I've been using my FTIIIXL for about 6 years. Never had any problems, other than replacing the master volume pot twice. This amp gets a serious work out! I take it on the road at least 80 shows a year and it has seen it's fair share of abuse. The other day my lead patch, which has been the exact same lead patch I've used since the begining, started giving me a very high-pitched squeal at certain times. This was a sound that I instantly recognized. It sounded exactly like when a 12ax7 preamp tubes goes microphonic. If there were a tube to change, that would be my first go-to troubleshoot! However, no tube!

Anyone have any thoughts on what this could be?

Any help from anyone at Line6 who happens to be lurking about would be appreciated as well!

Thanks!