Hi Jeff,
> My guess: Design the plate output circuit to resonate with a bit less
> than the rated output cap of the plate plus strays. Then put a series
> capacitor in the plate to tank coil lead to reduce the effective cap by
> the "caps in series" formula. This enables the 813 to operate at the
> highest possible frequency but grossly limits ability to band switch to
> lower frequencies ( where the series cap becomes much more significant
> impedance). It doesn't matter if the plate output is PiNet, tapped coil,
> link coupled. This approach allows the design to eliminate the
> traditional output tuning cap to ground and use tube cap instead.
I've seen it suggested in VHF amplifier articles that this reduces
tank Q. That isn't true.
You can make the amp run on a higher frequency....maybe....but
the tank circuit is a mess. The tube looks into a series tuned
circuit with low impedance, lower than if you didn't have the series
cap. Q of the *system* is actually increased over using less L and
not having the capacitor.
> An alternative exists as shown in some 6M amp designs...use
> no plate cap to ground, no variable series cap. Instead tune
> the plate tank coil as a variable inductor, implemented by
> swinging a shorted turn in-out or around in the coil. Another
> builder showed a coil that used variable compression of the
> turns.
The above suggestion is the lowest-Q option.
73, Tom W8JI
w8ji@contesting.com
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