Tube Question

I recently installed an Air Tight ATC-1 preamp into my system. I noticed that the left channel was considerably noisier than the right but not so noisy that I couldn't live with it. Very recently, however, there is a low level, high pitched squealing sound from that left channel. Is this due to a bad tube? The line stage uses one 12ax7 for each channel. Thanks in advance for your help.
Certainly sounds like you have a problem with the tube in that channel. Since the line stage only uses 1 tube per channel, try reversing the tubes and see if the problem then occurs in the right channel. If so, you've identified the source of the problem.
Sdcampbell, Thanks for the advice that's a good idea. This is going to make me sound like an idiot but I don't know how to change the tubes, it didn't come with a manual and I can't figure out how to remove the top cover. I ordered a manual from Axxis which should be here next week though and I'll try it then. Thanks.
Try switching the position of the two tubes to see if the noise changes channels. If the noise follows the tube, e.g. if it then appears in the other channel, you've got a bad tube. I have a variety of NOS and new Sovtek 12AX7 tubes for sale if interested.
Just cured this exact problem (Atma-sphere MP-3) with tube replacement. Same family of tubes...mine were 12AU7's. I highly recommend N.O.S. Make sure that wherever you get them from, you have a return privilege, in case they turn out to be noisy or microphonic. You'll know this immediately! For more help, you can e-mail me. Happy Tunes!