N_Cook wrote:

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
Well, that's where you will find the problem, almost for sure.
Hum that increases over time is very likely to be bad electrolytic
caps.

Jon