"Reg Edwards" wrote -

Take a look at their DC characteristic curves. Internal resistance
is
the SLOPE of the anode (collector) current versus anode (collector)
volts characteristic for constant grid (gate) volts (or constant
base
current).

=================================

And, incidentally, this is the internal resistance which determines
whether or not a Conjugate Match exists between the PA device and its
load resistance.

Note that it is not under the control of the circuit designer or
tuner-upper who is not permtted to change tubes during a tune-up
operation in order to obtain a 1-to-1 SWR.
----
Reg, G4FGQ