ikiru

Using power transformers as output transformers
« on: June 14, 2004, 09:05:05 PM »
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

Using power transformers as output transformers
« Reply #1 on: June 14, 2004, 10:05:14 PM »
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
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

NewYorkDave

Using power transformers as output transformers
« Reply #2 on: June 14, 2004, 10:20:14 PM »
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-***** 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

Using power transformers as output transformers
« Reply #3 on: June 14, 2004, 10:26:00 PM »
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


ikiru

Using power transformers as output transformers
« Reply #5 on: June 14, 2004, 10:50:28 PM »
Sorry to keep pestering, but what would happen if the 70v line tranny was used single-ended?

BradAvenson

Using power transformers as output transformers
« Reply #6 on: June 14, 2004, 10:51:05 PM »
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

Using power transformers as output transformers
« Reply #7 on: June 14, 2004, 11:02:52 PM »
Cool scope!
Whats up with the db axis's !
Where can I get one!
cj
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

BradAvenson

Using power transformers as output transformers
« Reply #8 on: June 14, 2004, 11:12:24 PM »
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

talema as output transformer
« Reply #9 on: June 15, 2004, 01:45:23 AM »
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

Using power transformers as output transformers
« Reply #10 on: June 15, 2004, 03:05:48 AM »
ey...

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

gyraf

Using power transformers as output transformers
« Reply #11 on: June 15, 2004, 05:28:19 AM »
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.
..note to self: don't let Harman run your company..

sismofyt

Using power transformers as output transformers
« Reply #12 on: June 15, 2004, 09:55:18 AM »
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

Using power transformers as output transformers
« Reply #13 on: June 15, 2004, 10:14:00 AM »
Quote from: "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.

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

Using power transformers as output transformers
« Reply #14 on: June 15, 2004, 10:19:06 AM »
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

Using power transformers as output transformers
« Reply #15 on: June 15, 2004, 10:26:29 AM »
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.
..note to self: don't let Harman run your company..

sismofyt

Using power transformers as output transformers
« Reply #16 on: June 15, 2004, 10:29:22 AM »
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

Using power transformers as output transformers
« Reply #17 on: June 15, 2004, 11:07:42 AM »
Quote from: "sismofyt"
I'll see if I can find the mag at the library  :wink:

I have either the mag or photocopies here somewhere if you want a copy...

Best regards,

Mikkel C. Simonsen

mcs

Using power transformers as output transformers
« Reply #18 on: June 15, 2004, 11:09:18 AM »
Quote from: "sismofyt"
What I meant about DC was that they shouldn't be used in a single ended configuration.

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

Using power transformers as output transformers
« Reply #19 on: June 15, 2004, 11:19:16 AM »
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?


 

Related Topics

  Subject / Started by Replies Last post
1 Replies
1764 Views
Last post April 23, 2005, 03:41:06 PM
by gyraf
9 Replies
2885 Views
Last post April 13, 2008, 06:31:06 PM
by radiance
2 Replies
1461 Views
Last post August 11, 2008, 12:05:01 PM
by maxime
5 Replies
2957 Views
Last post June 09, 2011, 08:45:22 PM
by opacheco