On Jun 2, 3:43*pm, Keith Dysart wrote:
....
From the 8146B spec sheet, depending on the operating point, Rp (i.e.
slope of plate E-I curve)
is between 10k and 30k so for the two tubes in the circuit it would be
5k to 15k ohms. But this
number is not much use for computing dissipations.

...Keith

Indeed, Keith...nor is it much use, by itself, for computing the
source impedance seen looking back into amplifier's output RF
connector, at the output end of the network that transforms a (50 ohm,
or whatever) load to the impedance seen by the tubes driving that
load.

If the tube is operated with cathode (RF) grounded and driven with a
stiff voltage source, the impedance looking back into the plate side
will be the plate resistance, shunted by stray and tube capacitances;
but put a bit of unbypassed cathode resistance in there and the
resistive part seen at the plate will be higher, possibly quite a bit
higher. Use a triode instead of a pentode, and the plate resistance
will be much lower, even though the optimal load presented to the
plate may well be quite similar to the optimal load presented to a
tetrode. Run the tube grounded-grid and the impedance seen looking
back into the plate will be much higher than if it's run grounded-
cathode.

I think it helps to understand that the "optimal" load presented to
the tube depends on what you want to accomplish: you want a
reasonable amount of RF output power with reasonable efficiency and
tube operating conditions that will yield good tube life (normally, in
our circles). If it's supposed to be a linear amplifier, you want to
keep (intermodulation) distortion products below some level. All that
has very little to do with the impedance seen by the load looking back
into the tube.

Of course, essentially the same situations exist with transistors and
FETs as with tubes.

Cheers,
Tom