I have a 1982 (i think) Marshall JCM 800 4210 50 watt amp that produces a lot of background noise, loud hum which is noticeably worse at higher volumes and higher gain levels. I'm useless with soldering joints, know nothing about tubes (although they've been replaced in the last 6 months). It's also much worse when I plug my Telecaster into it. Whereas my Les Paul isn't as bad. Obviously this is due to the singlecoils in the Tele, but I thought I'd mention it anyway.
Has anyone any idea what's causing this?