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Supro Output Transformer

This is the Push-Pull Output transformer that came with this Supro.

I found no shorts between leads, so I applied 5VAC to the secondary. In the primary side, I got 309V between BLUE and BROWN, 165V between RED and BROWN, and 144V between RED and BLUE. I don't have the original wiring harness, but both speakers are 3.2ohm DC, so the secondary is either 8ohm series, or 2ohm parallel. Parallel was more common than series, plus these are lighter duty speakers. so the math:

309 ÷ 5 = 61.8

2ohm × 61.8² = 7442ohm

 

The same math with an 8ohm secondary comes up with 30K:8ohm, which isn't going to be correct. 7.4Kohm:2ohm has got to be the original spec to this thing.

As for the imbalance of the center tap of the primary... I'm not sure how to figure the acceptable variance. When I use the above math to calculate the winding ratio, I get 2.1K:2ohm and 1.6K:2ohm, the difference being 509 ohms, which is about 6% of the total 7.4K. Which might have been acceptable for a 1950's budget amp. Am I figuring that correctly? I'm not sure...

An interesting note is that the OT was once mounted to the speaker frame. I can see the original rivets, and the OT's frame shows signs that the OT was once wrenched from the speaker. the baffle board also shows signs that a 12" speaker might been fitted to this, and then the original 6x11"s were later returned to the baffle board. Anyhow (on to my point), the original OT has long leads to the plates and the B+ (RED,BLUE,BROWN), plus a yellow speaker wire ran into the chassis as well. This must have been a ground wire. If it was NFB, a black wire would have have run as well (maybe it was and it went the way of the chassis...).

 
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