[Amps] Glow, gettering, and voltage

[Manfred Mornhinweg]

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

========================
Visit my hobby homepage!
http://ludens.cl
========================