[Amps] tube impedance, figuring tank circuit values

[Peter Chadwick]

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