Reg, G4FGQ wrote:
"The active device generally behaves as a current source."

As Reg also wrote:
"I can`t imagine why this conversation has continued for so many years
by more or less the same group of experts."

Agreed! Reg seems to have answered his own question.The same people
recite the same arguments in hopes their view of reality will be
accepted. Fat chance! Time has inured them.

Reg has faithfully proposed constant-current behaviour from all vacuum
valves and transistors as I recall. I agree that most of these devices
have extremely high plate ond collector resistances as linear
amplifiers. Current through them is almost constant regardless of anode
voltage.

As most transmitter power amplifiers exceed 50% efficiency by a good
margin, these devices are not operating as Class-A linear amplifiers.
They instead operate as HF switches. These are turned-off most of every
cycle and are only on for short pulses. Harmonics and other noise is
cleaned up by output filters. It`s the only thing which makes the output
linear.

During the output device`s conduction, its saturation volts are very low
and its current is very high, giving the device a very low impedance
while switched-on. You may not infer a low impedance from the d-c volts
and amps feeding the final amplifier. These are the averages, almost, of
the device amps. The device saturation volts sre what counts toward its
dissipation and loss. The transmitter usually has no built-in indicator
of saturation voltage. It wouldn`t read much anyway.Device
impedance depends mostly on its ratio of off to on times. This is a form
of lossless resistance. Dissipation is zero in a sewitched-off device.
The d-c volts and amps are related to the output device(s) internal
impedances used as a switch when the transmitter output is considered. A
high voltage and a low current accompany a high internal impedance but
they won`t be nearly so high as the spec sheet plate or collector
resistances.

We have d-c power input to the amplifier. We can measure HF power
output. The difference is dissipation, but loss resistance does not
represent the total source resistance because we have non-dissipative
resistance in the device off-times.

There have been measurements of transmitter internal output impedances
which indicated that they did indeed match their loads. I have not done
it myself but have no reason to doubt the reports.

Best regards, Richard Harrison, KB5WZI