WSQT wrote:
This is what is meant by "Class F." A Class F amplifier has traps
tuned to various harmonics to allow the use of a longer conduction
angle and/or square wave drive with high efficiency and excellent
modulation characteristics.

Class F, class X, class Y.

They are all recent "names" hung on what are really just class C
amplifiers.

I can do the exactly the same thing without a so called "trap" so long
as the output device sees a high impedance at the third harmonic or
better yet at all off harmonics.

That isn't a trap anyway, it is a resonator. The function is to provide
a high impedance at the third harmonic so the waveform slope is steep.
It allows the output device to go through the transitional state where
it is dissipating maximum power very fast. The goal is NOT to allow a
longer conduction angle. The goal is to have the output device either
on or off and spend less time in the high dissipation area where device
resistance is neither very low or very high.

The ideal duty cycle is 50%, with half time totally off and half time
in saturation.

This is why a very low mu trode with tons of negative bias worked so
well for RCA. The grid and anode 3rd harmonic resonators allowed the
grid and anode to see a very high impedance at the third harmonic, and
this sped the transition time up.

73 Tom