I wish to buy a power supply delivering up to 500V for experiments with thermionic valves. What would be a suitable current?

It would be helpful if you would list some of the valves numbers and how many valves you plan on operating at one time. Do not forget that you need the filiment voltage also. That could be 5, 6.3, 12.6, just to start with. You may need other voltages, but they are the most common. The current for one valve could run from 50 to 200 ma.

So far as making ones own is concerned, I found this while looking for something else.

Power supply current?

Apr 09, 2020 24 Replies

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Phil Allison 6 years ago

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** Fair enough - the OP has been very vague.

** But not *power* triodes - like the 300B.

They like 700 to 800V.

** Only IF you use toriodals.

The magnetising current with cheap E-cores will spoil that quite a bit.

Look, I admit have done it myself for a low current, tube pre-amp with a pair of 12AX7 / ECCL83s.

FYI:

I admit I have done many things myself I would never recommend on an internet forum while ever I use my REAL name.

There is so much risk of *misinterpretation* of a seemingly good idea.

One reason being, 500VDC is dangerous, concieveably fatal for anyone with little experience.

BTW I think you are one of the better, more serious posters here.

.... Phil

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Phil Allison 6 years ago

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** I do know it can - but no so much at higher currents.

** Well, it in fact does.

AC supply transformers are NOT intended to be run backwards.

Take a 15VA, 240V to 6.3V tranny - the off load voltage is gonna be about 7.5V. The gives a turns ratio of 32:1.

On load, 6.3V multiplied by 32 = 201V - so not 240.

Now, the approx 30mA magnetising current has moved from the primary secondary, so multiplied by 32 = 0.96 amps.

In reality you will be lucky to get 180V before rectification, filtering and load.

Been there, done that.

... Phil

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Pimpom 6 years ago

OK, I'll take your word for it. As I said, I did that "many moons ago" - decades in fact, and I don't remember the details.

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Jeff Layman 6 years ago

About right. Over 30 years ago I used back-to-back toroidals to get a safer supply to a 230V pond pump 10 metres away in the garden.

The matching toroidals were 230:18-21-24-27-30V and rated at 3A. I used the 30V output of the first, ran it through 10m of 1.5mm^2 cable, and connected it the 27V secondary of the second toroidal. The pump was rated at 50W, and, from what I remember, I got about 220V out from the primary of the second toroidal when under load. It worked that way without problem for many years.

Jeff

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Peter Percival 6 years ago

So far as making ones own is concerned, I found this

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while looking for something else.

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