[morzh]

OK, just to make sure you understand - transformer does not care about the DC voltage. It cares about AC voltage. The AC voltage is the current the tube puts through it times the load (the speaker's voice coil impedance) reflected to the primary (that is squared turn ratio times that voice coil impedance).

We can assume that the voltage across the primary is in tens to a hundred plus of volts. The output voltage is in units of volts. Say if your voice coil impedance is 1 Ohm it will take 1V to produce 1W of the power. For 8 Ohm it will take about 3V to do the same.

General rule is: the primary in the tube amps has a high AC across it and the secondary usually low.

So, take 20-30V AC from your variac and apply it across the primary. Watch the secondary give you some meaningfull voltage of probably a hundred to 3 hundred millivolt.

MAKE SURE you know whichh the primarfy is. If you switch them and apply 30V to the secondary, the primary may give you too much voltage and if you touch it.....notwithstanding it can damage your meter.