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

Another method, if you have an LCR meter (like the BK Precision 878) can be seen at my page https://www.wa2ise.com/radios/opt-imp-LCR.htm

More directly
A method of testing output transformer impedance. Assuming you have a LCR meter that uses audio frequency tones to measure resistance, here a BK Precision 878. Set it to resistance, and put its clip leads across the primary. Have the speaker on the transformer secondary. You should see a resistance reading indicative of the impedance the output tube will see. This tester does two different frequencies, 1kHz and 120Hz. You'll hear weak pulsing beeps from the speaker. The differing values you get at 120Hz and 1kHz reflect in part the varying impedance of the speaker itself. A dummy load resistor should show less variation.

Tubeactive wrote: Using an LCR for judging the apparent or implied fidelity of output and interstage transformers. In Primary Inductance mode, the test leads are connected across the primary of the transformer, with no connections on the secondary. This is very important, as any added resistance across the secondary will indefinitely load down the primary Henries reading. The higher the Henries, the better, concerning Primary Inductance.

Leakage Inductance can be tested with the same test lead connections, but the secondary gets shorted, thus reading the leakage reactances in Henries. A good transformer can read 20 to more than 100 H with an unloaded secondary. Shorting the secondary leads, if that transformer now reads under 100 mH, depending on the impedances and primary inductance, this indicates a good likelihood of high fidelity. Of course, under 50 mH of leakage will be much better. The HiFi opts. could read over 40H on primary inductance, under 20mH leakage inductance; some of the best are under 10mH leakage! I would chart out the leakage readings on all the speaker and line output taps, for comparison and quality judgements...

A word of caution is in order, to avoid misleading results. When measuring transformer in an amp, in circuit, the amp needs to be unpowered, of course. Just as important, the feedback loops may need to be unconnected. If there is a feedback connection from a speaker tap or a tertiary winding, the Primary Inductance Henries reading will be artificially low. If the feedback resistor goes to a preceding stage's cathode, across the cathode resistor, maximum H readings will be low and thus misleading...

Public Address and Sound Reinforcement grade interstages and opts will read closer to 10 or less Henries on the primary. However, their leakage inductance can read incredibly low, some under 20 mH! This would indicate a serious lack of low bass, but could imply good high freq extension. Bi-amping would be a decent application for such a transformer, used in the midrange/treble amp...

Some opts will flash OL with certain digital meters, while measuring their primaries. The inductive kickback fools the meter in question. A hookup to a trusted LCR can enlighten quickly. Unpower the amp, discharge the power supply caps safely, remove the speaker load and then test the Primary Inductance. Note that if you check each half of the primary winding, the Henries will read 1/4 of the full primary reading. This is a good indicator of the opt transformer primary having balanced windings.

Maxhifi wrote: A four inch radio speaker probably has a low frequency resonance of about 200Hz, so the 120Hz reading is probably representative of the nominal impedance of the speaker since it is far away from resonance. The 1kHz reading is higher due to impedance rising with frequency.