Using power transformers as output transformers

[ikiru]

I know this is probably not the best use of a power transformer, But I am thinking about doing some experiments with 12u7 and 12k5 tubes for guitar amps (that run on 12volts heater and b+) and want a cheap, locally available transformer to fiddle with. I've heard that it is possible to use power transformers as output transformers to interface with say an 8ohm speaker. How do you figure out what the impendances of a power transformer would be? Thanks,


Jay


(btw there is a tube called 12el6 that has a mu of 55 and is a 12 volt tube...seems like it could be interesting to build a low volt tube pre!)

 

[CJ]

Let's see, a 120 to 12 volt woulfd have a turns ratio of 10:1, so yes, you could probably get some audio out of it.
It is not interleaved, so the freq response is going to be questionable.
Try it.
cj

 

[NewYorkDave]

Not only that, but since they're designed for single-frequency use (60Hz), they're wound with no regard for winding and leakage capacitance. They're also not designed to carry direct current, so they will saturate like a son-of-a-bitch if you try to use them in a single-ended output stage.

Since the object with guitar amps is usually some kind of distortion, though, you might get a usable sound of some kind. For cheap output transformers, though, you'd probably be better off using a 70V speaker line transformer (as long as you operate it in push-pull).

 

[ikiru]

Thanks for the the responses cj and dave.

dave what would the impendance of a 70v line tx be? or how do you figure this sort of stuff out!


thanks.
jay

 

[NewYorkDave]

http://www.qsl.net/wa7zcz/area2/page11.html

 

[ikiru]

Sorry to keep pestering, but what would happen if the 70v line tranny was used single-ended?

 

[BradAvenson]

You might think about using a toroid. In fact I think I'll see what the response would be of the talema I have right here.

Ok so this is a response curve for the talema 62032 12V toroid transformer. I used a 10ohm load on the secondary and a low impedance drive on the primary. Looks pretty good considering it only needs to work at one frequency.

Just for fun ran a THD+N plot. Seems like it could work as long as you got the loading correct.

Now I can't say how much my test method really resembles the use as an output transformer, maybe CJ can comment on that. I only put 21 dbu into the transformer, so not very much level for a power output. I was dissipating .06W into 10 ohms.

Oh well, enough of a break for me, back to work.

 

[CJ]

Cool scope!
Whats up with the db axis's !
Where can I get one!
cj

 

[BradAvenson]

This is a Neutrik A2 Analyzer.
http://www.nt-instruments.com/X0-ASP-pLngCateId_134-pIntLevel_4-X1-default.htm

The response is plotted on a 2db per division scale with +2dbu at the top line and -8 dbu on the bottom line.

The THD+N is 0.2% per division with 1% at the top and 0% at the bottom.

 

[edanderson]

actually, a few people in the "hifi" world have dabbled with using those talema toroids as OPTs.

http://home.t-online.de/home/MHuber/pre-5687.htm

and steve bench even uses them for both input and output transformers in this design:

http://members.aol.com/sbench/50c5.html

so, you'd have to order them, but you could always find a use for them as power transformers later!

ed

 

[daArry]

ey...

just stumbled accross this site: http://www.diyaudio.8m.com/Ot/ot_en.html

 

[gyraf]

Yes, toroids often has surprisingly good frequency response. But used as output transformers they need to have the differential bias current adjusted very precisely, as they saturate the core very easily because of the very effecient magnetic circuit.

For a 6K:8 Ohm transformer, your transformer should be:

6000/8 = 750 - impedance ratio
sqr(750) = 27 - turns ratio

Say you use a 2x110V primary transformer - because you'll need to run push-pull to balance DC currents out.

110:27= 4.07 - secondary voltage

So using a 2x110V:5V power transformer should get you in the ballpark..

Jakob E.

 

[sismofyt]

I have measured a bunch of of transformers and yes, you can find some that are usefull, and yes, mostly toroids. Many moons ago there was a DIY EL34 PP amp in the danish mag "Ny Elektronik". That used dual power toroids for the output. Worked quite well.

And as others have said, don't run DC through 'em!

 

[mcs]

[quote author="sismofyt"]Many moons ago there was a DIY EL34 PP amp in the danish mag "Ny Elektronik". That used dual power toroids for the output. Worked quite well.[/quote]
No, they actually used standard EI transformers (from VRT) - and they did run DC thrugh them! They used a pair of transformers for PP, so both transformers had DC running through them - but they said it worked, so I guess it did...

Best regards,

Mikkel C. Simonsen

 

[sismofyt]

I'm pretty damn sure it was dual toroids :? but it was in the late 80's and I don't have the mag any more. What I meant about DC was that they shouldn't be used in a single ended configuration.

VRT is cool. He makes all my trannies :thumb:

 

[gyraf]

No, I once built that construction - it simply dosen't work.

I don't know what they were thinking of, but they used two EL-core 12V transformers - both of which had full DC running through core.. well, I should have guessed that it wouldn't work, but at the time I had very little experience with tubed stuff..

Jakob E.

 

[sismofyt]

Hmm, I think I remember that there were quite some letters to the editor and an update later on. I do believe people made it work though, but gee I was only like 15 back then and I don't remember the details :? :green:

I'll see if I can find the mag at the library :wink:

 

[mcs]

[quote author="sismofyt"]I'll see if I can find the mag at the library :wink:[/quote]
I have either the mag or photocopies here somewhere if you want a copy...

Best regards,

Mikkel C. Simonsen

 

[mcs]

[quote author="sismofyt"]What I meant about DC was that they shouldn't be used in a single ended configuration.[/quote]
When you use two output transformers in a PP amp, you make it a dual SE amp transformer wise...

Best regards,

Mikkel C. Simonsen

 

[sismofyt]

I'd luv to see that mag! Brings back ol' memories :grin:

But if it was true dual singleended fashion, they'd have to be gapped, wouldn't they?