[morzh]

Remember: you have not idntified the problem until you have identified the problem
Now roughly, 41 tube has about 8k load at this voltage, and say the voice coil is 4 ohm, so the ratio is 2000 and square root of that is roughly 45.

So, roughly speaking, if you apply 45 V from the variac between the centertap and eityer end you shoud see 1V output. Bring it up slowly, listen to buzz and look for heat and smoke.

If the voltage is higher or lower it is OK as long as it is sensible.


Another way is, connect the full primary (ends) in series with 110V light bulb (table lamp, low wattage, like 10-20 watts) and plug it in isolated variac at 110V. Keep the seconday open. The lamp should not glow. Then short the secondary. The lamp should start glowing.
If the lamp glows with the secondary open, there is a turn(s) short, no matter where. Then shorting the secondary should have no effect.

Be careful and observe safety precautions. Do all cconnecting and disonnecting after first bringing the variac voltage to zero.