Hey guys. I just bought a lovely BR6 from pepperco (great guy to work with btw). The amp works fine for about twenty minutes or so, then the volume drops a bit. I'm just trying to diagnose the problem so I can get the correct parts. Part of me has a hunch it could be the tubes, since I wouldn't doubt if these were the original set. The tube make-up of this amp is: (from L to R in back) (1) 6SJ7 preamp (1) 6SN7 phase (2) 6V6GT power (1) 5Y3GT rectifier Any suggestions? thanks guys!