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Originally Posted by davidgem1406
Today the progress slipped. First I found the output transformer primary had gone O/C since removing it from the chassis.
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Before scrapping the transformer have a careful check of the connections. It seems odd that the transformer should go O/C just by removing it from the chassis. Quite possibly one of the connections to the transformer tags have been disturbed when it was disconnected and it may be possible to 'remake' the connection. Worth a try before trying to fit another transformer.
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The question is about replacing the output transformer. I have several available that I can make fit in place of the original. However I do not know what the primary DC resistance should be.
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As has been said, it's not the DC resistance that is important. It's only given in service information as a guide but in reality the DC resistance has no bearing on the transformer ratio.....basically thickness of wire, size of transformer....there is a lot more to it than that....
The transformer ratio you require is 40:1. This is worked out by knowing the anode load (usually shown as Ra, not the same as ra) and the speaker impedance. You already know it's 5K (or 5000 ohms) and the speaker is 3 ohms, so if you divide 5000 by 3 this equals 1666.6666. Take the square root of that and you get 40.824....just ignore everything after the decimal point, so just 40:1
Mike has given you a way of measuring the transformer ratio. Basically you feed a known voltage into the primary and measure the secondary. Don't put too high a voltage in since it's only an output transformer. I use an audio generator connected to the primary with a scope connected to the primary and secondary. Adjust the generator to give 1V p-p on the secondary. Measure the voltage going in to the primary....for instance if the primary says 25V p-p and the secondary shows 1V p-p then the transformer ratio is 25:1