Any ideas, it does it regardless of whether an instrument is plugged in, and even with both volume and gain turned down. I know that it's mains hum, but what could be causing it. I've tried plugging it in to different sockets, and I've tried using different speaker leads. It's an old series 6, 200 watt head, with the 12 band graphic. It's not very loud, just damn annoying. I've had a quick look inside to see if it's anything obvious, but I don't really know what I'm looking at, I should point out that I have the technical knowledge of a tadpole.