You can do that but you'll want to increase the current limit since the MOSFET will have to handle peak ripple current instead of steady DC. Use a beefy MOSFET and change R4 to 1ohm 1W.
Alright, picked out some stuff. Got a 1kV NMOS in a TO-247 package, 1kV diode and the first NPN I found in a TO-220 package. Thinking I'll bolt the transistors to the back of the case with a mica plate for insulation and nylon hardware 🙂
Oh, well maybe I'll grab something super tiny then and just have that floating around in the mess of wires that inside of my amp sadly is atm 😂
Why not just reconsider using the PT CT and changing the plate load resistors? There is a simple road to your objective and not a major design remake. Your methods to get your voltage down look like just playing with off the wall guesses. Building a chutes and ladders game.
Please point to the plate load resistor:
This worked pretty well, though I think some resistor values need to be changed as I got 550V after the diode and over the reservoir cap. Didn't explode or anything 😁 Thought that maybe the MOSFET would go as it is only rated 5A but it held up at least this time. Used the tiniest little NPN I have ever seen in a THT package and it seemed just fine!
I assume the 330k/1M divider sets the voltage? If so then I'll have to double check those resistors. Maybe they were sorted wrong or something, didn't double check the values of anything but the 1k.
Found what was wrong with it, I forgot the ground to the 1M. Not sure how it even did anything but that's been rectified now and I now have 600Vac into my rectifier and 430Vdc over the cap. Maybe I should swap the 330k with a pot to set the right voltage? Or should I just swap it for a slightly smaller resistor, like 220k?
Yes you can trim the 330k/1M to set whatever voltage you like. You could just add a resistor in parallel with the 330k if you don't want to be too invasive.
Ended up going for 180k instead of the 330k for about 480V out with an RC filter to drop it the last bit. Not really sure why it's so far off the maths though, that's like 30V off what I'd expect..
Either way I think this PSU is solved, just need to add back my bleed resistor for the big cap and I'll be ready to hook up the PSU to the tubes 😄
It's because you put the circuit before the reservoir cap so the cap is no longer being charged in the standard way, which alters the loading on the transformer.