Hi Cathy, Jim, and all,
I agree that a part of the higher dissipation is caused by higher idle
current, multiplied by higher plate voltage.
But I think that you are wrong in a few points here, Jim, although most
is correct:
IF you run the same po in both settings, the plate current will be a
lot less in the higher B+ setting. Your plate load Z will have
skyrocketed.
Only if Cathy re-tuned the tank circuit for optimal efficiency at the
higher voltage and the same power. If instead she kept the same settings
for the tank circuit, then the plate load Z will remain the same, the
RF voltage on the plate will also remain the same, but being centered on
the higher supply voltage the tubes work farther from saturation,
and thus more linear but at less efficiency.
The actual load Z on the plate depends on the antenna impedance and the
PI tank values that have been set. Not on the supply voltage and desired
power! But of course the supply voltage and desired power define what
the best load Z would be.
And there is one mistake in this:
With higher plate load Z, the tank coil has to have twice the uh. The
tune + load caps have to be reduced to one half their values. Since
that is not done, your resulting tank circuit Q will have
doubled..... like from 12 to 24. Sky high loaded Q = lousy
efficiency. Lousy eff = higher plate dissipation.
The mistake is that a very high loaded Q will cause poor efficiency due
to increased loss in the tank coil, and to a lesser extend perhaps the
capacitors and switch, but not in the tubes! Except for the fact that if
this is taken to an extreme, the tubes needs to deliver more output
power to supply the tank's losses in addition to the desired output
power, and this may, or may not, incur in higher plate dissipation.
So, Cathy, did you do that test while keeping the plate and load
settings constant, or did you retune for the higher voltage?
Manfred
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