[GrimJosef]

As others have said, picking the wrong end of the transformer secondary would certainly induce howling. But it's not out of the question that something else could be causing it and in that case moving the wire from what is in fact the correct end to the wrong end could make the howl so bad that something is damaged. So I'd recommend that to begin with you just disconnect the wire and try the amp without feedback at all. If this doesn't stop the howling then we'll need to have another think about the possible cause. Of course disconnecting the feedback will make the amplifier very much more sensitive. So do take care to feed it only with a small input signal.

If the howling stops when you disconnect the feedback lead then you can power down, connect the wire to the other end of the secondary (and swap the secondary ground wire too) and switch the amp back on. Keep your finger on the power switch so that if howling starts again you can turn it off immediately. Running a 30W amp as a power oscillator can fry the tweeter in your speaker very quickly indeed !

I've not seen a copy of the book you mention. The only online link that Google finds for me is to a Gramophone magazine column announcing its publication in Jan 1953. So that will pre-date the EL34. Are you using EL37s as the power output valves ? Also did the original design have an 8ohm secondary on the output transformer ? If not then you might find that the feedback level is different from what the designers intended.

Cheers,

GJ