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.
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.
Jon
Jon