I am building an amplifier for 6m with a 3-500Z. I have 2.6kV EHT, plenty
of drive and am looking for something like 500W of output. I can get 300W.
I have had two problems.
Firstly a nasty parasitic at 190MHz that did not go away with the usual
shunt resistor on the anode link. It required an HF style parasitic choke
made from 2 turns of 1/4 inch copper sheet close wound on 3/8 dia, shunted
with 2 47ohm 2W metal film resistors. This works, but I worry about losses
at 50MHz.
Second problem is the output circuit. It tunes OK but either the grid
current is too high, or the efficiency too low. To me this implies the
impeedance transformation is not correct. The circuit is a pi-L with (up
to) 15pF Tune, 4T dia 2" of 1/4" pipe, 200pF Load and then 5T 1/2" close
spaced.
I think I might need 5 turns on the first tank coil, but then the tune
capacitance will then be very low. The tune capacitor is a silver plated
wide spaced butterfly of 30pF per section with a very low minimum C.
Good advice appreciated.
Mike
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