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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
notserp300
stevecham
3,340 posts
 
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.
notserp300 OP
3 posts
 
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!
mcintech
128 posts
 
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.
notserp300 OP
3 posts
 
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.
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