Richard Clark wrote:
"The presumption (forced or otherwise) is that the output is sinusoidal.
In fact, the cathode current of the amplifier proves quite positively
that only a pulse in, 180 degrees of sinewave, or even less, is
sufficient to generate a remarkably clean sinewave at the final`s
output."

That is a remarkably clear statement of the behavior of a Class C
amplifier. The amplifier acts as a generator of a sinewave which is
synchronized by its input signal instead of being an accurate reproducer
of the waveform at its input.

The less than half wave of current flow of the Class C amplifier allows
an efficiency exceeding 50%. Walt Maxwell`s tests show that the Class C
amplifier sticks to the parameters of a Thevenin source.

The question of "what is the source impedance" presented to a load by
the amplifier? is answered, not by magic, but by the maximum power
transfer theorem. The amplifier must be adjusted to deliver all its
available power. Then, the output impedance of the amplifier is simply
the conjugate of the load impedance which is easily measured.

Best regards, Richard Harrison, KB5WZI