This provides a low source impedance bias supply with only two components, which is necessary to maintain
low inter-modulation distortion in the push-pull stage output.
2-39. Voltage feedback for transistor 1A11Q2 (through resistor 1A11R10, capacitor 1A11C5, and
inductor lA11L3) and for transistor 1A11Q3 (through resistor 1A11R14, capacitor 1A11C6, and inductor
lA11L4), and, current feedback for transistor 1A11Q2 (due to unbypassed emitter resistors 1A11R12 and
R11), and for transistor 1Al1Q3 (resistors 1A11R15 and R 16), provide negative feedback to stabilize the
gain of the push-pull stage. The output of the push-pull stage is impedance transformed to 50 ohms by two
wideband transmission line transformers, wound on toroidal cores, 1A11T2 and T3. The overall gain of the
two-stage driver is 25 dB with a nominal output level of 14 watts. The driver is protected from damage
due to excessive RF input levels up to two watts because the currents and voltages of the Q1 first stage are
limited and the Q2, Q3 second stage can handle the saturated output of Q1.
2-41. The output
of RF Driver
1Al 1 is amplified
to the rated 400 watt
1 dB power amplifier output
level in the grounded-grid class AB tube amplifier stage which consists of Tube Socket Assembly 1A14A2,
Tubes 1A14V1 and V2, and Plate Assembly 1A9. Tubes 1Al4V1 and V2 are two ceramic triode vacuum
tubes, type Y-621B, which are connected in parallel with their grids grounded directly to the chassis. The
cathode bias for V 1 and V2 is developed across a 4.7 volt zener diode VR1 in Tube Socket Assembly
1A4A2. The RF input from the driver stage to the parallel connected cathodes of V1 and V2 is coupled
through a low-pass filter network 1A14A2-L1 and C4. The plate voltage (+600 Vdc) is series-fed through
RF chokes L1 and L4 of Plate Assembly 1A9 (See figure 3-73). A CATHODE SAMPLE signal is picked off
at Tube Socket Assembly 1A14A2 by a capacitive divider network and applied to Tune Detector 1A10 as
one of its inputs. A PLATE SAMPLE signal is picked off at Plate Assembly 1A9 by another capacitive di-
vider network and applied to Tune Detector 1 A10 as its other sample input.
2-43. A pi-type impedance matching ( 1600 ohms to 50 ohms) Tank Circuit Module 1A8 is RF coupled to
the plates of tubes 1A14V1 and V2 through capacitors C2 and C3 of Plate Assembly 1A9. The tank circuit,
along with Harmonic Filter 1A15, attenuates the harmonic output of the RF signal to the antenna coupler.
The Tank Circuit consists of a continuously variable coil series element (1A8A5L1) and bandswitched input
shunt capacitors (1A8A1C3, C4, and C6 through Cl4 and output LC shunt networks 1A8A2L1 through L1
and 1A8A2C15 through C24). In normal operation (tune condition) the proper shunt elements have been
selected and the 1A8A5L1 inductance has been adjusted automatically to match the tube impedance to 50
ohms at the desired operating frequency as described in Servo Controlled Adjustment of Coil 1A8A5L 1.
2-44. The output of Tank Circuit 1A8 is applied through Harmonic Filter 1A15 and the energized con-
tacts (pins 4 to 6 and pins 3 to 5) of relay 1A7A1K1 to antenna coupler RF connector 1A14J4. Relay
1A7A1K 1 is energized by the Tx/Rx LINE Signal from System Control Assembly 1A3, during the Trans-
mit mode of operation. The output of the power amplifier is kept constant by the ALC (Automatic Level
Control) circuit 1A7, which is pre-adjusted to provide a 400-watt nominal output into a 50-ohm load. In
the Transmit mode of operation, the receive RF bypass line is grounded by relay 1A1K1 to prevent un-
wanted signal feedback to the input of the power amplifier.
2-45. For system operation,
the RF output
of the power amplifier
is normally connected
to the antenna
through Antenna Coupler CU-2064/GRC-l93 (see figure 1-2). Direct connection to a 50 ohm, nominal.
antenna system is also permissible, but a dummy antenna coupler plug is required at 1A14J1, wired to feed
back to the power amplifier a COUPLER READY ground signal. Otherwise, the power amplifier will not