How to calculate the output transformer impedance of an OPT with unknown values

Working with tube amps you may need to calculate the output transformer impedance especially when we have an old transformer or a transformer with unknown values.
All we have to do is to calculate the turns ratio of the transformer. To do this we need an AC voltage source and an AC voltmeter.
So we apply an AC voltage to the primary of the transformer and we measure the output voltage to the secondary.