I was messing around with my amp today and decided to try all of the inputs. Everything is fine with the exception of the bright input on channel 1. While playing the volume will drop to a very low level,not completely off but way off the mark from where it should be, then it comes back up only to drop again. I gave the strings a good thump to see if it reacted like a bad joint but it doesn't change the low volume at all.All the other inputs work perfect. Any ideas? Tube, bad solder joint?