Having an intermittent rustling static noise, like a component is taking charge ( a faulty cap arrgh ), but not settling down. This is on my MK11 SB 100 which has had a hard life and was first owned by Lemmy and has done many miles since, it's just the most in y'r face sounding amp's I've had and so aggressive, I use it for Bass guitar I should add. So I check all the pre amp valves for what seems like an eternity, sometimes it's better but never fixed, all this time I'm using the standby switch. Hmm so I think try Deoxit on the switches, bingo and all's good. Until the next day I switch it on, noise is back again so I check the switch with a DMM and find I'm getting a resistance of between 10 and 50 ohms. Right, switch is coming apart. All becomes obvious now. It's full of dust and crap that has mixed with the switch grease and this has compromised the contacts, the deoxit only added fuel to the mixture, I made tools for cleaning etc and it wasn't that difficult once you're in there. Here's the rest of the pic's, inc mains switch. The amp now ( and another JCM800 with same problem the other day ), is so quiet and full, dare I say it, smoother, so much depth, I know this sounds a lot like audiofoolery but I do know this amp, having gigged it for the last 10 years, but basically it's happy and running on all 4 again.