• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Output transformer

Would someone suggest a reliable way to test an output transformer.
I know I can use an ohm meter to verify shorts or opens but that does
not tell the whole story. I'm not asking how to determin impedance. Replacement is a good way I know, but I dont have a spare to do that.
If I could determine whether or not the OPT is good or bad I might save myself buying one I dont really need.
Measure primary inductance, with the secondary open and then short circuit. There should be a big difference between these two measurements. If not, it has a shorted turn somewhere.

Connect a 6.3V AC heater supply to the secondary, via a wirewound resistor (5-10 ohms, 5-10 watts). Measure the AC voltage across the secondary, and then the AC voltage across the primary (which is used as the 'secondary' in this test -i.e. we use it back to front). The first voltage should be somewhere around 3-6V, the second voltage should be 100-300V. The exact values depend on the transformer details. If they are much lower than this then it probably has shorted turns.