Possible uses for 70V transformers?

[teemuk]

I have a bunch of (approximately 5-watt) coupling transformers lying around that were torn from speakers of old 70V paging system. They are just robbing space since I can't figure much use for them. I was hoping that maybe folks have built something with such and could suggest possible uses for such items.

I was initially thinking... well... audio coupling but - to best of mt knowledge - the transfer ratios and impedance levels are poor match for conventional amps, whether tube or solid-state (too low for tubes, too high for solid-state).
They are too big and "hifi" for any saturation effects so use in low voltage circuitry seems kind of pointless as well. (Plus the overall transfer ratio is inconvenient for such task). Thoughts?

I was thinking of maybe using one wired as autoformer impedance matching device that only employs the secondary winding. 4 or 8-ohm tap would be wired to output of (low power) solid-state power amp, COM to amp's ground. Would this work? Seems plausible to me but any practical experience?

Another idea (I have about six of these transformers) is to employ one in an octave up effect. (You know, one with transformer phase inverter and rectifier diodes). Input to secondary side, rather high voltage output from primary side so the diode forward voltage is less concern. Plausible idea? Definitely way bulky for the application and doesn't fascinate me much.

I was hoping to hear more bright ideas.

[Eb7+9]

when I met Roly in 2014 he told me about the work he did on 70v trannies ...

https://www.ozvalveamps.org/optrans.htm

there's some mention of the collaboration between him and Paul Cambie, the original dude :

https://www.diyaudio.com/forums/instruments-and-amps/301873-output-tube-op-amp-post4946150.html

if you go to AGGH or Ozvalveamps you'll find more info lying about ...

[Marcos - Munky]

I think you can use them as the output transformer for a 3W to 5W tube amp, using a EL84, 6V6 or some tv tube like the ECL82. To be sure, we'll need to know how's the turns ratio. Measure the continuity between each end of both windings to check if they're not open, then measure the continuity between both windings to check if they're not shorting together (or if it isn't an autotransformer). Then feed the primary with let's say 9V-12V AC (from a small transformer), measure the voltage on the primary and on the secondary. This should give enough info to be sure if it could be used as an ot transformer.

Take a look at this video:
https://www.youtube.com/watch?v=-xrXjw7nt-g

[teemuk]

Thanks.

No center taps in these so it limits applicable circuitry. I have to do some math but the primary impedance is probably in the range of few hundreds of ohms so kinda low for power tubes and yet quite high for power transistors.

[Marcos - Munky]

No center taps mean they could be used for a single ended tube amp, if they have correct impedance ratio. While single ended and push pull amps do have their differences, a ECL82 will give you about 3W output and a 6V6/EL84 can give you something around 3-5W. It's very loud for bedroom playing.

Just get the voltages and I can quickly do the maths to discover the impedance ratio for you. I done this lots of time to try to find some use for random transformers, so I'm used to the maths lol.

[amptramp]

I have been using these as output transformers for years.  Use the equation
:
R = V²/P

If you want a 2500 ohm transformer for a 50L6/50C5, the 2 watt tap will give you 2500 ohms.
If you want a 5000 ohm transformer, the 1 watt tap will give you that.
If you want a 1000 ohm transformer for a triode output in an antique radio, the 5 watt tap will give you that.

The 70.7 volt transformer will give a numerator of 5000 for V².

You can do push-pull with a line transformer as well but the split may not be as accurate as with a proper output transformer.  One plate goes to the common lead, another is selected to get the impedance you need for one plate and it becomes the centre tap and the other plate goes to a tap with a quarter of the wattage of the one chosen for the centre tap.  Not all line transformers have taps at a given wattage and a quarter of that wattage, so not everything works in push-pull.

I have a Burtek T7010C in front of me and I have applied 1.7 VAC between the common (black) lead and the 5 watt (red) lead.  Black to yellow (the 1.25 watt tap) is 3.4 volts.  But beware: the turns ratio is approximately correct but the DC resistance of the windings from the centre tap to both plates may not be equal.  This example would give you a plate-to-plate impedance of 2000 ohms.  You could also do black (common) as one plate, orange (2.5 watt) centre tap and green (0.625 watt) as the other plate and get 4000 ohms plate-to-plate.  The transformer core is good for the highest tap rating, in this case, 10 watts.  The DC current rating may be in doubt but it appears to be able to support 10 watts as well.

The imbalance will give you extra even harmonics, not good for classic high fidelity but excellent for an instrument amplifier.

[PRR]

Copper transformers: 8 to 10 cents a pound at Rockaway Scrap Metals. (Price expires the day you come in.)

Price of iron/steel is WAY down since a few years. But be of good hope. I'm paying to haul-away a hundred pounds of iron scrap; as soon as it leaves on someone else's truck the price will spike.