I was able to find the schematic online:
http://www.westebbe.com/Vox/AD15_30Servicemanual.pdf
It doesn't look like the tube's anodes are operating at very high voltage (15-30v if I'm reading the schematic right. The tube is operating at starved plate voltages and is there to add some distortion.
The limiting factor is the filament voltages. The closer the voltage is to 6.3v (or 12.6v depending on how the tube is wired) the longer the tube will last. A 12AX7 tube operating at 6.3v will last decades if the plate voltages are low enough (< 250v or so).
If the amp sounds the same as it always has, I wouldn't worry about replacing the tube. If the distortion gets kind of weak, or the sound changes a lot, then you may look into getting a replacement.
tung
Originally Posted by marnold