Interesting transformer construction.

[Diabolical Artificer]

I found two identical tfmrs in my stash and tried to figure out what they are intended for. One winding, the primary I presume has three taps, total R is 75r, next at 74, then R drops to a few ohms. I took this as being something like 250,240, 10, 5, 0 volts.

The seconday has one tap and when the pri has 240v on it we get 7v6 and 6v8. What's interesting and unusual is that there appears to be a winding of smaller gauge wire on two of the pri taps, IE 74r and 2.5r, see pic two, tap is second from camera. Why would this be done?

The sec voltages are odd too being a volt apart, unless that is these are actually SE UL OPT's, a test with DC should settle that one. One last question are tfmr's with copper bands on mostly found on mains tfmr's? Why have a screen on an audio OP tfmr? To stop EMF is probably the answer, sometimes it helps thinking when you type something out.

Andy.

[trobbins]

The belly band and plastic former would indicate a more modern transformer. Was the 240V level applied across the total primary winding?

Another measurement option is to drive a secondary and see if the primary winding voltages align with your anticipated values.

Solid state PA amps can have a low impedance push pull winding, with a variety of options on the secondary, although your voltage ratio is a little high to suit a PA OPT and the voltages are noticeably different (not sure if that could be an outcome if you were saturating the primary at the time).

[Silicon]

Are the laminations the E & I type and are they interleaved?

Have I understood the secondary correctly? - there are 3 tags, maximum voltage is 7.6V but a 6.8V tapping is available.

It could be designed for a 6V lead acid battery charger.
7.6V for fast charge and 6.8V for trickle.

[G6Tanuki]

I wonder if it could be an amplifier-end "100V line" transformer? For 100V line usage you don't need isolation between the line and the drive-signal so an autotransformer (or sharing a tag between the primary and secondary windings to save money) can be used.

Could the little winding be for negative-feedback?

[m0cemdave]

A copper band on the *outside* of the transformer is intended to short out the leakage field and cut down on hum induced into nearby circuitry. Unlike an interwinding screen, the ends are joined to form a complete loop.
It would normally indicate a mains, rather than audio, transformer.

[Diabolical Artificer]

"Another measurement option is to drive a secondary and see if the primary winding voltages align with your anticipated values." I'll do that and report back.

"Are the laminations the E & I type and are they interleaved?" It's a bit hard to tell, will have a good look today. "Have I understood the secondary correctly? - there are 3 tags, maximum voltage is 7.6V but a 6.8V tapping is available." Yes.

"Could the little winding be for negative-feedback?" Don't think so, at least not in the conventional sense where there's a separate sec wndg. This winding seems to be in parallel with the thicker gauge wndg; why would you use to different gauges of wire and only on one part of the primary. I guess I'd have to unsolder the wire from the tags and measure them separately.

"A copper band on the *outside* of the transformer....." Thanks Dave.

Andy.

[Craig Sawyers]

The name for the copper strip is a "hum band". The idea is that it is a shorted turn for leakage inductance, and therefore reduces radiated 50Hz and harmonics.

Craig

[trobbins]

Also referred to as a flux band, belly band, hum strap, and gauss band by the various application users. Simplified diagrams of influence on leakage flux prepared by Nave: