power supply

[DaveC]

Mark - thats an interesting approach. The ripple frequency for a full wave rectifier is 2x mains frequency per phase...so for 3 phase, you would have a 300 to 360 Hz ripple which is much easier to filter. The unfiltered ripple voltage for three phase is only about 13.4% of peak voltage (100 - 86.6) rather than 100% of peak, so you gain there as well.
With 415 V (rms ? ) input you have 586 volts DC available for primary which means the turns ratio is quite modest for an output of 20 or 30 kV.

Tubes also are nice for direct line connection in that the higher plate resistance means more protection against shorts and overloads.

With the transformer cores you mentioned, you could build the supply as a low frequency Tesla coil, and let it free run. The cores would keep most of the EM fields inside, so noise would not be too serious. You would no doubt need a good shielded enclosure, for safety, anyway.

The only issue on frequency that comes to mind is the diodes for HV rectification. They typically like frequencies below about 50 kHz for the fast recovery ones. (Microwave oven diodes are mains frequency types. But the 10kV fast recovery diodes are only a couple bucks each and they are good for 20 -30 mA)



The only issue

[guest]

I spoke today to a radio ham friend who is a bit doubtful about using rectified 415 Vac through 833B tubes, he tells me they are happier with around 2000 volts to run off which would require an input step up transformer and would start to make things pretty complicated. I scored three steel cabinets from the rubbish dump today, they are 8 foot tall modular units in grey powder coat and lined with refractory composite insulating material about 3/4 inch thick throughout. Apparently a local electrical engineering firm went bankrupt and these items weren't bought at auction and so were dumped along with a spool half full of co-ax cable about an inch in diameter (looks like foam dielectric). I bought them (brand new) along with a 5 Kw three phase transformer for $50 Australian which is a bit over $25 US bucks!!!. Anyway whatever power supply I end up with, I now have a cabinet for it and maybe some cable to connect it up with.

[DaveC]

The cabinet find sounds great. I would be careful with the coax, to find out if it is good for HV applications. Foam dielectric cable is usually used for low loss characteristics. It is typically NOT suitable for HV work. For HV work, you need a solid insulation (like PE or PTFE) is you are going above a few hundred volts. It's fine for other thing , of course. Just watch the applied voltage levels.

Regarding the tubes preferring 2kV for plate voltage, that would mean your plate current is limited to about 500 mA, so at 580 or something VDC you would only get about 1/4 to 1/3 the power. Also the transconductance will be lower at the lower plate voltage, meaning maximum plate current could be lower still.

So to follow your original idea about 3phase 415, you would need to consider a pretty stout voltage quadrupler circuit. Not impossible, but a bit more involved.

Dave C.