Hum particularly on clean channel builds up from nothing ,over half an hour,
and very intrusive 0.3V rms of hum over 8R speaker load and still rising.
Putting a signal in Return, for PA only, is fine but opening guitar input
hum returns.
Owner had replaced all the valves and exactly the same hum.
Hum is negligible on the downstream HTs from HT1. What sort of grounding
problem increases with warmth?
I've not started exploring the low voltage electros around V1 yet, I'm
letting the amp cool down