• 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.

Transformer faulty?

I tested the transformer last night and found something I don't understand.


During the tests all secondary windings are not connected to anything but the multi-metre terminals.

Why there are such high, stable voltages on the secondaries against the ground point?

I don't think the capacitance between windings would create that sorts of voltages. I guess the multi-metre has about 1M input impedance... but not much more.

Can't work out from your diagram where the 'ground point' is connected do you mean mains earth?
Most digital multimeters (I'm assuming that's what you are using) are quite high impedance devices and can play all sorts of tricks in high voltage circuits. When I was a tv repair person, we used analog avo 8 meters partly for that reason. A digital would sometimes 'find' a voltage at the end of an open circuit fuse, broken wire or burnt out component confusing us all somewhat.
Since you have 800v flying about, I'm not surprised by the odd readings. If you are worried try checking between the windings and transformer frame with an insulation tester (megger) - obviously mains must be disconnected for this and adequate precautions taken to avoid being zapped by the tester's output voltage.