Quote Originally Posted by Gutmann
Feedback without attached guitar probably is caused by a microphonic tube.
Try to hit the tubes slightly with a pen while the amp is running. If one is making a strange noise, it might be a microfonic one. It could be a preamp or a poweramp tube.
Yep, I had the same increasingly loud squeal with no input on my Gries 5.

tap, tap with a pencil eraser & lo & behold the 12AX7 was obviously microphonic. I changed that & problem solved.