Dan:
Someone suggested that you use 8x811A in parallel. Before starting to
punch out the holes for all of those sockets (not to mention the 200W
filament transformer) I would suggest you consider the following basic
electronic facts that would confront you.
Assuming anywhere nearly a normal operating regime, the optimum plate
load for that bank of tubes operating anywhere near their maximum, will be
in the range of 500ohms. At 10 meters, for a properly designed PINET plate
circuit, you will end up with a target L value of arounf 300nH, around 500nH
for 15M.
Next consider the fact that with that physically large bank of tubes,
the total anode lead from the distributed anodes to the output capcitor, and
through that to return to the GG, is not likely to be less than a foot long.
With reasonable size conductors, you will be facing the nominal 25nH per foot
of inductance, which when multiplied by your ca. 1 foot of wire, uses up your
ENTIRE budget of inductance. In other words, most of the L in your tank
circuit will be the anode wiring itself. This is necessarily a low-Q way to
build an inductor and will result in considerable losses. Hence, your 10 (and
possibly 15) meter efficiency will stink.
I have found this basic issue slightly troublesome in the 4 tube
designs, which always tend to show decreased efficiency on 10. Good designs
get the 10 meter tank coil as close as possible to the anodes to reduce this
effect.
The solution is obvious - you need to raise Rplate, by raising the
plate voltage and lowering the peak current. This will lead right to the 572
or similar. This also reduces the drive requirement considerably by not
having to drive the plate current as high (for a given tube).
73
Eric von Valtier K8LV
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