Owen Duffy wrote:

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The question is can the plate (or the whole transmitter for that matter)
be accurately replaced by an equivalent series circuit of a fixed voltage
generator and fixed equivalent series resistance (independent of load).

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The answer is NO!!

Power Amplifiers are not linear devices! [Regardless of manufacturer's claims].

My AL-80B puts out ~850 watts with ~1300 watts input. IF a conjugate match
existed the output would be ~650 watts. The Zo of the 3-500 varies depending on
the phase of the pulse signal. The amplifier provides a pulse of energy into the
tuned circuit, tank circuit, tuner, antenna, where the reactances maintain the
current/voltage under resonant conditions.

My understanding is that reflected energy is also coupled into the tuned
circuits. The active device, power amplifier, may be cutoff, saturated, or
somewhere on the active load line. The energy in the tuned circuit will increase
the Vmax and/or Imax depending on the circuit QL. This increase in stored energy
provides the extra stress on the active device. If the QL 10, nominal design
value, then 90% of the reflected energy is re-reflected back towards the load.
The missing 10% produces heat in the tuned circuit.

Devices that operate from saturation to cutoff are by definition NON-LINEAR.

The tuned circuits store both forward and reflected energy. Ultimately all the
power is radiated, either as a rf field or as heat.