Well I have an old Cort electric guitar which was repaired so it could be playable. Now I have a Hughes and Kettner Warp 7 amp which hasn't given me trouble in the past; however, at my last band practice, there was a lot of feedback when I turned up the gain and volume. Now the volume and gain weren't turned up halfway and there was feedback even in every pickup position. I also tried playing the guitar through my friend's Marshall Valvestate halfstack and I got no feedback even with the volume and gain cranked up. Can someone help me diagnose the source of the problem?