I have a 50 watt valve amp which seems to have blown its output transformer. A new one is on order, but I was puzzled by some of the reading I got off it and wondered if someone could give an explanation. All the measurements were made with the transformer *_completely out of circuit_*, both primary and secondary.
The transformer is from a push-pull standard guitar amp (Trace Elliot 50 watt combo, 2 EL34s) and one of the valves was glowing red hot before the fuse popped. Checking the bias voltages, I had around -46v on the grid of each of the El34s, so that was a reasonable figure. I then removed the output transformer and did some resistance measurements on it. When measure from the centre tap to either of the primary outer connections, the reading was 69 ohms for each. But when I measured across the primary outer connections, instead of the expected 138 ohms, I got an open circuit. I rechecked it numerous times but with the same result. I even removed the cables from their connector and measure across the bare wires, but still the same result,
69 ohms from the centre tap to the outers, but open circuit between the outers.On the basis of these bizarre reading, I assumed the transformer is faulty and ordered a new one, but can anyone offer an explanation as to why I got such resistance readings? I have a vague recollection of similar results on a small 15 watt amp I repaired many years ago and replacing the transformer fixed it and it went on to give good service, but I would like to know what is actually going on. Anyone?