Power transformers as output transformers?

[brett]

Hi
Has anybody else tried this?
I'm building a simple Class AB amp using transistors push-pulling an output transformer.  It's early days, but it's all going well - it's such a simple, elegant type of amp.

BUT, they need an output transformer, and a new Hammond output transformer costs OZ$60, and I'm not going to pay that for one part for an experimental 5W transistor amp.
 
So I thought about using a power transformer instead.
First I tested the throughput of a little 6VA 24V job, about 1 inch cubed.  It obviously didn't have enough iron and windings, as it wouldn't work much below about 250 Hz and it picked up or resonated at mains frequency (50 Hz).
But then I tested an old 72VA 24V transformer, about 2 1/2 inches cubed and half a kilo.  There was no mains interference, the bass -3 dB point was at about 70 Hz, and the -6dB point at about 50 Hz.  At the top end it worked well all the way up to 20 kHz.  All in all, an excellent result from a transformer that costs OZ$15 new (I bought two on special ages ago for $8 ea !).  There must be heaps of these old transformers in old hi-fis and stuff, so I'm sure they could be got for virtually nothing.

So far, I've only driven a volt or two through these transformers, but it's looking good.  I figure it might give me grief when I crank it, but that 72VA one will hardly be saturated.  It will be interesting to hear how it sounds when fully installed in an amp.
cheers

PS the inspiration for this amp is the Pignose 3W, which is similarly a class B push-pull with an input and an output transformer.  Those amps always sounded good to me.

PS PS Unfortunately, the large voltage step-down (eg 240:24) means that any amp needs a high operating voltage if it's going to put any major voltage out.  To help overcome this, I'm planning on using a 4 ohm speaker.   

[Doug_H]

Search the archives at the ax84 forum. Others have done this and reported on it. As long as the impedance ratio is correct for your design and the xformer can handle the current it should work. However, others have reported that the frequency response is not very good, and it won't sound as good as a real OT.

[DDD]

Russian DIYers are fond of using PTs as OTs in guitar amps. They report it's OK and frequency response is quite suitable for guitar sound.

[MartyMart]

Brett, I have a little OT from a 5 watt Epiphone valve amp
It's 7.5 k and has 4 /8 /16 ohm outputs, you can have it if you
cover the shipping from the UK ?
Perfect for a 3 to 5 watter
PM me.

MM

[Threefish]

Have you tried Altronics? They have some low cost OT's intended for use as 100v PA line transformers. There's a few (aussie) articles around on the net about them being used as OT's for hifi and guitar valve amps. I have a few of them at home. An interest (project) I'll probably never get to....
Here's one I found with a quick google search using the words "altronics transformer valve" and the "Pages from Australia" box:
http://home.alphalink.com.au/~cambie/6AN8amp/6an8amp.htm
IIRC there were a few articles around this concept at that website.
Ah here it is - a larger one from the same site with more info. Google search "M1115 transformer":
http://home.alphalink.com.au/~cambie/EL36.htm
I'm pretty sure there was something in one of those pages (somewhere in that website) or a link off of it where someone had tried a few of those types of transformers in a small valve amp, though can't remember if is was a push-pull or not...anyway, they're really interesting articles.
There's also an article somewhere out there I can remember that was all about using a toriodal transformer as a OT (another silly aussie trying something different...)....but they can be almost as expensive, so there's probably no point pursuing that.
Keep us informed! By the way, do you frequent the Aussie Guitar Gear Index forum?? http://www.guitargear.net.au/discussion/ -
there's an "In the shed" section there, for stuff like this.

[brett]

Hi
Thanks for the very generous offer MartyMart.  It'll get some good use, for sure.

Thanks heaps for the useful info, too.  Those line transformers look really interesting (I looked at one in the Jaycar catalogue and thought "Maybe that could work....Nah!! Just another one of those things you buy that won't work").  So I've ordered the 15W one from Altronics.  $12 for a 15W output transformer seems top value if it is any good.

Most power transformers seem to have a voltage ratio of at least 10:1 (ie mostly less than 24V out for 240V in), which means that the voltages out are miserably low.  But that line transformer from Altronics has taps that give 12:1 (no good), or 5:1, which is a lot better.  With a 30V supply (from an old inkjet printer), that would give me a peak of almost 6V.  With a 4 ohm speaker, 6V peak gives almost 5W.

I'll report in the New Year how it's all going.
Thanks again and Merry Xmas. 

[JimRayden]

Brett, I have a little OT from a 5 watt Epiphone valve amp
It's 7.5 k and has 4 /8 /16 ohm outputs, you can have it if you
cover the shipping from the UK ?
Perfect for a 3 to 5 watter
PM me.

MM

If brett relinquishes the deal, I'm in a desparate need of one. Will pay shipping. (europe)

----------
Jimbo

[MartyMart]

No problem Jim, I'll let you know

MM.

[GFR]

Everything you've always wanted to know about using power transformers as output transformers

http://www.prodigy-pro.com/forum/viewtopic.php?t=436

[MartyMart]

Jim, it's gone ... sorry !

MM.

[JimRayden]

It was well worth a shot...

[edit] I'll look into that power tranny thing. I once actually did but the guys at ax84 told me it would have a really really sucky freq. response.

----------
Jimbo

[MartyMart]

Jim, check out transformers on Ebay, I recently got a PT thats 250-0-250 with a 6.3v heater
tap, looks almost new and was £16
Picked up a single ended OT ( EL84 ) for £8 which was a little larger than the VJ OT

MM.

[brett]

Here's some info I found in various wanderings:

First, a tiny PT used in a valve hi-fi:

Ok, I just come from my lab in the basement, where I have listend to some Gentle Giant, Neil Young, and classical solo piano music over that tiny power transformer at various levels. I was surprised about the quality - better than I had expected! Not really good, either. A bit thin and tinny. Certainly not the right choice for a LA2A output transformer. But it's remarkable how the whole audio range comes thru.

Concerning their technical suitablility (incl capacity to carry DC current):

Anyway, these things are all built as CHEAP as possible. With minimum material. Minimum copper leads to low leakage inductance and low capacitance. To keep it from burning up on power, they used fairly high-inductance iron, and it is the ratio of iron permeability to air permeability that sets the frequency range of a simple transformer. So for simple commercial reasons, they are not bad; it would cost more to make them NOT pass most of the audio band. Also the smallest trannies, around 10VA, are very often run with half-wave rectifiers that induce DC in the core. Since this is obviously the cheapest class of work, buyers don't expect precise operation, but the iron must not DC-saturate, suck huge AC current, and burn-up. So there must be some tolerance for DC flux at least in the smallest trannies.

thanks for the help and suggestions.

PS I wound the power transformer for this project last night.  I took a Dick Smith 30V 15VA ttransformer (M2860)  and took it apart.  I replaced the secondary with 25g of 0.25mm diam enamelled wire (Jaycar ww4012).  The secondary now gives 50V into a 1.5k load (33mA).  Perfect for a  high-voltage, low current project like this.  50V AC should rectify to about 70 V DC, which is nicely under the 80V maximum voltage rating of TIP31B and similar power transistors.