I have an Egnater Tweaker combo, never had a problem before. Last night while playing I noticed a sort of whine coming from it. Almost like the ear-ringing from tinnitus. It was totally independent of the guitar, or any guitar settings. No pedals being used. It wasn't loud, but it was there. This morning it was there again, from the get-go this time. After maybe 20 or 30 minutes, the guitar sound all but cut out, and the whine got louder and more like feedback. I suspect this may have something to do with the tubes; some quick web-searching suggests a microphonic or otherwise "bad" preamp tube. But the fact is I know nothing about this. Any ideas what could be wrong, and how to fix it?