Toroidal transformers for tube pedals

[R.G.]

Yeah, I know there's some ones from EH. Those are fine.

But you can also get toroidal transformers suitable for powering a tube or two in an effect from a 12VAC wall wart at Digikey.

See http://dkc3.digikey.com/PDF/T051/1378-1379.pdf for the AMVECO mini toroidals. The TE62012 is a 12:12:120:120 vac 3.2VA unit that sells for $13.68 and should power a couple of tubes.

You hook the 12VAC windings in parallel to be powered from the output of the wall wart, and that 12VAC also powers the tube filaments in series connection. You have two choices of high voltage: primaries in parallel for about 13ma of DC at 160V or series for about 6ma of 320Vdc.

This one is 42mm diameter by 17mm high - that's 1.65" by 0.67" for us non-Metricians; small enough to go into a pedal or a wah rocker.

[birt]

i knew about the wallwart thing but i still don't get how you get the high voltage out of it :?

oh and if you take the rectifier out of a 9V wallwart you get 6,3Vac  :wink:

[object88]

You get high voltage DC by trading amps for volts, essentially.  The wallwart should be able to procide a small (DC) voltage at a high amp rating, and the transformer turns that into a high (DC) voltage with a low amp rating (which tubes perfer).

The one thing I'm not certain about is the voltage that the transformer can take.  If the transformer is rated to take 12Vac, then isn't the mean voltage it seems something more like 6V?  So would you want to use a 6V wallwart?

[R.G.]

Transformers produce a specific ratio of voltages on every winding, as long as the power-supplying voltage is below some maximum they're designed for. For instance, a 120V:12V transformer has a voltage ratio of 10 to 1, and the maximum voltage you can drive the "120V" named side with is -- 120V.

You can also run it "backwards" and put 12Vac on the winding named "12V", and you'll get 120Vac on the winding named "120V".

If I put 90Vac on the "120Vac" winding, the "12V" winding produces 9Vac.

If you put 120Vac, or in fact any voltage much higher than 12V on the "12V" named winding, you get smoke. Likewise, if you put voltages much over 120Vac on the "120V" winding, you get smoke. If you put DC of any voltage on either winding, you get smoke.

So - a transformer rating is a ratio and a maximum for each winding. To a first approximation, the transformer doesn't care which winding you drive with a power source, just don't put too much voltage on it, and the transformer will then do it's voltage ratio thing on all the other windings.

Now to the questions:

i knew about the wallwart thing but i still don't get how you get the high voltage out of it

You run it backwards. You use a second transformer, a 120V to 12Vac wall wart. It's important to use a 12v **AC** output wall wart, or you get smoke. If you put this 12vac on the 12V winding of the toroid, the toroid makes the proper ratio voltages appear on the other windings. In this case, there are two "120V" windings, and so they both have 120Vac across them. You are then free to make DC from that AC voltage by using rectifiers and filter caps.

The rated AC voltage of a transformer is the "root mean square" value, chosen because that amount of voltage in a sine wave causes the same heating as a DC voltage of that numerical amount. The peak value of this sine wave is 1.414 times as big as the RMS value. If you rectify AC, the rectifiers pick off the peak voltage in a capcitive filter, so you get 120*1.414 = 169.68Vdc minus diode forward drops and resistive losses, or about 160 to 170Vdc after filtering. You can do that from one "120Vac" winding, both separately, or both in series, so you can have 1x165Vdc, 2 x 165Vdc independent of each other, or 330Vdc with them stacked in series.

oh and if you take the rectifier out of a 9V wallwart you get 6,3Vac

Yes, you do. And for the same reason I just explained. To get 9Vdc, you only need 9V/1.414 = 6.36Vac on an RMS basis.

You get high voltage DC by trading amps for volts, essentially. The wallwart should be able to procide a small (DC) voltage at a high amp rating, and the transformer turns that into a high (DC) voltage with a low amp rating (which tubes perfer).

I talked too fast, I guess.

You get high voltage DC by starting with low voltage AC from a 12VAC wall wart, and trading amps for volts there. You then have to make the high voltage DC from the high voltage AC with rectifiers and filters. Transformers cannot transform DC.

The one thing I'm not certain about is the voltage that the transformer can take. If the transformer is rated to take 12Vac, then isn't the mean voltage it seems something more like 6V? So would you want to use a 6V wallwart?

