A plate (and screen) modulated AM tube Class C amplifier needs an audio power of half of the carrier-level DC input power to the PA.
For a 500 kW carrier output, the input power is around 650 - 700 kW, and the modulator output needed will be around 350 kW, whatever means are used to create it.
The RF excitation and output tank must be dimensioned to create
4 times of the no-modulation carrier power at the positive modulation peaks (PEP, peak envelope power).
Sorry, Phil. I meant Doherty amplifier. Been too many years since I worked as a transmitter "baby sitter".
This is very apparent when a modulation transformer is used.
When an audio controlled power supply is used, the whole power flows through the power supply, Compare it to a HVDC generator. Instead of generating a 50/60 Hz sine wave, a biased audio waveform feeds the output tube. Both the switching mode power supply as well as the class C tube can run at a high efficiency.
A 100% sine-wave modulated AM signal has an average power of 1.5 times the carrier power. The extra 50% comes from the modulator, regardless how the modulator output stage is configured.
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