KT-88 diagnosis

[wanders]

The project was opened ended, not a defined kit or even bom. I've replaced a couple of caps/resistors with different specs; worked a combinations of power and bias settings; tried different output tubes, etc. Nothing was done to amp1 that wasn't done to amp 2 first; and amp 2 is working fine. I've been planning to install GNFB and wanted everything precisely the same and optimum in both units before I started to experiment with feedback. The risk of tinkering is that you might create a short or knock something loose. I've had that happen a few times, bur was always able to figure it out -- until now.

 

[JMFahey]

Must have forgotten to mention the 8 ohm dummy load

Good, I was worried.

That said, an OT "singing" (that is the usual word) is absolutely normal, lamination does exhibit a slight magnetostrictive effect which means they will mechanically vibrate, hence produce ¨*sound* .
Low level, poor efficiency but sound nevertheless.
Tightly pressed laminations or good potting will kill that, a bump may loosen them enough to be audible, even annoying.

But they are a mechanical problems located at or around the OT, you are wasting time by

replaced a couple of caps/resistors with different specs; worked a combinations of power and bias settings

eliminated the preamp as a cause

testing shorted inputs and eliminating the pre ...... work back through all of the output wiring, checking for broken/cold joints, shorts,

swapped front end tubes and tested with open and shorted input - amp sits quiet. Wavegen input w/o pre: same as with preamp: opt noise and wave deterioration; however the gen does supply about 600 mA.

You mean 600mV?

In any case:

I've eliminated all the suggested alternatives:

does not surprise me, none of those has any relation with your slight problem.
I bet if you vacuum pot your OT "singing" disappears.

Not suggesting you do that, but suggest you live with your very slight problem, which to boot will be inaudible when playing Music through real speakers.