Ive been playing around with some old UPSs that are the switch mode
type, the ones without the big iron transformers. I m not sure if I
have a handle on how they work or not but it seems very similar to a
Class D amplifier amplifiying a 60HZ input, well sort of. It seems
like they dont actually input a 60Hz signal rather they just go
directly from digital by generating the pulse width modulated
waveforms with a computer. I was thinking that a transformer might be
placed before the output low pass filter to step up the voltage for
tube circuits, maybe even a TX PA. No doubt for this to have any merit
the LPF would have to be redsigned.

Jimmie