So I haven't played this amp in a while, fired it up , let it warm up (it usually sounds muddy for the first 20 or so minutes) and played it, so far so good.
Now, you know when you switch on the reverb and it fades in rather than just bang its there? well , this took a little longer to fade in but loads of loud feedback , like a woooooooooooo , faded in with it, and just kept getting louder, so I switched it off.
Reverb was on half way, it happens with the guitar connected or not connected. Volume, tone, touching strings , nothing makes a difference.