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).