(from TM 11-866)

21.  POWER SUPPLY UNIT. -- Power Supply Units RA-74-C,
RA-84-B, and RA-94-A are alike except for variations in the
power transformers (T1, T2, and T3 respectively).  These 
differences are described in detail in PAR. 2 and PAR. 7e.  The 
power unit furnishes "A", "B", and "C" voltages for the receiver.
The "A" or heater voltage, is 6.3 volts a-c obtained from a separate
secondary winding (1-2) on the power transformer.  "B" voltage
is obtained from the center-tapped high-voltage secondary (7-8-10)
connected to the plates of the "B" rectifier tube VT-145 (V1).  After
this voltage is rectified, it is filtered by the combined action of the
first filter choke (L1) and the first two 8 microfarad sections of filter
capacitor C1.  This provides 380 volts d-c for the plates of the
power output tubes in the receiver.  Further filtering by the second
filter choke (L2) and another 8 microfarad section of C1 provides
250 volts d-c for the plates of the remaining tubes in the receiver.
Approximately 100 volts d-c for the screen grids of the receiver tubes
is obtained from the tap on the bleeder resistor (R1), which is 
by-passed by the remaining section of C1.  Negative "C" voltage
is obtained from a tap (9) on the high voltage secondary connected
to the filament of the "C" rectifier tube VT-80 (V2).  The rectified
output from the plates of this tube (V2) is filtered by the three
8000 ohm sections of resistor R2 and the four 3 microfarad sections
of filter capacitor C2.  When connected to the receiver, the voltage
at the end of this filter is approximately minus 50.