"T N Nurse" bravely wrote to "All" (25 Sep 04 14:35:33)
--- on the heady topic of "Transformer question"

TNN From: T N Nurse
TNN Subject: Transformer question
TNN Organization: Malathion Church of Holy Redemption
TNN Xref: aeinews sci.electronics.repair:39778 sci.electronics.misc:18316

TNN I have a 50 watt valve amp which seems to have blown its
TNN output transformer. A new one is on order, but I was puzzled
TNN by some of the reading I got off it and wondered if someone
TNN could give an explanation. All the measurements were made
TNN with the transformer *_completely out of circuit_*, both primary
TNN and secondary.

TNN The transformer is from a push-pull standard guitar amp (Trace
TNN Elliot 50 watt combo, 2 EL34s) and one of the valves was glowing
TNN red hot before the fuse popped. Checking the bias voltages, I
TNN had around -46v on the grid of each of the El34s, so that was
TNN a reasonable figure. I then removed the output transformer
TNN and did some resistance measurements on it. When measure from
TNN the centre tap to either of the primary outer connections, the
TNN reading was 69 ohms for each. But when I measured across the
TNN primary outer connections, instead of the expected 138 ohms,
TNN I got an open circuit. I rechecked it numerous times but with
TNN the same result. I even removed the cables from their connector
TNN and measure across the bare wires, but still the same result,
TNN 69 ohms from the centre tap to the outers, but open circuit
TNN between the outers.

TNN On the basis of these bizarre reading, I assumed the transformer
TNN is faulty and ordered a new one, but can anyone offer an
TNN explanation as to why I got such resistance readings? I have a
TNN vague recollection of similar results on a small 15 watt amp I
TNN repaired many years ago and replacing the transformer fixed it
TNN and it went on to give good service, but I would like to know
TNN what is actually going on. Anyone?

The only thing I can think of is that (burnt) oxidized metal to metal
contacts can form a rudimentary semiconductor and will not allow current
to flow in reverse just like a diode. Try swapping your ohmmeter wires.

A*s*i*m*o*v

.... Are part-time band-leaders semi-conductors?