B. Cathode bias is

The setting of the GAIN control for

developed by the voltage drop across R410,

this stage affects the total gain of

w h i c h is bypassed by C426. Part of the

both the preamplifier and the five-

a m p l i f i e d voltage across plate load re-

stage if. amplifier (para 48). Sec-

sistor R411 is coupled back to the grid by

ond if. amplifier grid resistor R426

capacitor C425 and neutralizing coil L417.

i s c o n n e c t e d by feedthrough

These components are utilized to prevent

c a p a c i t o r C446 to a voltage di-

the triode from oscillating. Resistor R411

vider consisting of R467 and R468.

and capacitor C480 provide decoupling for

T h i s voltage divider develops the

the plate circuit. The bifilar wound coils,

fixed bias for the second if.

L 4 1 9 A and L419B, together with bypass

amplifier.

capacitor C448 and choke L421, decouple

(3) T h e first if. amplifier screen re-

t h e heater circuit from the 6.3-volt ac

sistor R422, is bypassed by C435.

Supply.

a n d the second amplifier screen

b . Cascode Amplifier V403B. The sec-

resistor, R429, is bypassed by

o n d half of triode V403 operates as a

C442.

grounded-grid amplifier. The rf input

(4) T h e first if. amplifier plate load

v o l t a g e is developed across coil L418.

c o n s i s t s of R423 shunted by the

Cathode bias is developed by the voltage

t u n e d network, Z402. Resistor

d r o p across R413, which is bypassed by

R425, which is bypassed by C437,

C428. Resistor R412 serves as a parasitic

d e c r e a s e s the plate voltage. The

s u p p r e s s o r in the plate circuit. Resistor

second if. amplifier, the plate load,

R415, in shunt with if. tuned network Z401,

c o n s i s t s of if. transformer T404.

forms the plate load. Resistor R417,

R e s i s t o r R432, which is bypassed