>>ZL1AXB has successfully used a circuit which tunes the filament
>>and this improves the efficiency dramatically. This tuned cathode
>>network takes the form of a bi filar circuit, and eliminates the
>>usual filament RF chokes. ZL1AXB demonstrates key-down >400W
>>output on 10m with a single 813 with Pv of 3000v.
I have done the same for a 50 MHz TB3/750 amplifier I made some years
ago. I used a bifilar inductor, a shunt C to resonate the lot and power
feed was somewhere 2/3 up from the cold end, indicating an input
impedance of around 100 Ohm. Worked okay, but I don't recall any great
improvement in efficiency. I wouldn't want to make a circuit like this
for a multi-band amp, but in a single band amp it works fine.
Gerard
PA3DQW, AA3ES
>
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