You have a potential replacement transformer (such as a PA 100V line transformer) and you want to know the impedance it presents to the output valve. Here is a simple method that doesn't require sophisticated gear.

All you need is a signal generator, a 100k pot and an AC voltmeter (such as a DVM).

1. Connect the sig gen to the transformer primary, with the pot in series with it.

2. Connect the secondary of the transformer to the speaker you will be using, (or use a resistor of the same impedance.)

3. Set the output voltage of the sig gen to 1 volt and the frequency to 1kHz. If the gen won't do this output voltage, set it for maximum anyway and take a note of the reading you get.

4. Measure the voltage across the transformer primary. Adjust the pot to get half the sig gen voltage, say, 0.5 volts

5. Disconnect and measure the resistance that the pot has been set to. That is your transformer impedance.

Most PA transformers have several secondary taps. You can select the one that gets you closest to the desired primary impedance.

PA transformers generally don't have core air gaps which means the output valve anode current can drive the core towards saturation, which will cause distortion. In my experience this has never been a problem, but if you wish you can restack the E and I laminations with all the Is together and use a piece of insulation tape to set the gap.

Simple but effective! Good one to take note of.
Thanks for sharing Ian