I wonder whether someone can help me to clear an issue regarding AM signals
produced by a (class-C) tube transmitter.

It is well known that the peak-envelope power of a carrier 100%-modulated by a
pure sine wave is four times the unmodulated carrier power. This is simply
because, according to the Ohm's law, doubling voltage power gets four times
higher.

I would have no problem if a tube would behave like a normal resistor. But a
tube instead behaves like a differential resistor, and this makes a lot of
difference.

If you consider the plate characteristic of a typical pentode, you see that
plate current does not change much increasing plate voltage. So, doubling the
plate voltage, power would not get four times higher, but just a little more
that two times.

The triode plate current is instead much more dependent on plate voltage so
that, doubling plate voltage, power would grow probably at least three times.
But definitely not four times.

Can someone pinpoint what I am missing?

73

Tony I0JX