1296 MHz Quarter KiloWatt Cavity
amplifier
Updates notes and additions to the original article as it
appeared in QST , March 1985:
Also see article "1296 high power ring hybrid combiner" for additional info on combining and tuning of this amplifier
Always use Machlett or GE 7289 tubes. I don't use any other brands or tube types as most exhibit very poor thermal stability; 3CX100s. Don't use any pulse tubes: 7815, 7211, same thermal problem. One must remember that this amplifier is running the tube at 4 to 5 times the specified performance limits!
Spec: 1000 VDC at 125 mA
running: 1400 VDC at 450 mA
Always keep the input return loss (SWR) optimized and do it at low power levels (1W) initially. This can even be done correctly without any high voltage. All that's required is filament voltage....try it!
Most people who have called with a problem haven't read the article throughly and haven't tuned the input for best SWR (carefully). If the input SWR isn't tuned correctly, the amplifier will seem as though it's tuned to a different frequency (anode tuning). Once the input tuning is optimized at low power (1 watt), don't detune it very far when driving the amplifier to high power levels, it may arc the ptfe tuner.