I'm hoping someone will know this since i cannot seem to get a straight answer from anyone locally ... i'm wondering if it is ok to swap a 12ax7 for the 12at7 phase inverter tube in the preamp section of my '71 bassman 100 head. I tried it and it sounds really good but I don't want to fry the amp. I am running 5881 power amp tubes, 2 12ax7s in the preamp and there is a 12at7 in the PI. I just want to know if it's ok to go all 12ax7s???