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Tube Power Supplies (Design)

5/18/1999 12:56 AM
Jim L.	Tube Power Supplies (Design) I want to build the Mesa V-Twin Pedal but I am unsure about designing a power supply for it. I would like to use a AC adapter and then step that up. Can I use a 12 volt AC adapter and then have a transformer inside to step that up? What about currant, will I have enough to make the circuit operate properly? What are some guidelines I should follow?? Has anyone here done this before? I need some help as I have never worked with HV transformers (yes, I am aware of the dangers) I have ordered the small Allied Transformer Steve suggested will that work,...RG suggested I use IN4007 diodes for the recifier section...is there anyone that can draw up a nice 2 tube power supply schematic that uses a 9-12 volt AC adapter? I would appreciate any help! Jim

5/18/1999 12:56 AM

Jim L.

Tube Power Supplies (Design)

I want to build the Mesa V-Twin Pedal but I am unsure about designing a power supply for it.

I would like to use a AC adapter and then step that up. Can I use a 12 volt AC adapter and then have a transformer inside to step that up?

What about currant, will I have enough to make the circuit operate properly?

What are some guidelines I should follow??

Has anyone here done this before? I need some help as I have never worked with HV transformers (yes, I am aware of the dangers)

I have ordered the small Allied Transformer Steve suggested will that work,...RG suggested I use IN4007 diodes for the recifier section...is there anyone that can draw up a nice 2 tube power supply schematic that uses a 9-12 volt AC adapter?

I would appreciate any help!

Jim

5/18/1999 8:07 AM

R.G.

quote:
"Can I use a 12 volt AC adapter and then have a transformer inside to step that up?"

The devil is in the details again. Maybe.

For a single 12AX7 (sorry, I'm kind of blushingly ignorant about what is in a V-Twin) you'll need about 1ma per section. This is probably full wave rectified, so you could use a 12VAC to 120VAC transformer to get about 170Vdc or a 12VAC to dual-120VAC-primaries to get about 320Vdc. This would require about 4ma at 120VAC or 240VAC, so the 12VAC primary would need about 40ma or 80ma in the case of the 300+ volt supply.

In addition, if you run the filaments from the 12VAC, you'll need another 300ma of 12VAC, so the adapter supplying the box needs 340 to 380 ma to run one tube. Add more if the 12VAC into the box also supplies DC to other stuff in the box.

I'd say that a 12VAC/500ma adapter might do what you want.

Here's where the devil comes in. Small transformers have a higher proportion of wire resistive losses to voltage than big ones do. Since they lose proportionately more in resistive losses, the makers increase the *no-load* voltage so that at *full-load* they have the right output voltage.
That 12Vac/500ma transformer may have 12VAC out at a half amp load, but may put out 14 or 16 VAC at no load. The same is true of the step up transformer in the box, so it may actually be a 15Vac to 120/240 transformer, which means that the plate power will be accordingly lower. You may only get 120-130Vdc from the "120Vac" version or 240-260Vdc from the "240Vac" version, as the AC adapter coming into the box is kind of fully loaded and the step up section is not as big a step up as you might have intended.

5/18/1999 9:19 AM
Santiago	The V-twin´s power supply has a step-up transformer with a turn ratio of aprox. 1:18, which means a 12V to 220V standard transformer. It has a full-wave rectifier with four 1N4007 and a 10u cap. The filter is built with three RC stages of 4K7 and 10u each one (a total of four caps including the first one after the filter). The filaments are run in DC with a half wave rectifier (a 1N4007) with two caps of 1000u each. The diode has a 2.7 ohms 5W resistor in series with it. As R.G. said a 12VAC/500mA will do.

5/18/1999 9:19 AM

Santiago

The V-twin´s power supply has a step-up transformer with a turn ratio of aprox. 1:18, which means a 12V to 220V standard transformer. It has a full-wave rectifier with four 1N4007 and a 10u cap. The filter is built with three RC stages of 4K7 and 10u each one (a total of four caps including the first one after the filter).

The filaments are run in DC with a half wave rectifier (a 1N4007) with two caps of 1000u each. The diode has a 2.7 ohms 5W resistor in series with it.

As R.G. said a 12VAC/500mA will do.

5/18/1999 6:40 PM
Mike Burgundy	12VAC/500mA is fine for one tube + ic´s, but the VTwin uses two 12AX7´s, I´d say use a 1A supply. It´s essentially the same trick the McTube, you can use this for any supply you want. A 120V-12V with a 120V-6V will leave 240, etcetc..

5/18/1999 8:36 PM
R.G.	If you use a 12VAC supply in and a 6V to 120V stepup transformer you'll get - smoke! You should never put more than the rated voltage on any winding of a transformer. The windings are calculated to keep it from saturating at that voltage and the assumed power line frequency. If you use a 6V/120 to get 240, you'll saturate the 6V side with the 12VAC you're driving it with. That's why I said to use a 12VAC to dual-primary. The 12VAC is important. However, if the V-twin uses two tubes, yes, absolutely, you need 12VAC 1A (about) for the AC adapter.

5/18/1999 8:36 PM

R.G.

If you use a 12VAC supply in and a 6V to 120V stepup transformer you'll get - smoke!

You should never put more than the rated voltage on any winding of a transformer. The windings are calculated to keep it from saturating at that voltage and the assumed power line frequency. If you use a 6V/120 to get 240, you'll saturate the 6V side with the 12VAC you're driving it with.

That's why I said to use a 12VAC to dual-primary. The 12VAC is important.

However, if the V-twin uses two tubes, yes, absolutely, you need 12VAC 1A (about) for the AC adapter.

5/19/1999 3:39 AM
Mike Burgundy	Thanks for the pointer! Glad I didn´t get to the point I started smelling funny things yet... Completely missed it. It would however be possible to use a 120V-12V into a (European style)230V-12V I guess? Don´t know if you can get those, but you probably can. I can get 120V mains transformers for the same price as "regular" ones over here. We´re lucky over here (Netherlands) to have 230VAC out of the wall. That´ll power mostly anything, and I can use the "McTube-back-to-back-trick" on both the VTwin (which is a Mesa "three"-channel tube od box, I have a schem if you want it. There. Now you know what it is.) and the Matchless Hotbox...

5/19/1999 3:39 AM

Mike Burgundy

Thanks for the pointer! Glad I didn´t get to the point I started smelling funny things yet...
Completely missed it. It would however be possible to use a 120V-12V into a (European style)230V-12V I guess? Don´t know if you can get those, but you probably can. I can get 120V mains transformers for the same price as "regular" ones over here.
We´re lucky over here (Netherlands) to have 230VAC out of the wall. That´ll power mostly anything, and I can use the "McTube-back-to-back-trick" on both the VTwin (which is a Mesa "three"-channel tube od box, I have a schem if you want it. There. Now you know what it is.) and the Matchless Hotbox...

5/19/1999 7:14 AM

R.G.

quote:
" It would however be possible to use a 120V-12V into a (European style)230V-12V I guess? "

Yes, that will work fine.

We don't get many 240V primary transformers, only "universal" ones with two 120Vac primaries that can be put in series for 240 and parallel for 120Vac.

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