Ok. Here is a mystry to me, but I am sure you guys know - How and when should you change your 12AX7 PreAmp tube? How long do they last and What kind should I use. I have an SWR Bass 750 (Pre-Fender). I have had it since new, about 4 years. I have never changed the tube. I have played on average of about 14 hours a week, traveled 8 states. I know nothing about tubes other than my amp has one.