Well if the tube has been in that amp for seven years then, yes, it's possible for the tube to fail. Even small signal tubes wear out and need to be replaced, usually after 8-10,000 hours. The question is, was anything else damaged when the tube failed? Try installing a new 12AX7. I recommend the JJ803S. Hopefully it works and away you go. You can also email the good people at Vox and ask them.
12AX7 preamp tube failure last night
I was playing a gig last night with my Vox AD50VT amp which has one 12AX7 preamp tube. Suddenly I lost all sound. Is that a normal failure mode for a tube? Is that uncommon? I bought this amp new almost 7 years ago and love the sound of it. I have a recording of the song when it failed and it seems like you can hear the tone breaking up and getting scratchy right before all sound was lost. Any ideas appreciated.
Bill
Bill