Bogner Ecstasy Squealing

[rf7]

I put new tubes (Winged C) in this head, biased them, and burned them in. The owner used the amp for 3 hours and then it started squealing, even with the strings on the guitar damped with your hand. I've got it back from him and it seems to be affected by the proximity of your guitar to the head. The closer the guitar is to the head, the more likely the amp will squeal. He's using the high gain channel, with the gain almost cranked.

Whats causing this? Is it the pickups in the guitar? The head will start to oscillate a bit with the volume and gain both cranked in the high gain channel.

[dai h.]

squealing when putting the pickup(s) close to the head (or speakers) is normal. It's feedback. Need to get some distance from it to reduce the interaction.