Re: HV regulation.
I confess that I don't like using solid-state devices (apart from rectifiers) in power supplies for valve equipment. I see too many blown ones come in for repair. I'm not at all surprised you've already snuffed a transistor out. If they fail short-circuit then that can put a lot of volts onto valves which might not be expecting them.
As G6Tanuki has suggested you can take a valve-based approach. They can fail too but because the supply and the load are operating on more equal terms it seems to happen less often. The noise from VR-tubes takes a little handling but it's perfectly possible to achieve very good performance. The two things I would say are i) that the regulator can draw significant current itself, which might be an issue with a weedy mains transformer, and ii) that depending on the circuit chosen one or more valves might need isolated heater supplies, which again can raise mains transformer issues.
Cheers,
GJ
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