HV power supply for stomp pedals??

[bacanador]

For a while I've been looking at alternatives for making an HV power supply small enough to fit inside tube pedals.

I have found a few designs with switching power supplies, but recently I found this thread.

http://www.diystompboxes.com/smfforum/index.php?topic=29134.0

Where you can use a regular transformer backwards to generate HV AC and rectify that. Like the Electro Harmonix Hot Tubes. The issue is I wouldn't want to run the tube heaters with AC.

So my question is would it be OK to rectify the 12VAC on the primary to get DC voltage to the heaters.

[Jarno]

A more precise wording would be: put 12VAC on the secondary of a transformer to use the HV AC off it's primary to feed the tubes. The 12VAC you put on the secondary of the second transformer (because you need a first one to get the 12VAC in the first place) is ok to rectify and regulate, but you will need regulation because you end up with 14-16.8VDC and that's too high. Also you can only use this with tubes having center tapped heaters.

[bacanador]

Thanks for the response.

Sorry about that. Even tho I worded it weird you understood it.

A more precise wording would be: put 12VAC on the secondary of a transformer to use the HV AC off it's primary to feed the tubes.

Yes, that is what I meant by using it backwards, I would get a 120VAC to 12VAC wallwart and connect the 12VAC to the secondary of this transformer, and get 120V on the other side.

The 12VAC you put on the secondary of the second transformer (because you need a first one to get the 12VAC in the first place) is ok to rectify and regulate, but you will need regulation because you end up with 14-16.8VDC and that's too high. Also you can only use this with tubes having center tapped heaters.

Yes I would use a regulator to get either 12.6VDC or 6.3VDC for the heaters, I was just wondering if it has any effect on the rest of the circuit.

[The Tone God]

If you don't need over 120v you can use a transformer with dual primaries but only wire one primary to the line. If you use say 12v secondaries you can use those to power the filaments and the unused primary becomes your isolated line voltage AC. The only major caveat is you have to use a transformer with double the current rating on the secondaries because you are only using one primary so the secondary gets half the current. Doubling the rating compensates.

Its a great trick for quick for getting above 100V B+ for preamp circuits. If you want an example I used this in the "Black Top" design.

Andrew

[Gus]

google
"Royer tube microphone mod"
Look at the power supply circuit

[Jarno]

You could have a look at the DIY projects on Gyraf.dk as well, but those are aimed at those of us with 220-240VAC.

[amptramp]

Same idea we were discussing here:

http://www.diystompboxes.com/smfforum/index.php?topic=86354.msg724607#msg724607

I have a supply for antique battery portable tube radios that was made using the back-to-back transformer technique.

[davidallancole]

If you can find some, the old "Razor Only" plugins in the bathrooms were usually isolation transformers.

[Gus]

If you google for what I posted before you will note the power supply uses one 24VAC transformer not two back to back.  Clever circuit, the RCs slow start the heater and you can get about 100VDC for the B+.  I have built a power supply like that in electrical outlet box

Building a DC heater supply looks easy but there are design rules you need to understand.  It might be better to use the 12VAC with twisted heater supply wires and maybe elevate the the heater supply above ground.   And if you are using 12VAC for a 12VDC supply there is a good chance by the time you have it adjusted for low noise with RCs filters and/or a regulator the supply will be lower than 12VDC.

Also you need to be mindful of the transformers VA ratings if you want to do the back to back circuit.

[defaced]

maybe elevate the the heater supply above ground.

That's what I'd do. AC and elevate.  DC heaters look good on paper, but they require more parts, are larger because of the filter cap size, and grounding can be sensitive. 

[tubelectron]

Hi bacanador,

So my question is would it be OK to rectify the 12VAC on the primary to get DC voltage to the heaters.

Yes. Indeed. It works well. And with a single 12AX7, I use only RC filtering, no 7812 or LMxxx regulator : 4700µ/25 - 2x10ohms/3W - 10000µ/16. Doing this, I have 12,5VDC 0,3A with negligible ripple on the scope (inaudible) from a transformer delivering 13,1VAC and 15,7VDC after the bridge rectifier.

Tip : if the stompbox transformer is close to the audio circuit, chances are you will need to shield the Xfo with 0.5mm aluminium folded sheet or encasing, as it is done in my tube stompbox below :



the shielding sheet is upper left, the RC filtering as described is upper right on the picture.

A+!

[bacanador]

Thanks for all the info guys.

@tubeelectron: That is a pretty amazing job you did there.

I was planning on using an LM317 to regulate the DC but I might try straight AC or non regulated DC.

[tubelectron]

Hi again bacanador & thanks,

I was planning on using an LM317 to regulate the DC but I might try straight AC or non regulated DC.

I had problems with regulators/tube association (insulation, cooling, erratic operation, death...) so I am not akeen to use a device which makes me lose my time for unreliability - Even if I know that some of you use it satisfactorily with tubes.

The non regulated/only RC filtered works very well if you use rather big caps and enough watted resistors (don't forget the ground point at heater center or between 2x100ohms resistors if there is no center). It doesn't take so much room compared to a regulator and its mounting requirements... And it's reliable.

I also experimented the direct "usual" AC heating in overdrive and preamp/booster stompbox, and - indeed - it's more simple and room-effective, but there is often a little AC hum impossible to balance due to the serial operation of the 2 heaters of the double triode. So it is worth to heat the tube in DC for hum-quieter operation.

A+!