• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Hot power trafo problem

In my case, I would be very surprised if it was a crossed winding, I think more of an isolation fault on the wire itself + extensive operation of the final stage of the amp.
The guy in question was a very rigorous career soldier with a great winder but tended to believe a little too much in the extreme reliability of the components, or to push them too hard according to different opinions...
I should repair it one day but it stinks so much that I can't get around to it.