[sdiy] makeshift audio output transformers

[harrybissell at wowway.com]

Signal Transformer is indeed a brand name.

Most audio output transformers are designed for quite high
voltage on the primaries. The secondaries are designed to match
the load impedance of the speakers.  Power transformers mith not
do that properly.

Most Audio output transformers have the primaries and secondaries 
wound over each other on an E-I core (thats the shape of them). The
"split-tran" units with the four coils on a semi-toroidal core might
not work too well. I don't know that a gapped transformer is an advantage
except for high standing DC currents. The gap guarantees non-linear
performance....

Small power transformers run on the 'edge' of saturation for highest
efficiency... not what you'd want for audio. You need to derate them a lot
to get reasonable performance.

Also, look for any old tube gear in the pawnshop etc (if there ~is~ any)
that you might remove output transformers from...

Triad is another manufacturer to look for... they used to make audio
output transformers.

Go ahead and try your power transformers... but remember that most were
desined for 240V max with the primaries in series.  Most guitar amps are more
like 450V ....

H^) harry






On Mon, 14 Jan 2008 11:57:04 -0500, anthony wrote
> 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
> 
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Harry Bissell & Nora Abdullah 4eva