[Tubesforme]

It looks the problem is after the 24 but before the 27. It could be lead dress. Try moving the 27 grid lead to see if that makes a difference.

Steve

That has been done several times, in fact the lead was replaced with a shielded one, grounded only at the one end. That didn't help so the original black wire was reattached. It was long enough to move around. The area of the problem that you suggest is exactly what I had come up with early on. I doubt that #16 is defective. The interstage transformer, while new, was manufactured overseas and I wonder if there could be leakage in that, but that seems unlikely. The hum sounds like 60Hz. I have ordered a 100uf @350V and once I get it to bypass point B to ground and see what happens.