Reg Edwards, W4FGQ wrote:
"---when the load resistance is changed, the tube`s operating conditions
also change, and so does the tube`s internal resistance."

I would think that change in operating conditions result from
readjustment of the amplifier, not from changing the load on it.

Most of the output impedance of a Class C power amplifier tube does not
come from the saturation resistance when switched on, but comes from the
duration of the period in which it is switched off. Until you readjust
drive, the on and off periods and even the saturation resistance of the
tube remain unchanged.

Best regards, Richard Harrison, KB5WZI