You have to figure on the supply volts under load.
Let's assume we're in AB1 for an example. From the tube data sheet, we can
choose the plate voltage when the grid volts are zero - usually just about the
knee in the characteristics. That tells us that the load impedance is equal to
the supply volts minus the knee volts divided by the plate current at Vg=0.
Exactly the same principle applies if running AB2 or Class B, except you have
to decide the positive grid volts that represents the peak of the cycle. Then
you have the peak plate current, and the minimum plate volts. Again, the
difference between the supply volts at that particular amount of current load
and the minimum plate volts, divided by the peak plate current, gives you the
load resistance.
The factors relating the peak plate current, conduction angle and the DC drain
on the PSU are more complex....
73
Peter G3RZP
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