I have a custom made tube amp (5E3/5F1 clone-mix) that is really acting up... I was playing it the other day and everything was great - volume, tone, etc... But after about 10 minutes of playing, the volume all of a sudden decreased by about 90%. Just one minute everything was full power, and the next not. All of the controls still worked - the tone pot, the volume pots too...just everything was really quiet. The sound coming out of the amp retained the same overall character too - I was playing with the volume dimed, so a good amount of gain, and the gain was still there after the drop. So, I shut the amp off, let it cool and tried it again an hour later. Same thing happened - played great for about 10 minutes, then massive volume drop. So the next step was to replace the power tube - so I let the amp cool off for another hour and dropped a new 6v6 in, came back...same exact results, good for 10 minutes, then a drop. So I left the amp for the day. I came back the next day to play and now it just started off quiet. It never played again at full volume/full power....still hasn't... All the pots still work. I can still control the tone. I can still control the volume - granted, it's not much anymore, but I can definitely hear changes in the volume and gain when I move the knob around. I looked at the components inside - all the capacitors and resistors look good (none are bloated or have black spots). I tapped on every piece inside the amp with a chopstick to see if anything would pop or crack - nothing. I tried moving every solder joint to see if something came loose, but I didn't find anything. I tried reseating tubes, putting all new tubes in every position (except the GZ34) and that didn't do anything either. I know that my guitar, cables, cab all work because I tested them with another amp. Basically, I'm at a loss for what could be causing the volume drop. The builder suggests that maybe it's the output transformer...could that be the case? I always thought if that goes then no sound would come out... Any suggestions or do I just have to literally check voltages across every single piece inside the amp?