Wow - he really is a good tech if he didn't charge you.
It's crazy what happens with amps. There are so many things which can cause problems. 99% of the problems with tube amps though are tubes. I built an amp for a guy early this year and he lives two hours south of me. He drove up today to drop off the amp because the volume took a dive. I spent about 15 minutes swapping tubes and sure enough (and slightly unusually) it was the PI tube which was on the way out. Stuck a new one in and it's back to sounding great.
So while I had it I did a couple of minor tweaks for him because his needs have changed a bit. Now it's going to sit in my hallway for the week until he comes back next weekend to get it.
