"N_Cook" wrote in message
...
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




This might be the infamous output PCB fault.
Keep metering the bias voltage from the 3 pin connector provided, (Should be
90mV both sides).
If it starts to climb/runaway in sync with the hum increasing, then that's
what the problem is.

You will need to buy another PCB from Marshall if so.



Gareth.