Universal Tube Power Supply

[mcgruff]

I hugely enjoyed building (and playing..) a 5F1 Champ last year and this has really got me interested in amp building. There are lots of different circuits I'd like to try out and I might even try designing/adapting some once I learn a bit more about tube electronics.

Do you think it's feasible to build a "universal" power supply? The idea is to have a single box which can supply a range of plate voltages for a workbench filled with half-a-dozen different circuits (supplying one amp at a time). This could bring down the cost of trying out a new circuit.

[nickb]

I hugely enjoyed building (and playing..) a 5F1 Champ last year and this has really got me interested in amp building. There are lots of different circuits I'd like to try out and I might even try designing/adapting some once I learn a bit more about tube electronics.

Do you think it's feasible to build a "universal" power supply? The idea is to have a single box which can supply a range of plate voltages for a workbench filled with half-a-dozen different circuits (supplying one amp at a time). This could bring down the cost of trying out a new circuit.

I think my approach would be low tech.

Use two transformers, one 3.15-0-3.5 at whatever current rating you need/can afford for the heaters. Then a second say 350-0-350 (or whatever you deem to be your [B]maximum[/B/ voltage) rated at whatever you want for a current rating that feeds a full-wave recitfier/ smoothing caps. Feed the second transformer only from a variac - you can get the quite cheap, even for new. Just be careful not to set the variac output higher than the transformer's input rating (115V etc...).

You could have a switched series resistor in the HT for sag, I guess too.

[olddawg]

Back in the old days test equipment manufactures marketed universal power supplies for troubleshooting and design applications, exactly what you are looking for. A quick search on EBay turned up this Fluke model. You can find Eicos, HPs , Sencores, and others for much cheaper if you look around and take your time: http://bit.ly/1iwElsI

[Enzo]

Regulated Power Supply IP-32 Equipment Heathkit Brand,

Like that?

Or
Heathkit IP-17 HV Power Supply

Kepco 605 Regulated Power Supply

[mcgruff]

Thanks for all the advice. I'd quite like to try building one to keep the cost down but I've a lot to learn first. If I can figure out something which could be useful and is easy to build I'll post a schematic.

[Enzo]

Steal the circuit. Any tube amp needs 6vac for heaters, and most any tube amp power transformer has that. You will need B+, preferably variable. If you are building power amps, then you will need maybe up to 500v. For preamp circuits 400v ought to be enough. There are various power scaling MOSFET circuits on the web, and one of those might be useful. You could also steal the B+ regulator circuit from a variable supply like the Heath units. Most such supplies also include a bias supply - a low current supply up to 100 or maybe 150v. Voltage and current meters are REAL useful.

So the power transformer from something like a fender Twin Reverb ought to be an ideal start for a bench supply

[Chuck H]

Can't add much to what's already been said. I might build in a power supply sag circuit, Something to account for different mA ratings and rectifier types for when an actual PT is purchased for an amp. A big ol 100W 200 ohm rheostat could make for good approximations.

[mcgruff]

There seem to be two ways I can approach the power supply.

(1) supply a single rectified DC voltage (for B+)

A normal circuit passes this around to all the tubes. The voltage drops a step for each resistor in series in the plate voltage circuit and some additional caps provide extra filtering. The first tubes (power tubes) get the highest, least filtered voltage and the last tubes (preamp) get lower voltages and less ripple. To change a B+ voltage for a single tube would require swapping out resistors in such a way that the B+ presented to other tubes is not altered.

(2) supply an individual B+ for each tube

Now you can easily adjust the plate voltages for a single tube without affecting the B+ supplied to other tubes. This makes it easier to explore and experiment.

Suppose I also supply bias voltages individually per tube with a battery and a pot to easily control bias voltage. Would this be a fruitful area for experimentation in order to fine tune the tone/headroom/compression etc or are easily tweakable plate and bias voltages simply not worth the effort?

[Enzo]

Don't get ahead of yourself. We are now apparently only considering the B+ part?


In a typical amp - Twin reverb say - the first stage of filter cap serves the power tube plates. And of course all the subsequent stages. it will have the most ripple, a few volts. In most of our amps, the output is push pull, so that ripple cancels out on the PP stage.

Through a resistor to provide isolation and some voltage drop, we arrive at the second node, which powers the power tube screens. By this point ripple should be about gone. We then move through some more resistors dropping the voltage further for the preamps. At these stages, the filter caps are not there to remove ripple so much as they are there to decouple the preamp stages.

Usually the voltage drop chain is a linear thing, but nothing stops you from getting to the screen node, and there branching out through individual dropping resistors to individual caps for each stage in your mock up. That would make it easy to alter just one stage.


Keep in mind that almost all preamp circuits are class A, and so have a steady current draw. So if you started with 400v and had nodes of 350, 300, 275. You could change that to 400-375-300-275 by changing two resistors. All of which allows you to get a lot of experimenting done with a single variable B+ supply.


Did you look at the commercial products? They include a low current variable bias supply. I would surely include one in my design if I were you. I would hope it was a totally independent circuit so I could ground it as I saw fir in use, but you could easily use a tapped B+ winding and make it a dedicated negative supply.

[olddawg]

By the time you buy all the parts, you would be better off just buying a bench supply like I mentioned before. There is no longer a demand for them. I see theme all of the time for around. $50 and they have accurate meters and controls. Also skip all of the time and headache if fabrication. Get an oscilloscope while you are at it.