Western Electric 407A-4, Instruction Manual

Page: 22 / 136

The direction of rotation of the blower motor will be
reversed at each change of phase connection to terminals
34, 35 and 36, but this can be corrected at the motor
terminals.
D-99120 Bias Rectifier Unit
(
Unit K, Schematic Figure 3
)
Circuit Description
Three rectifiers for supplying bias voltage to the
232B tubes in the D-151135 modulating amplifier and the two
298A tubes in the D-99118 power amplifier are mounted in
this unit.
The single-phase full-wave rectifier consists of power­
supply transformer TlK, filament transformer T2K, 249B
vacuum tubes VlK and V2K, choke coils LlK and L2K, filter
condensers ClK and C2K, load resistor RlK, and voltmeter
multiplier R6K
(
which is associated with protector PR2K
)
.
It supplies a potential of approximately 600 volts d-c for
biasing the grids of the 232B tubes in the D-151135 modu­
lating amplifier. Some adjustment of this voltage may be
obtained by means of the slider on RlK.
One of the three-phase half-wave rectifiers consists
of power-supply transformer T6K, filament-supply transformer
T5K, 249B vacuum tubes V5K, V6K and V7K, choke coil L5K,
and load resistor R2K. It supplies approximately 500 volts
d-c for biasing the 298 vacuum tube VlE of the D-99118
power amplifier. Filter condensers for this rectifier are
located in the D-99118 unit E. Taps are provided on T5K
for adjusting the filament voltage applied to the terminals
of V5K, V6K and V7K to a value as close as possible to 2.5
volts. Taps on the primaries of T6K permit a variation of
the rectifier output voltage. They should be set to obtain
a value as close as possible to 500 volts.
15