If your processor has tubes, the life of your 12AX7 tube(s) can vary greatly depending on use. Generally they'll last anywhere from 2-4 years. You'll start to notice the quality of the tube distortion isn't quite the same as it used to be. That's when it's time to change the tube(s). Any 12AX7 tube will work fine. Once you've gotten into the inside of your unit, simply pull the tube out of its socket and plug in the new one. Be sure the pins line up properly on the socket before you firmly press the tube into place.