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.
Turns Ratio = V in (to the primary) / V out ( measured in the secondary)
e.g. if we apply 5 Volts AC and we measure 0.21 Volts AC to the secondary the Turns Ratio of the transformer is :
5 / 0.21 = 23.8 That means a turns ratio 24:1
Knowing the Turns Ratio we can calculate the impedance ratio of the unknown transformer and the impedance in a given load to the secondary.
The impedance Ratio is the square of the turns ratio:24 X 24 = 576 that means an impedance ratio 576:1
So in a given load impedance of 8 ohms the transformer impedance is 8 X 576 = 4608 ohm ( it is about 4.5K)