I have a little problem with my 6505+, maybe somebody here could help me. On the clean channel, it seems like it wants to break up. It does the same when i turn the crunch on as well. When i go to the lead channel, i have almost nothing for sound. Could i have a bad tube? I checked my pre-amp tubes and the first 2 in the chain are real dim, but the last 4 seem to be ok. Any ideas?