[sdiy] makeshift audio output transformers

[anthony]

I have a large number of old transaformers: power transformers and audio 
transformers (I'm not even going to begin about the number of IF 
transformers I have because that doesn't concern us here...).

Anyway, I've read from time to time that power transformers can be coaxed 
into being audio output transformers, generally with poor results in the 
case of single-ended topology because of saturation of the core by the DC 
flowing through the primary. BUT I have this old power transformer that I'm 
pretty sure was the in an old acoustic coupler modem and it was definitely 
being used in the capacity of its power transformer (since the mains wires 
were run straight up to it...); but the point is that it has a serious 
airgap just like a small 5-7Watt output transformer. The DC resistance is 
about the same as that for the somewhat stout single-ended output 
transformer in the Wards Airline Radio 6V6GT amp that I mentioned making 
here. The secondary is center tapped so I'd have options for speakers. It's 
been my experience that with small amps you can use just about anything with 
close to the correct values. I used a tiny crappy output transformer with no 
visible gap that was actually originally used in a solid state audio amp 
(using high voltage transistors) that ran from a tap on the flyback - and it 
sounded OK. A little farty when you tried to run bass through it, but it 
worked.

But my real interest at moment is using power transformers with dual 
primaries or those split-core "signal transformers" is amplifiers with 
push-pull topologies. Probably this is just a stop-gap until I can afford 
the Hammon1609 that I WANT to use in my 2xEL84, 100% Class A, 10 Watt, 
Push-Pull amp. But I have several tranformers for 120V/240V service just 
depending on how the primaries are connected. I assumed if you got the right 
ends, and connected the two points in the center as the center tap, you 
should be able to use it in a push-pull amp. If there's a danger in damaging 
tubes, I can use my 6AQ5A's as guinea pigs (although I really hope I don't 
fry one). Seemingly lackluster 6AQ5's seem to really shine when you use them 
in push-pull amps - some say even moreso in triode mode. But the other amp I 
intend to use 6AQ5's (4 of them to be exact) is going to be more of a Class 
AB1 as opposed to the 2xEL84 amp that I want to run Class A.

I guess in the end there's no substitute for serious iron in an amp, but I 
am not shooting for hi-fi here. These are going to be amps for guitars and 
monosynths. I guess I'm curious about what is crucially different about a 
transformer, intended as a power transformer that can be configured to work 
like an audio output transformer and the actual center-tapped, push-pull 
audio transformer. My guess is that more care is given to giving in the 
right impedance at the right frequency response.

And then there are those split-bobbin transformers that I've pulled out of 
gear in which they are clearly being used as power transformers and plain as 
say it says "Signal Transformer" on it. My guess is that this is just a 
brand name, but is it? Do these have audio possibilities? If so, the ones I 
have probably couldn't handle that much power anyway.

Or can anybody direct me to a push-pull transformer that is better than a 
Hammonnd 125E and cheaper than a 1609? I don't need the ultralinear tap. I 
do OK finding decent single-ended iron for 5 Watt amps, so I'm mostly on the 
look-out for a few good push-pull models with power handling capabilities 
rangeing from 10 Watts to 50 Watts. I actually think that I can have better 
luck on eBay with the higher powered ones than the little 10 Watt push-pull 
I want to make. Maybe I could put a Bandmaster transformer in the small amp 
and just make sure I connect higher impedance speakers.

cheers,
Anthony