I've been experiencing issues where I get squealing from my blues jr. I thought it was an issue with my Tele but as it turns out its the amp. I've used 4 different guitars with it and its the amp. Some guitars appear to make it worse than others but it is not tolerable.
The amp squeals while I'm playing at band practice levels and is is worse when I am not playing. Even covering the strings with my hand does not curb the squeal totally. It only squeals when using a gain or distortion box in front of it. I am NOT using a lot of gain, typically 12 o'clock on my crunch box and plexitone. I don't have these issues with my fender super with the same setup. The reason for the jr, was to spare my back for practice. The more the amp gets warmed up, the worse it gets.
What could be causing this? could a bad tube do this? The amp is relatively new and I had it retubed recently. At this point I'm almost done with it.