Okay - so my beloved K2 is apparently having a problem with the factory tube. For the first 30 minutes of a session, it sounds fine. After that, the mic starts to get very thin and muffled to the point where it eventually stops receiving/sending signal at all. So, I'm looking for a replacement but I'm not good at which tube can replace what with minimal problem. The tube that they use is a dual triode 6922. I have several Mullard 12AX7s laying around which are also dual triode - will these work and if not, recommendations for a tube within the GT Gold series would be very helpful (as I can get those locally and they are easily stocked and I need it ASAP). Thanks! J.