The phase inverter is a hard worked tube. Perceived wisdom states the most important tube in a pre-amp stage is V1, the first valve the signal sees. well, to an extent yes, but the P.I is the tube that gets the work out. It may be worth changing yours out if you find you're amp is getting a little quiet, or your hearing is picking up some odd swirl artefacts. If your amp has gone silent on all channels, yet appears otherwise normal, check the P.I The P.I also has some bearing on sound. The higher the transconductance, the faster the valve can slew voltage. A 12ax7 is usually O.K in the P.I position. Fender specify a 12at7 for some amps, and for these amps, the Ruby is as good as any I've tried. My amp had become a lifeless thing. It is once again a lively bundle of loud and furious joy. ps.12at7 make rubbish pre-amp tubes. They are P.I and reverb tank drivers first and foremost.