300V Variable PSU for Tubes

284

03:57:39

Just a simple linear regulator with an IRF740 MOS and a small signal transistor like BC550. The maximum power dissipation of the MOS is around 40W, so either don't use low voltages at max current or supply it with a sufficient heatsink, to draw the heat. The current limiting is set to 130mA and can be prefixed with the 5.5ohm resistor. Come to think about it, you can substitute the 5.5ohm resistor with a 3.3ohm resistor for maximum current of 215mA and add a 22Ohm 1/2W trimpot in series to fine adjust the current. The voltage drop across the trimmer will be so little at such little current, that it won't burn up the trimmer. Keep in mind that at 215mA the maximum power dissipation for the MOS will become around 60W, so you decide how to keep it. All resistors should be 1/2W. There is current protection of course. As it is very much needed at such voltages. For the 1uF capacitor don't go above 2.2uF or the output response will be quite delayed. I recommend you keep it at 1uF and increase the 100uF to 220uF, maybe even 470uF. The diodes are 1N4007 for up to 220uF and 1N5408 for 470uF. The transformer is 1:1 isolation transformer. The upper potentiometer is fine voltage control and the bottom one is course.

published 10 years ago

giomix

10 years ago

I often appreciate your circuits, well described and mentioning real components. One year ago I realized a variable power supply for tubes, using ... a tube (Ecl86 triode-pentode). Output is 100 to 380 volt at 70mA maximum current. Unfortunately I forgot to provide PSU with a current limiter. So I'm searching for a simple solution to add limiter directly at the output + - , externally to PSU case because inside there is not enought space.

thebugger

10 years ago

Yeah you can absolutely make a completely tube circuit. With tube rectifier and tube regulator, but nowadays you can find the transistors for 50 cents each, so i guessed it would be cheaper to make a transistor power supply. The only expensive thing in the circuit is the transformer, but it's mandatory in both transistor and tube circuit, so not much of a difference there

thebugger

10 years ago

BTW the limiter ca be a simple fuse. You'd have to change it if it blows though

giomix

10 years ago

Yes , of course perhaps in my case is the simplest thing to do, thank you for idea.

thebugger

10 years ago

Actually if your circuit is similar to this one, MOS and Tubes and kinda similar i guess the same principles can be applied for current limiting, but transistors are better for regulators, because their current capabilities are limited by the current limiter itself, whereas the tube is limited by its internal resistance, and if you were to short circuit the output, the tube may fail, or even explode. I've heard of exploding tubes, although it's rare. Moat of the times they just crack up and lose transconductance due to oxidation.

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