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Edit 3: in summary I want a make microprocessor-controlled dynamically-variable voltage source. With a range of 100v to 350v dc which can supply approx 10-20ma. The voltage would change quite infrequently. From every few seconds to hours. I am not sure about the properties of switching e.g. slew rate, overshoot etc. but do not believe them to be too import. The slew could be quite slow. And if overshoot was a few volts that would not matter.

I am trying to build a variable power supply that goes from 0-240V (I actually don't need lower than 100V, so 100V to 240V would be fine). This would then be regulated to DC, and require very little current, as I am going to drive a couple of pre-amp tubes. So I don't want to use switch mode supplies, hence the transformer approach.

I tried the following (I used a B5K for testing):

ytested circuit

And it (kinda) worked. The response was not linear though, the higher voltages were in the last few degrees of the pot. So I assume this is not the best way to do it! (I'm new to AC)

My assumptions:

  1. On the secondary, the ratio of the input to output voltage is linear. (e.g. if I sent the secondary transformer 6VAC it would output 120VAC, 3VAC it would output 60VAC etc)
  2. using a voltage divider on an AC voltage will work in a similar way to a DC voltage divider (although I did read with an inductive load this is not the case, but did not quite understand the difference)

Is this approach ok?

If so, what would be a better way to vary the voltage in the 12V section?

Should there be other components in there too? Inductors etc?

I am thinking of 2 ways I would like to control it:

A) a simple pot - conceptually like in the pic

B) digitally - so something like a DCR or vactrol?

Edit: Output current need is about 10ma

The circuit will be used on two 12ax7 tube to vary their plate voltage

This is the design that I took inspiration from:

enter image description here

As it for audio, I am trying to avoid any digital components to reduce the risk of interference, noise etc.

Edit 2: Or would I be better off get the highest dcvoltage I need and using a voltage divider on the output?

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    \$\begingroup\$ You say "require very little current". How little? Could you spell the required number? \$\endgroup\$ Commented Aug 10, 2024 at 2:52
  • \$\begingroup\$ I believe 5ma would be about the maximum \$\endgroup\$ Commented Aug 10, 2024 at 3:14
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    \$\begingroup\$ How about a simple small variac? \$\endgroup\$ Commented Aug 10, 2024 at 3:23
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    \$\begingroup\$ And why do you need it variable? \$\endgroup\$ Commented Aug 10, 2024 at 3:31
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    \$\begingroup\$ To change the plate voltage on the tubes, as this affects the saturation and headroom. I wanted to be able to alter it digitally., i.e. have a microcontroller be able to change the value. \$\endgroup\$ Commented Aug 10, 2024 at 3:39

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