Western Electric 407A-4, Instruction Manual

Page: 47 / 136

when the thermostats are clos ed and curren� flows in the
heater cir cuits .
Each os cillator is adj us te d as a unit to the operating
frequency and will maintain its calibration well within ±
10 cycles .
The radio-frequency output obtained from terminal
3
of
each os cil lator is connected to the "Oscillator " switch D5A,
which permits c onnecting either oscillator to the grid cir­
cuit of the first amplifier tub e . Plate supply for the os ­
cillator is als o connected thr ough this switch and is ob­
taine d from a s lider on resistanc e R6 2A.
First Radio-Frequency Ampl ifier Stage
The radio-frequency input voltage to the first ampl i ­
fier tub e VlA i s impres s ed across resistance R3A. Condens ers
ClA, C 2A, and C3A provide radio-frequency paths to the cath­
ode of VlA and to ground. The gain of this stage and thus
the output to the transmitter is c ontrolle d b y changing the
setting of the "R. F. Output11 c ontrol R4A, which varies a
bias voltage applied to the grid of VlA through the resist­
ances R47A and R3A. The potential drop in R4A is obtained
thr ough the "Reduced Power Adj . 11 potentiometer R56A, from
the l owest s l ider on R6 1A. The plate of VlA works into the
anti-res onant circuit cons i sting of the inductance LlA in
parallel with the variab le condens er C 5A which is tuned by
the control 11Plate Circuit Tuning Vl .
u
Condens er C4A pro­
vides the radio-frequency return path to the cathod e of VlA.
Plate potential of VlA is supplied through tuning coil LlA,
choke coil L2A, and resistance R6A from a s lider on res ist­
ance R60A in the rectifier load circuit . Scre en voltage is
obtained through R5A from a s l ider on R60A. Condenser C 3A
provides
�
radio-frequency by-pas s from screen grid t o cath­
ode . Plate current of VlA is measure d by the potent ial drop
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