Just bought a used Pro Jr to use as a practice amp. I took it to an open mic last night and the thing was singing, and then all of a sudden, the volume just dropped to almost nothing. I turned it off, back on again, and the volume was fine for the first few notes, then quickly and smoothly dropped back to a level that was barely heard over the drum kit. We weren't playing that loud either. Any time the volume would drop, if I just stopped playing for 10 seconds or so, it would come back strong, but then gradually drop again. I am suspecting the first preamp tube, and was going to retube it with some JJ's anyhow, but I'd like to troubleshoot it over the next few days while I wait for the new tubes to arrive. Do you think I'm on the right track with the preamp tubes?