WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
However, Rp is always large cf good cathode follower Zo, so I think variation should only be small. Neither does gm vary much with Vp, a bit but not enough to worry about, or is this a different way of saying the same thing?
Anode impedance will not be large when compared to cathode impedance if mu is low. That is one reason why low mu valves are best not used as cathode followers - despite this being popular.
@DF96 - I suspect it's different ways of saying the same thing. Rp can be modelled as a resistor between anode and cathode in parallel with a voltage controlled current source gm mA/V ? So yes, Rp effectively is in parallel with the output impedance, as you say, for a cathode follower.
