It's been a while since I had the need to post.
This is a brand new amp, purchased November 2012 from Guitar Center. It was not a floor model, it came brand new in a box.
Lately, about 30 seconds after you power it on, I'm getting a strange buzzing sound that gets higher and higher in pitch and then fades out. This is with or without a guitar plugged into it.
I know this isn't supposed to happen as I've owned this model of amp previously with no such problem. Any idea what it could be? Maybe a tube needs replacing? If so which one?
I've uploaded a video of the problem to youtube, skip to 34 seconds if you don't want to wait for it to appear in real time.
As you can see, turning the knobs has no effect on the buzzing. Also you'll see that after i switch it off, and then back on, it makes even weirder sounds.