Hey everyone, I've been reading these forums for quite a while, but just recently got around to becoming a member and have a question about my amp. It's a Marshall Valvestate 8100 (not sure about the year, I bought it from a friend who inherited it). I recently decided to change out the preamp tube (Marshall ECC 83) for a new one. I brought the amp into a reputable shop in Portland, Oregon and had three or four of the employees there look at it, all of whom decided that the replacement tube would be a 12AX7 Tung-Sol. Following their instructions for the installation, I carefully installed it and let it warm up slowly for a good 30 minutes before playing through it at a lower volume. It sounded absolutely fantastic for about five or ten minutes, at which time I decided I wanted to hear how my Les Paul sounded through it. I powered down the amp, plugged in my other guitar, and powered it back up. Then something odd happened. The distortion was very quiet, almost inaudible. I turned it up a bit, and still it remained quiet enough that I could barely hear it. I decided to troubleshoot a bit and powered it back down, and installed my old tube, which worked fine (it was as loud as it was supposed to be). I reinstalled the new tube again and the same problem persisted. I thought maybe the tube was a dud, so I took it back to the shop and got a replacement, which again has the same problems. With both new and old tubes, the clean channel sounds fine, just as loud as it is supposed to be. Do they keep selling me the wrong tube? They look almost identical. They are the same size, etc. although the insides of the tubes look a bit different. The odd part is that with the new tubes installed you can actually hear the distortion, and it sounds fine, it's just very VERY quiet even at very high volumes. The music shop that I've been buying from is over an hour away from where I live, and it's growing very tiresome (not to mention expensive) driving up there to exchange items so often, but the music shops in the town I live in are pretty useless. Thank you in advance If I left out any crucial information, please let me know.