12AX7 preamp tube failure last night

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.