How to measure the primary impedance of an output transformer.
(See disclaimer below)

Following these steps you can measure the primary impedance of your output transformer without removing the chassis from the cabinet.

Required equipment: Signal Generator, Multi Meter (VOM),1/4 inch phono plug and Calculator.

Step 1) Disconnect all AC power from amp.

Step 2) Disconnect speakers from output jack and remove output tubes.

Step 3) Connect the signal generator across the full primary winding (pins 3 on both output tube sockets see drawing 1a).
If the amp is a 100W model use the two outside or inside tube sockets. (See drawing 1b)

Step 4) Set the output of the signal generator to 1kHz AC (1,000 cycles per second) @ 1 volt.
I like to verify the voltage output of the signal generator with the multi meter.

Step 5) Insert the 1/4 inch phono plug into the speaker output jack.

Step 6) Measure the AC voltage across the secondary tap. 4, 8 or 16 it doesn't matter which. (See drawing 2) I like to check each one.
Make sure the impedance selector is set to the tap that you want to check.

Step 7) Divide the input voltage by the output voltage. This gives you the voltage (turns) ratio.
The amount of turns on the primary to 1 turn on the secondary.

Step 8) Take the voltage (turns) ratio and square it (VTR x VTR). This gives you the impedance ratio.

Step 9) Take the impedance ratio and times it by the secondary tap rating (4, 8 or 16 which ever one you chose).

Step 10) You now have the impedance for the primary of your transformer.

Formulas

#1 Input voltage / output voltage = VTR
Example: 1 vac/ .035 vac = 28.571428 vtr

#2 Volts (turn) ratio x Volts (turns) ratio = Impedance ratio.
Example: 28.571428 x 28.571428 = 816.32649

#3 Impedance ratio x Secondary tap = Impedance ratio.
Example: 816.32649 x 4 = 3265.3059 Drawings   Disclaimer

Do not perform these steps without disconnecting the AC power from the amp.
There are deadly voltages inside of an amplifier that can kill you.
If you are not comfortable working on an amplifier have a qualified technician
perform this test for you.