Amp picking up tapping on chassis

[NickyBass]

When I tap on the chassis of one of my heads, the tapping gets amplified. It's an all tube amp with octal output tubes. What could cause this? There are wires touching the chassis. Could this be why?

 

[Jeff Scott]

Microphonic tube(s).

 

[NickyBass]

I think you might be right. But, what can be done about it? And does it really matter?

 

[Rick Auricchio]

Try replacing preamp tubes, one by one. Usually that's where the problem lies, not in output tubes. You can open the amp and carefully tap each tube with a long wooden or plastic stick. You'll know when you're tapping the bad one. Don't touch anything directly.

It isn't a problem till the microphonic tube gets real bad. You might be at a gig.

 

[NickyBass]

But aren't octal tubes more microphonic by nature? I will try checking the preamp tubes too.

 

[Rick Auricchio]

Not necessarily; any tube can become microphonic.

You might even use a pencil with an oversized eraser so you're thumping the tube with rubber instead of wood. Whatever works to let you tap without sticking fingers into the amp.

 

[NickyBass]

Cool. I found it. It was a 6SN7 tube. I swapped it out, and it's alot better. The tapping is still amplified a little bit, but not as bad. Is a little normal, or maybe I should pick up a third tube to test it out.

Thanks for all the help!!! This place is the best.

 

[runmikeyrun]

you really shouldn't have any. But think about what your tubes endure sitting on top of your cab hours on end... somethings bound to come loose eventually!