12AX7 preamp tube failure last night

Usually preamp tubes go out slowly. But if you move it around all the time, the trip may have 'helped' the tube to have a sudden failure.
Most of the time preamps just sit in one place forver.
If you move the stuff in your car etc, then the banging around would be a partial explanation as to why it went out with a complete fail instead of a gradual one.

I have also experienced sudden tube failure in preamps. It is always a good idea to keep spare tubes on hand.

Hi Bill, I have a Vox Tonelab, which i think uses a similar circuit to your AD50VT and uses one 12ax7 tube. Vox uses the tube more as a tone control than anything else ( as a driver for example) in the circuit. I have rolled the tube with various other 12ax7's and you can alter the sound of the Vox a little. However, I would say that your tube probably was failing for a while and you may not have noticed it, given the way the Vox works. Have you changed out the tube yet, because this may not be the cause of your problem.

It's happened to me too. In combo amps the vibration of the speaker interacting with the tubes is a part of the overall sound but the downside is that it puts the tubes at some risk of shortened life and catastrophic failure. Fortunately these are a cheap fix and I always keep a couple of spares handy just in case.

Thanks for all of the responses. I am awaiting a replacement tube (12ax7) and hopefully that will cure the problem. The amp lives in my car a lot and does bump around. That probably explains a lot of the wear........ I have received a lot of feedback suggesting that I try upgraded tubes for that amp. I am planning on purchasing a back-up tube:-) Thanks again everybody!