THEORY OF OPERATION
This chapter describes overall, interrelated, and individual mechanical functioning and electrical
theory of operation of the assemblies and circuits that comprises the power amplifier.
OVERALL FUNCTIONAL DESCRIPTION
The power amplifier consists of the following major and minor functional circuit groups as shown
a. RF Amplifier Circuits
b. System Control Circuits
c. Tuning Control Circuits
Power Supplies and Cooling Circuits
a. Power Supply Circuits
b. Cooling Circuit
c. Monitoring and Test Circuits
100 milliwatt input signal (J5) from the receiver-transmitter is applied through input relay
1A1K 1 to the input of RF Driver Module 1A11, where the signal is amplified by the two-stage transistor-
ized wide-band driver amplifier to an output level of about 15 watts. This output is the drive signal for
the grounded-grid output power amplifier stage, consisting of two parallel-connected type Y621B triode
vacuum tubes, plate circuit assembly 1A9, and a tunable pi-network tube tank circuit. The tunable tank
circuit transforms the 1600 ohm tube plate impedance to the 50 ohm output impedance and provides
selectivity to suppress harmonics. Tank circuit tuning is accomplished automatically in less than five
seconds. The shunt elements are selected in eight bands by the motor (1A8A4B1) operated rotary switches
(S l-A, S1-B, and S l-C). The series coil L1 is fine-tuned to the proper inductance by the servo loop and
servo motor (1A8A3B1).
The system control circuits perform the following functions: ON-OFF switching of primary power,
Rx-Tx mode switching, fault switching, and Automatic Level Control (ALC) generation. The first two
functions are initiated upon receipt of commands from the receiver-transmitter, while the last two func-
tions are initiated within the power amplifier.
The ON-OFF switching of primary power is initiated by the PA ON/OFF signal from the receiver-
transmitter and controls Power ON/OFF Relay 1A14K 1. This relay connects +26.5 Vdc to all circuits that
require +26.5 Vdc.
2-10. The Rx-Tx switching function is controlled by the KEYLINE signal from the RT. In receive, both
the input relay (1AIK1) and the output relay (1A7A1K 1) are de-energized, allowing the receive RF signal
to bypass the power amplifier circuits. In transmit, both relays are energized connecting the exciter