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29. If. Amplifier, 455-Kc, Detailed Analysis
the plate load for V8. A11 if. transformers
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
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
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.

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