"whit3rd" wrote in message
...

The filament winding is on the same transformer as the HV,
so the filament turns off when the transformer isn't powered;
and magnetrons are a diode, it ONLY has the HV electrode
available to control the power.

Ineed, the patent I referred to shows the filament secondary as part of the
main transformer. But the system presumably works.

If there were separate filament and HV transformers, the price
would reflect this 'feature' in unpleasant ways.

I can't imagine a separate filament transformer being /that/ expensive.