How to perform a frequency response test in a tube amp output transformer?
You can use a signal generator and oscilloscope. Set it up like this: -



Connect the scope's 2 channels to A and B with the scope inputs grounded at the commoned point of the transformer and signal generator.

One channel (A) of the scope verifies that what you are inputting stays constant and you note down the value (p-p) on the scope channels connected to (B)

This test only works at small input levels but if you are trying to compare two transformers it'll work fine.