Reg, G4FGQ wrote:
"What allows a class-C amplifier to exceed 50% efficiency is a small
operating angle."

Exactly, and during the majority of the degrees it`s switched completely
off. It draws no current and suffers no "IsquaredR loss" during the
amplifier off-time. Impedance is approximately E/I, but I is the average
I, which is much less than the bursts of I during the conduction angle.

The switched-off time makes the I in the denominator of E/I very small
indeed and the solution to Ohm`s law is a high impedance without the
dissipation of a resistance that remains in place continuously while
agitating the atoms of a poor conductor to limit current.

Instead, we have a low-resistanc in high conducton for short spurts.
On-time is limited, instead of conduction, to produce a certain
effective resistance.

An automobile Kettering ignition system may use a dwell-meter to
indicate how much of the time the points are closed. An ohmmeter
indicates the resistance between its test prods. The two test circuits
are almost the same although limitation of the deflection of the
dwell-meter is different from limitation of the deflection of the
ohmmeter due to the difference between limited conduction angle ignition
points, and the continuous conduction through a current-limiting
resistor. There`s an analogy between Class C and Class A amplifiers in
there somewhere.

Best regards, Richard Harrison, KB5WZI