measuring Audio Transformers ???

[bluelagoon]

I have a handful of antique audio Transformers, Was wondering since I dont know the specs for them, is there any way to do a test on them to acquire the specs? And just what could this type audio transformer be used for.??
I see most audio transformers are pretty costly, so these would have to have some value I would expect.
Thanks.

[Rob Strand]

Measure the DC resistances that will tell you what windings go together and you can also guestimate the winding impedance (which is always much more than the DC resistance).

When you have an idea what you are dealing with inject a 1kHz signal on one winding and measure the voltage on the others.

You can do a bit more or a lot more depending on details but that's a way to put a loose box around what you have.    The size can sometimes help workout maximum drive levels.

Good chance at least one of the windings will be 600 ohm perhaps tapped.

FWIW,  I haven't had much luck finding info on that brand of transformer.

[bluelagoon]

Thanks Rob. There are 6 leads coming out so would that indicate centre taps on both sides or what might that indicate?
Cheers.

[Rob Strand]

Thanks Rob. There are 6 leads coming out so would that indicate centre taps on both sides or what might that indicate?
Cheers

Best guess would be yes but it's more reliable to follow the resistance measurements.   With transformers there's always room for surprises with funky windings.

[amptramp]

At the London Vintage Radio Club in London ON Canada, we recently got a group buy together to buy ten Hammond single-ended driver to push-pull grid interstage transformers for CDN $60 each.  This item used to be in their catalog and in fact, they at one time had both the transformer at something like CDN $28 and a coreless transformer with just windings on a bobbin for CDN $13, used to repair transformers with bad windings.

I have a United Transformer Co. Minerva radio from 1938 that uses a 6C5 to push-pull 6F6G outputs through an interstage transformer that showed my daughter (who is a guitarist) why pro guitarists still use tubes.  Although the output is nominally 11 watts, it is louder (even with its old electrodynamic speaker) than most transistorized 50 watt amplifiers.  She has this radio in her home office now.

[Electron Tornado]

Here are several videos that will help:

[mozz]

I usually measure output transformers with a lcr meter at 1khz and a 8 ohm load. Did the turns ratio thingy but lcr meter is much easier. As to small signal transformers, i think you may need a close load on both sides to get a better reading. I have used my USB scope with a bode plot to give me freq curves.

[Rob Strand]

Here are several videos that will help

I usually measure output transformers with a lcr meter at 1khz and a 8 ohm load. Did the turns ratio thingy but lcr meter is much easier. As to small signal transformers, i think you may need a close load on both sides to get a better reading. I have used my USB scope with a bode plot to give me freq curve

With output transformers there is an assumption about the speaker impedance side.  When you know nothing about the transformer the DC resistances are your best bet pinning down what the impedances are.   You need that starting point.

As you make more detailed tests you can refine your results/conclusions.   However it can be difficult to split hairs between a 2dB insertion loss 300 ohm transformer and a 1dB insertion loss 600 ohm transformer.  You have to inflict some other specs on the transformer.    Ultimately it comes down to not what the original transformer was spec'd as but how you intend to use it.

[PRR]

An Australian electronics company is still trading under the Selectronic name. Might ask them to look in the old files?
http://www.selectronic.com.au/contact/

[Rob Strand]

An Australian electronics company is still trading under the Selectronic name. Might ask them to look in the old files?
http://www.selectronic.com.au/contact/

It wouldn't hurt.

When we actually made stuff in Australia (say pre 80's) we had transformer manufacturers who produced standard products (custom on special order) and others that only did OEM.   Selectronic and a few others only really did OEM.   Selectronic also did smaller coils for switchmodes.  I think they got a lot of business from the telecom industries (who made their own equipment).  Even back in the day they didn't give out info because they were custom parts for customers.   I used to ring these companies up never got anywhere.