Couple of Super questions.

[didit]

Having taken this 2nd harmonic product for PP class A as given from books, my confidence of my understanding is fading.

Even without clipping, plate voltage vs. grid voltage is nonlinear.

Definitely.

In a Class-A amp the top and bottom halves of the input waveform are reproduced by different parts of the power tube characteristic curves, so second-order harmonic distortion is produced.

True definitely for SE. What's given pause is that, like AB, in class A push-pull there is a pair of tubes; but each conducting simultaneously and each moving through that nonlinearity in opposing polarity. The sum won't perfectly cancel, but the waveform will be roughly symmetric, as pdf64 & JTU suggest. I'll need to think on this, perhaps reread and likely recant. Intuitions now lean to minimal if any output stage generated 2nd order distortion, and instead possibly some soft 3rd order.

All kind of off-topic of the original post but fun.

Best .. Ian

[martin manning]

My understanding has improved a bit... I agree with you about the p-p power stage cancelling its internally-produced second order distortion products when in Class-A due to the recombination of the inverted waveforms in the OT. However, quoting Merlin Blencowe: "...guitar amps rarely use matched valves and usually have a less-than-perfect phase inverter. (Even if the phase inverter is perfectly balanced it will inevitably go out of balance when the power stage is overdriven.)." In p-p Class-A these products will not be cancelled.

[Firestorm]

I wonder, too, about the ability of tetrodes and pentodes to pull their plates to zero. They can't, so I question if there is 2nd order possibility in the varying abilities of tubes to hit the zero node.