Microphonic Tube Feedback

[Bishop5150]

Rescently replaced the preamp tubes in my amp before we went on a road trip from Houston to Indy and back for a show. When we got back I started noticing that when I would put something on top of the amp like a pedal or something I'd hear a ringing type feedback and when I'd tap the top of the amp or lay something on it. Then I started noticing that my high gain channel would start to feedback even with the guitars volume completely turned off. So I put some different preamp tubes into slots V1 and V4 and that seems to have taken care of the problem. I didn't notice this before we left for the road trip. Do you think the bumping and shaking from the trip caused this? I had the amp in a flight case with padding underneath the amp and on top. This is microphonic tube feedback correct? What causes it and how do you prevent it?

 

[D-Rock]

It definitely sounds like a microphonic tube. I'm not sure that something causes this to happen though. I think it's just different from tube to tube at the time of production. Some tubes will last year and years, but some will "slip" out of their proper performance rating and go microphonic.
Maybe someone else can chime in regarding the matter.

 

[Bishop5150]

I forgot to mention it's my 5150 III 3 channel amp. I went ahead and swapped out the tubes in slot V1 and V4 but went back and put the tube back into slot V4 And it's still working fine so it was the tube in slot V1.

 

[japetus]

Yeah man, it happens. Fragile beasts, preamp tubes are.

 

[boof]

sound like this? your description sounds like pre amp tubes gone bad.

 

[Bishop5150]

Yep that's pretty much it. It's worse than that though since it's my high gain channel it's more like a true feedback sound. What's funny about that clip is that I bought the tubes from them! LOL!