How to measure output transformer?

[disco]

That could be at quite low rms voltage for easy listening level, or it could be at max rated signal level at onset of output stage clipping where amp distortion is naturally a high level and your ears are complaining. Hence the generic use of a commonly available test voltage (5Vac 50/60Hz).

The mistake made is to test the primary for inductance with this low voltage, like mentioned a couple of times already in this thread.

 

[trobbins]

Why is it a mistake? As long as the test conditions are identified, it is informative.

 

[disco]

*ding* You should test the live conditions: low voltage on the secundary only.
Read post #37.