Matching tfmr?

[Diabolical Artificer]

Probably being a bit slow here but here goes. Going through my tin of small tfmr's I have found several matching tfmr's, which I presume like your average audio tfmr it's function is to match impedance, however AFAIK it has no primary and secondary windings.

So how does the thing work? Like an auto tfmr? Also how/why were these used? In early tranny amplifiers?

Just curious, Andy.

[Ed_Dinning]

Hi Andy, yes an autotransformer will work for impedance matching bit not provide isolation.
It was an economic solution where isolation was not required as it is more economical on wire sizes that double wound.

Ed

[Diabolical Artificer]

Thanks Ed. What were they used on, valve stuff? A couple I have are Wharfdales so wonder if they were used in speakers?

Andy.

[radioman]

Hi Andy,
I do know that Tannoy used matching transformers for the tweeters in some of their Hi-Fi speakers, so presumably Wharfedale did so as well.

Andy

[Terry]

We used to fit RS 'Boost' transformers in TVs, either to isolate the heater in the rare cases of breakdown of the heater/cathode insulation or to boost a flat tube to get a bit more life out of it.

These had mains powered primaries and were designed to have a very low capacitance between windings.

I did though once come across an anonymous boost transformer in a set that came to us which was just an auto-transformer inserted in the heater chain. No use for isolation, of course, but I would imagine a much cheaper solution for the usual boost function.

As I say, the maker was anonymous and I only ever saw that one example.

Auto-transformers were also widely used for running US 117V kit off 240V mains. Woe betide anybody who also thought they were buying an isolating transformer because they weren't!

[Nick]

Audio 100v line output transformers were often autotransformers to save money as galvanic isolation was not always needed.