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DRIVER STAGE,
A 5763 tube operating with a combination of fixed and automatic bias, is used to drive the final
amplifier. As the stage has plate voltage supplied and the cathode cireuit closed, sufficient bias
is necessary to keep it from drawing plate current during key up or standby conditions. The ad-
ditional automatic bias establishes the operating bias during "key down" condition, A meter
shunt resistor is permanently connected in the cathode circuit and the meter swilched across it
when reading driver current. The resistance of the shunt is such as to cause the meter to read
50 milliamperes full scale. Potentiometer control of the driver screen voltage allows control
of the grid drive to the final amplifier, The plate circuit of the 5763 is shunt fed through an RF
choke. Pi network interstage coupling is used between the driver and a final amplifier with the
input capacitor of the pi section variable and the output capacitor fixed. The pi section indue-
tance is tapped and the proper tap for each band selected by a section of the band switch. The
use of pi network interstage coupling helps reduce the harmonic output of the transmitter. The
fixed output capacitor of the pi interstage coupling appears from grid toground of the final am-
plifier and shorts out the higher frequency harmonics.
FINAL AMPLIFIER
'Two 6146 tubes operating at approximately 740 volts are used as the power amplifier. Here
also, a combination of fixed and automatic bias 1s used to establish the operating and standby
condition. In addition to the fixed bias, the screens of the tubes are clamped by a 6AQ5. The
action of the clamp tube is as follows:
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