Western Electric 407A-4, Instruction Manual

Page: 49 / 136

Third Radio-Frequency Amplifier Stage
Res i stances R93 . 1A and R93 . 2A constitute the grid load
of the third amplifier tub e V3A. The plate c ircuit contains
the anti-s ing as s embly Ll9A and R83A operating into the
transformer L9A and into the coupling circu it through con­
dens er Cl9A. The secondary of L9A in s eries with the adjust­
able condenser Cl5A provides a c ircuit , the contro l of which
is designated "Neutral iz ing 3rd Amp. 11 • The coupling c ircuit
cons ists of trans former LlOA, the pr imary of which is tuned
by condens ers C 51A and C20A, the control of the latter being
designate d "Plate C ircuit Tuning 3rd Amp. 11 • The adjustable
coupling o f this trans former is terme d "Output C oupling 3rd
Amp. " . Its secondary is tune d by inductance LllA in s eries
with condens ers C22 . 1A and C22. 2A, which are shunted by the
adjus table condens er C21A, the control of which is des ig­
nated "Output Circuit Tuning 3rd Amp. " •
Audi o Amplifier
The amplifier which provides the necess ary audio-fre­
quency modulating voltage has one transformer - and two re­
s is tance-coupled s tages employing tube V6A in the first
stage, tubes V7A and V8A in parallel in the s econd stage ,
and V4A and V5A in parallel in the third s tage. The input
to the s ec ond stage is the difference of two voltages, one
the s ignal from the speech-input equ ipment through the first
stage acro s s res istance R70A, the othe r a portion of this
signal fed back from the output of the transmitter, demodu­
lated by a ful £-wave rectifier V9A, and applied acros s re­
s is tances R48A and R50A. The s peech-input circu it cons ists
of the input transformer TlOA, the resis tance networks R73A
to R82A inclus ive , and the contacts of relays SSA and S9A.
The grid b ias of eac h tub e is obtained by the potential
drop across resistances b etween each tube cathode and
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