20s Audio xfmr with deliberate shorted turns Why?

[RadioHist]

Just now cleaning-up a Seibt EI-472 (EJ-472) of 1927/28 vintage for exhibition. It has one feature I find more than curious and is beyond me to speculate...

There is an audio transformer between the detector and first audio valve. It is branded N.T.IX. There are markings of P 7,000 and S 19,000 (presumed turns)

What has me scratching my head is a separate four turn, deliberately shorted, winding elsewhere on the transformer core.... What in the heck is that doing?

Enlightenment welcome...

Robert

[G6Tanuki]

Could it be some sort of 'hum bucking' arrangement for when the transformer's used in something powered from a battery-eliminator?

Feeding a bit of AC or ripple through such a small winding could be enough to null-out hum in the stage the transformer is feeding.

[kalee20]

Bizarre to me! I can't conceive of any reason to do this. It will clobber inductance.

[TrevorG3VLF]

Perhaps to damp the RF signal, coupled to the signal by the leakage inductance.

[Terry_VK5TM]

As to the why, no idea.

But is it original (as in the transformer manufactured like that) or was somebody experimenting with some idea or other at some stage?

[Radio Wrangler]

It could have been a very misguided later addition.

There were some rather strange and thoroughly wrong ideas circulating back then. For an example, look up 'Stenode reception'. So things from the past don't have to work or make sense. Someone may have tried a strange idea out and it didn't catch on simply because it didn't work.

As Kalee says, it'll clobber inductance and ruin the low frequency response. In later years we'd have wondered what they were smoking.

David

[Graham G3ZVT]

It reminds me of the shorted turn on a shaded-pole motor, but how that helps a transformer I couldn't say.

[RadioHist]

Well folks thanks for your comments...

I have no reason to think the transformer had ever been removed just to add this mystery shorted winding after the original build....

I have a very faded sketch of the schematic... I guess that needs to be my next task so I can post... The sketch makes no acknowledgement of the shorted winding.

I have not found any Web pictures showing the internals like this picture of my set.

Robert

[Radio Wrangler]

Could it be set up for some sort of trade test? A set that's been subtly banjaxed by an instructor to see if an apprentice can troubleshoot it?

David

[Terry_VK5TM]

You can get a good copy of the schematic from https://www.radiomuseum.org/r/seibt_ei472.html

No indication of the shorted winding there either.

There's also some internal pics but I haven't downloaded any to see if there is a clear shot of the transformer.

[Graham G3ZVT]

Originally Posted by Radio Wrangler

Could it be set up for some sort of trade test? A set that's been subtly banjaxed by an instructor to see if an apprentice can troubleshoot it?

David

You jest, but my C&G practical using the then new Thorn 1500 included one with the DY86 heater o/c. Probably shouldn't have, but I unscrewed the ceramic valve base to confirm, and remember struggling to get it back in, in time.

I suppose you lost marks for sabotaging the next candidates chances.

[PJL]

Could it have been part of the manufacturing process that should have been removed?

[fetteler]

Hmmmm... I wonder if the wire used for the 'shorted' turns has any significant resistance? Not that it suggests any particular use or advantage to me but more that it might stop them being such a definite bad idea

Steve.

[turretslug]

Good call- perhaps an added resistance wire winding was a slightly despairing attempt to tame an unstable circuit! Albeit a drastic and somewhat crippling one.

[RadioHist]

Originally Posted by fetteler

Hmmmm... I wonder if the wire used for the 'shorted' turns has any significant resistance? Not that it suggests any particular use or advantage to me but more that it might stop them being such a definite bad idea

Steve.

Great minds think alike a former co-worker just posed the same thread.
Is 4 T winding copper or resistance wire of some sort? Quote:

I would expect a copper 4 T to reduce winding inductance and reduce gain at low frequencies.

A resistance wire 4 T would be more like a shunt resistor across primary or secondary. This could damp out a resonant point caused by transformer L and stray C. This would reduce audio gain a bit. End quote..

Tomorrow afternoon I'll try to figure that out... Right now I'd say that the wire is ordinary double silk wrapped magnet wire from the snapshots I'm looking at... Time will tell...
Robert

[Maarten]

It doesn't look original. Not by the transformer manufacturer. Maybe, just maybe by the set manufacturer.

[Hartley118]

I guess that the way to test all these imaginative theories would be to test the transformer with and without the shorted winding.

These early coupling transformers were far from ideal by today's standards. They needed high inductance windings because they were fed from a high source resistance (e.g. a specified 18k for the anode resistance of an HL2 valve) and the inadequacies of the core were compensated by lots of turns of thin wire, resulting in high leakage inductance and self capacitance, resulting in a self-resonant frequency typically falling in the audio band.

I can only guess that the set sounded better with the transformer damped by those shorted turns. There may have been plenty of gain available from the valves to compensate for the reduced transformer effectiveness.

As you say, those four shorted turns with their self-resistance load of say 0.1 ohm would have a damping effect similar to connecting a load resistor across the 19000 turn secondary. A rough calculation comparing the shorted turns with the 19000 turns on the secondary, suggests the impedance ratio would be (19000/4)squared, or 475,000. Assuming that the shorted turns measure around 0.1 ohm, that would give an equivalent secondary load of 47,500 ohms - a not unreasonable figure to provide significant damping.

It would be great if you could get the set working so that you can try it with and without the shorted winding. If this isn't practicable, then you could simulate the effect by taking a frequency response measurement by feeding the primary from an audio oscillator via (say) 20k ohms to simulate the driver valve equivalent anode resistance and measuring the secondary output voltage into say 1Mohm. Doing this with the four turns open and then shorted should answer the mystery.

Martin

[RadioHist]

Martin,
Thanks for the thorough suggestions on how ti investigate... This afternoon I started to think about pulling the chassis out of the cabinet and remembered how very difficult it was to put back. The Oak cabinet has shrunk and panel is now firmly press fit in place!
I plan to put it on exhibition March 26-28 and think I should not risk a hurried attempt to extract and make measurements before the exhibit. What I will do is to do a write-up of comments to place in my presentation note book.
I sure seems to me now that the purpose is most certainly to normalize frequency response one way or the other. There is supposed to be another version of this chassis where R-C coupling replaces the transformer. Apparently they judged the loss in over-all gain of the radio a penalty they could afford.

Again thanks for all the comments...

Robert

[Radio Wrangler]

It could have been a clever alternative to having to have a high value resistor as an anode load to define the stage gain. Back in the day high value resistors were known to be difficult to make reliable, and they were expensive and noisy.

We've all become too used to seeing valve stages with no overt anode load defining resistance and stages run to gains set by field feedback in triodes (mu-limited) where the designer was after gain at all costs.

This could have been a case where the designer had plenty of gain and could opt to trade some for a flatter audio response. Later changing it to RC coupling would support this. Finding the wire to be manganin or nichrome would fit.

David