Tubes are always a good place to start. Underwood HI-FI uses Mullard CV4003 in their Unico SE upgrade.
Hum in Unico SE
I have a Unico SE hybrid integrated amp that suddenly (sort of) developed a hum. I've removed all inputs except the speakers and the hum is still there. I picked up and separated the bi-amped speaker wires but no change. As I added back individual devices no change. What do I try next. I don't have replacement tubes to try but I will get some. Also, what would be best tubes for replacement.
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