I think that's because you're confused about AC and DC. The transformer is rated to take 12Vac, so you have to feed it 12Vac, or a lower AC voltage. DC will kill it.

[birt]

if i get this right:

here in belgium the mains is 230V.
i take a 9V wallwart and take the rectifier out of it so it has 6,3Vac wich i can use for a couple of tubes and i make that 6,3Vac go in another wallwart (also without the rectifier) in the secondary.
the primary of the second wallwart will give me 230Vac again and i can rectify that and get 325Vdc.

and i won't lose any of the power?  

[object88]

I think that's because you're confused about AC and DC. The transformer is rated to take 12Vac, so you have to feed it 12Vac, or a lower AC voltage. DC will kill it.

Ah, you're so right.  I forget things.  Thanks for the refresher!    (Oddly enough, if I had stopped to think about the OT on a tube amp, I would have remember immediately.)  And I automatically assumed that the wallwart in question was putting out DC.

[Paul Marossy]

Thanks for the tip RG!  

[R.G.]

here in belgium the mains is 230V.
i take a 9V wallwart and take the rectifier out of it so it has 6,3Vac wich i can use for a couple of tubes and i make that 6,3Vac go in another wallwart (also without the rectifier) in the secondary.
the primary of the second wallwart will give me 230Vac again and i can rectify that and get 325Vdc.

Theoretically, yes.

However, the devil is in the details. Since every wall wart maker makes his stuff differently, I would only try that with two identical wall warts, if I tried it at all.

And I wouldn't try it at all. I would instead use wall warts that were **AC** output, not DC, so I didn't have to bust them open to get at the transformer AC. Any time you open a wall wart, you compromise the safety of the setup. The loss in your safety ( and the safety of anyone who touched the equipment ) is not worth what you'll save. Never, never, never open a wall wart and expect it to be safe to use it again.

If you used two 6.3Vac wall warts or better yet, two 12Vac wall warts, yes, to a first approximation, it works like you say.

and i won't lose any of the power?

Of course you will. Nothing is perfect. But losses should be low. How low?

An AC transformer in such small sizes loses from as little as 4-5% to as much as 10% of the power that goes through it. The first transformer, the one that plugs into the wall, has to carry the entire power, high voltage DC and filament power as well. The filaments of a 12A?7 device use 150ma each, or 300ma if wired in parallel for 6.3vac. If you use 1.5ma plate current in each section, you have to supply 3ma at that 325Vdc. It's a peculiarity of full wave rectifiers to pull about twice as much RMS current as they supply DC, so you will be pulling about 6ma RMS on the 230Vac winding. The 6.3V "primary" has to supply 230/6.3 = 36.5 times as much current, or 109.5ma of RMS current to supply that 3ma of DC.

So the first transformer, the one plugged into the wall, has to supply over 400ma of current to make this work.

But that's if the transformers are perfect. They're not, and you'll still lose about 5-10%, so you can count on needing 450ma out of the first transformer. The voltage on the high voltage winding will also sag a lot. Count on 280Vdc, not 325vdc.

But yes, it works.

[lightningfingers]

I would buy a valve transformer.......

[R.G.]

I would buy a valve transformer.......

I would, too...

- IF a valve transformer could be found at all ( this is 1000% better today than it was even five years ago)
- IF a small enough valve transformer could be found to fit inside pedals
- IF a toroidal valve transformer with the right voltages could be found to eliminate 60 cycle hum problems inside a pedal
- IF a valve transformer did not require either wiring power line voltages to the pedal or wiring high voltage AC or DC out of the valve transformer over to the pedal.
- IF this magic valve transformer that did all this was affordable.

The devil is always in the details.

[mlabbee]

Okay - I'm trying this in a tube project and I'm omlyu getting about 110 out of the high side.  I've got the secondaries in parallel hooked up to 12VAC, the primaries wired in series and I'm reading 110 VDC on the output of the rectifier.  I've double checked the wiring - maybe I should be running the secondary in series?

[Karmasound]

This thread is great. I'm getting ready to build a Real McTube and a  Hotbox.

The Hotbox only uses one transfo right? The Hammond 269ex how much voltage does this put out. I can't find as much info on it as I can the McTube.





Thanks

[Paul Marossy]

Karmasound-

Check out the transformer section here: http://tubesandmore.com/
The basic specs are listed on it.