12AX7 test power supply

[JJH0906]

Hey group,

Has anyone got a suggestion of how I could create a selected voltage to power a group of 12AX7s for testing? The current test set up I have is throwing out like 496VDC, which is unloaded so that's to be expected, but I've got to knock that down to 270-300 VDC.

I'm using some amp books to figure out a lot of things but I've been having a hard time trying to nail down how to do that. Is it just a matter of figuring out the voltage drop between Rl and Rk to get the current and then figure out the node resistor to supply the tubes from there?

Attached is my little test set up that I've put together. Basically all I have is power, I put in some 12AX7 tubes with a filament voltage but don't have anything else going on at the moment.

Any advice would be appreciated!

[Phil_S]

If you must use that PT, take the power supply from half the HT secondary. In other words, use Red and Red/Yellow, or Red/Yellow and Red/Green. Instead of 325-0-325, you'll have 162.5-162.5. Full wave bridge, cap input should give you about 230VDC, if my recollection about FW Cap input is right. You'll need a rectifier that doesn't need a CT.

Another thing you might do is to use a voltage divider. I don't like that because it is a lot of heat to dump somewhere.

Otherwise, select another PT with a lower voltage HT secondary.

[JJH0906]

Oh gosh, why didn't I think of that?!

I have all kinds of electrical test equipment but I do not have a voltage generator. So I will have to use the power transformer that I have. I agree about the voltage divider, in fact I try to do that and things got quite warm.

I don't have another rectifier but I think I've got some diodes laying around so I'll just rewire things to create a full wave rectifier with the 162 volt. This is just for some testing to try out some circuits with the try out so as long as I can get to a decent supply, I should be all right.

[martin manning]

There is no 162 VAC. The secondary is 325-0-325.

[Stevem]

In terms of having a near 120 hz hum free amp it’s a good thing that you have all of that voltage because you can use large value ( resistance ) resistors feeding your filter nodes which helps to make any value of Uf that your filters have more effective.

[Phil_S]

There is no 162 VAC. The secondary is 325-0-325.

If he uses 325-0, is that not the same as 162.5-162.5? Curious minds want to know if there's a difference.

[martin manning]

325-0 (VAC) will have peak voltage at 460. There is no center tap. Any way you rectify 325 VAC (half- or full-wave) it will produce approximately 460 VDC (higher with a light load).

[bepone]

Has anyone got a suggestion of how I could create a selected voltage to power a group of 12AX7s for testing? The current test set up I have is throwing out like 496VDC, which is unloaded so that's to be expected, but I've got to knock that down to 270-300 VDC.

mosfet regulator (simliar to VVR) , any voltage can be adjusted on the output for thesting 12AX7

[JJH0906]

I tried looking through eBay and some other sources last night for a voltage generator. I found some at an extraordinarily expensive price.

Then I got to thinking just as you had suggested, perhaps building a small power supply with two outputs that could be adjusted between 200 and 250 volts on one, and then 250 to 350 on the other. That way it could be used to power a couple different branches of the tubes.

Building one in a solid state format like you suggested would not be that expensive and pretty handy!

[bepone]

money you dont need. all is very cheap. actually you need 2 x 250-300VDC, isolated windings, and one 0- -14VDC with some positive LM317 or negative LM337 or similar for negative bias supply, i have built several testers like this

[JJH0906]

Hmm... Interesting! Maybe I can explore that as well.

I think it would be pretty handy to have, thanks to many suggestions by the group here, I've taken a step back and tried to thoroughly understand the various circuits before actually going into a build.

That way I at least know what kind of results I should expect, rather than just building with family and having it not work and not understanding why.

[Andy Le Blanc]

You can use a single 12ax7/12at7/12au7 as an adjustable voltage regulator. You'll lose around 50v off the top, with the insertion loss, and a gain a usable voltage range. Simple circuit, tube/socket, a 100r and a 10k ww pot. Has around 5w capacity. From there you can step up the watts by using a pentode like a 6v6 or 6l6 for the same application. I'll have to go digging for the scheme,..

[Phil_S]

325-0 (VAC) will have peak voltage at 460. There is no center tap. Any way you rectify 325 VAC (half- or full-wave) it will produce approximately 460 VDC (higher with a light load).

Martin, asking for your patience here, I'm feeling a bit thick on this one and I'd like to understand. Or maybe I'm not communicating my point clearly. I am thinking "325-0" wasn't what I meant -- that suggests not using the CT, but using the full winding.

We have a transformer with 120V primary and 325V CT secondary, or 162.5-0-162.5. Let's color the secondary windings to aid in clarity, making them Red (162.5)-Yellow (0)-Blue (162.5). Let's ignore load related losses. I think it is clear that there are always some load related losses.

If I run 120V into the primary, cap the Blue secondary wire with heatshrink, and take the secondary output from Red-Yellow, I expect to see 162.5V. Is that right? Assuming it is, rectified with FW Cap input, it will yield 180*1.41 = about 229VDC. Am I missing something?

[sluckey]

We have a transformer with 120V primary and 325V CT secondary

That's where you are mistaken. It's not 325 CT. Read the label on his transformer...

290dx.png

[Phil_S]

We have a transformer with 120V primary and 325V CT secondary

That's where you are mistaken. It's not 325 CT. Read the label on his transformer...
290dx.png

I've only been to college 3 times. You'd think I'd have learned to read the instructions. Thank you for point this out.