Stand-Alone Bias Supply..? (for bench work)

[lonote]

I am trying to organize/supplement my bench equipment to be able to more or less breadboard some lower-powered amp circuits & realized that I do not have the means to provide a negative bias voltage, short of building the actual PSU (or going cathode bias).

Seems like there should be a simple lo-tech way to provide that voltage, but it is not coming to me. I am wondering what people are using in that situation?

[martin manning]

What are you using for the B+ supply?

[lonote]

I have ended up with a retired 0-250V DC medical power supply that I thought I would try out. Not a high B+ but I also have a few sets of lower voltage power tubes (6AQ5, 275V rated) I would start with. I already had a separate filament source on hand.

This is all in the thinking stages, sort of going through the steps in my head before I put anything together.

[martin manning]

I'd look for a 24VAC transformer and put a voltage doubler on the secondary. That should get 60-65 VDC.

[lonote]

I'd look for a 24VAC transformer and put a voltage doubler on the secondary. That should get 60-65 VDC.

Thanks, that sounds simple enough. I will head that direction.

I have a idea & design for a kind of "universal" bench amp power supply; essentially a higher voltage,regular 4 or 5 node supply but with adjustable dropping resistors & built on a smaller module-type plate.

I included a bias circuit on that plan, but it only just occurred to me the medical unit alone (290+V out unloaded) could cover any number of smaller circuits, given a separate filament supply & a bias supply were sorted out.

[sluckey]

I included a bias circuit on that plan, but it only just occurred to me the medical unit alone (290+V out unloaded) could cover any number of smaller circuits, given a separate filament supply & a bias supply were sorted out.

Most small amps that operate with 250V on plates of output tubes will also be operating with cathode bias. I doubt you will ever need a fixed bias supply with that medical supply.

[lonote]

Most small amps that operate with 250V on plates of output tubes will also be operating with cathode bias. I doubt you will ever need a fixed bias supply with that medical supply.

You are probably right, but I wondered about how to do it if I wanted the option.

Looking at Matchless Spitfire/Fender Harvard sized stuff, sort of where you could go either way with biasing.

[TUBEDUDE]

You can use 33 9V batteries for B+, 2 6V lantern batteries for heaters. Bias with small assorted batteries. I did this for a while until I found a surplus Hearh power supply.

20241223_180835.jpg

[LOUDthud]

Those old Heathkit supplies are so cool. If you can find a Ham Radio swap meet, you're sure to find one cheap.

[lonote]

Those old Heathkit supplies are so cool. If you can find a Ham Radio swap meet, you're sure to find one cheap.

That is a pretty cool PSU. But I don't think they are giving them away, only a couple on eBay at the moment, listed at either side of $300.


I might have to revise my own PSU build into a similar cabinet.

[TUBEDUDE]

It'd be hard to build this for $300.
Mine was $300 a few years back from a surplus house. The pass element is an RCA blackplate 6L6GC.

[R.G.]

Mouser 546-187A56 $12 > Hammond 187A56 56Vct, 40ma: FWR > 77.8V;
That's the chassis mount version. If you can stand to build it all one perfboard/PCB, the board-mount variants of 48 to 60V small transformers cost as little as sub-$7.

100Vdc or higher rated 47uF filter cap
4x 1N4004 diodes

Regulator of some sort: I'd use a 75V 1/2W zener, Mouser, $0.12. If you need more stable than a zener feeding a pot for setting, I'd use a zener feeding a pot feeding a MOSFET pass element.

The really costly stuff is, as always, the externals:
- box to put it in
- meter, if you want to read it on the box; digitals are cheap these days
- plug in terminals
- line cord/socket/fuse

[jbrrrrr]

Mouser 546-187A56 $12 > Hammond 187A56 56Vct, 40ma: FWR > 77.8V;
That's the chassis mount version. If you can stand to build it all one perfboard/PCB, the board-mount variants of 48 to 60V small transformers cost as little as sub-$7.

Welp, I guess I just got handed a new project that I must embark on... thanks for the BOM, R.G.!

[lonote]

Mouser 546-187A56 $12 > Hammond 187A56 56Vct, 40ma: FWR > 77.8V;
That's the chassis mount version. If you can stand to build it all one perfboard/PCB, the board-mount variants of 48 to 60V small transformers cost as little as sub-$7.

100Vdc or higher rated 47uF filter cap
4x 1N4004 diodes

Regulator of some sort: I'd use a 75V 1/2W zener, Mouser, $0.12. If you need more stable than a zener feeding a pot for setting, I'd use a zener feeding a pot feeding a MOSFET pass element.

The really costly stuff is, as always, the externals:
- box to put it in
- meter, if you want to read it on the box; digitals are cheap these days
- plug in terminals
- line cord/socket/fuse

Thanks for that, I appreciate you taking the time.

I will add it to the list, most of the bits already on hand.