12AX7 preamp tube failure last night

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.

Yes, it is normal, as Hifigeek said, for a preamp tube, such as your 12AX7, to fail after even fewer hours. 8-10,000 hours is pretty good, only the sturdiest of tubes were advertised as being 10,000 hours tubes.
There are many options these days to choose from amongst current production tubes. I don't know what would be appropriate for an instrument amp. I have used the JJ803S and am OK with it. I am curious about the Gold Lion re-issue 12AX7.

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!

May I also recommend allowing the amp to cool down before moving it, the tube filaments are at their weakest when transitioning from hot to cold or vice versa. I have seen most tubes fail DURING use if the amp was bumped or slammed whilst it was on, especially large triodes like 845, 211's.

I used to emphasize with my Nagra VPA customers to NEVER move them when on or cooling down.

Thanks Mcintech. That makes good sense and I have broken those rules on occasion with this amp :-)
Still waiting for the replacement tube to arrive and anxious to find out if it was the tube............or something else like a capacity, resistor, or broken solder joint.