[Amps] Plate Load Impedance

[David C. Hallam]

I am going through the exercise to design the Pi-L network for a new
amplifier.  Calculating the plate load impedance by two different methods, I
come up with two vastly different answers.
With a tetrode and the formula  for class AB1 operation, I get about 3400
ohms.  I realize that this formula is an approximation and the factor of 1.3
might not be quite right, but.

Using the constant current curves in the Eimac data sheet for the tube,
4CX1500A, and the method given in their book, I come up with a much lower
value, around 2500 ohms.  I also realize that there are some approximations
in this method too.  However there is a significant difference in the two
methods.

Does anyone have any ideas where the truth lies so I can get on with the
construction of the Pi network?

David C. Hallam
KC2JD