12AX7 preamp tube failure last night

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.