Is there any way to visually tell a 12ax7 from a 12 au7? Well, I had my Ampeg preamp serviced for buzz a while back, and they replaced 3 of the tubes, including the Au7. I noticed that all the replaced and new tubes have a kind of double stack in the middle... while the au7 has a single stack. I think they replaced the au7 with an ax7. The replacement tubes are totally unlabeled. Now, I've been experimenting, and there doesn't seem to be a difference between an ax7 and an au7 in that slot, but it's a little disturbing. I'm sending back to the service center anyway because of a totaly different issue (distortion/weak signal that goes away after a few minutes of being on and only comes back after it's off for a while) I'm gonna ask these guys when I take it back. Any advice? What do you think?