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Microphonic Amps?

I have a valve amp (EL34 in ultralinear config).
I have read that valves are microphonic...what does this mean exactly. Will outboard vibrations be picked up (why?) and transmitted as sound deficiencies (again how?) ?
If this is the case, do i need to take equal care isolating my amp as much as I would, say, a turntable?

thanx for any help
gaffer74

Showing 2 responses by eldartford

eldartford
4,262 posts
 
All tubes are microphonic to some extent, but the only time that it becomes a real world problem is with high gain amplification stages, such as a phono preamp. Output tubes like EL34 should not be a problem, although the input stage, 12ax7 for example, might be. Usually avoiding the obvious (don't put the amp on top of a speaker) is sufficient. Various devices that mount on the tubes (dampers) are helpful.

As Gregadd mentions, microphonics may increase as the tube gets old.
eldartford
4,262 posts
 
Gaffer74...Think about it...a phono pickup is designed expressly to pick up small vibrations, whereas tubes are designed to minimize microphonics.

Turntable vibration control is essential although IMHO it calls for common sense more than expensive equipment. Amp isolation is something you can play around with when all the significant problems of your system are solved.
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