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
Microphonic means that the device is susceptible to converting the electro-acoustic vibrations that it picks up into an electrical signal and introducing that signal into the system as a form of distortion. The initial acoustic based vibrations that it picks up are much like a "microphone" converting acoustic energy into electrical energy, hence the name of the term.

As a side note, most all devices ( including cabling ) is microphonic to one extent or another. Some devices, such as tubes, are more susceptible to such problems. Having a microphonic device in a circuit with high gain, let alone being centered in the device itself that generates that signal gain, can be quite problematic. This can make the situation far more noticeable than if the source of microphony were in a less conspicuous location in a lower gain part of the circuit. Sean
Sean, great answer, as always! There are even some pre-amps that will produce an audible "thunk" through the speakers when switches are thrown. Also, the louder you play your music, the less isolation that you provide for your preamp, the higher the gain of your tube circuit, then the more problems that you will have with good tubes being microphonic.

There are some tubes (small signal tubes, usually NOT power outputs like the EL-34's) that are susceptible to being microphonic. There are some rather unscrupulous dealers [no names!] who will not check for badly microphonic tubes, which will test out just fine on their tube testers. I highly recommend Andy at Vintage Tube Services...he's the best, and has helped me to undo the mistakes of another well known tube guy! http://www.vintagetubeservices.com/
Hi Gaffer,

A tube that is exhibiting a microphonic condition adds an unwanted contribution to the signal flowing through the component. The microphonic signal added to the original signal takes us further away from faithfully reproducing the instrument that has been captured in the recording.

A tubed component that is subjected to floor-borne vibration that is entering through the equipment support can cause tube microphonics. In addition, air-borne vibration that is striking the component's chassis as well as internally-generated vibration (transformers) may also be contributing. Isolating the component from the floor-borne vibration and restricting the other two forms of vibration as much as possible will minimize the problem of microphonics.

Some vibration control products allow the component to resonate freely or do not restrict the air-borne or internally generated vibration from affecting the component. This allows the signal to become contaminated. Once that occurs it will be impossible to restore the signal to its original, unaltered condition.

Best Regards,

Barry Kohan

Disclaimer: I am a manufacturer of vibration control products.
Thanx for that everyone (nicely explained :-) )
In that case, do I take as much care as with turntable placement....would an anti-resonance base help or would tube rings be a better bet re: next purchase?

PS, my amp is integrated, with the pre-amp section being 4 small GEC valves, and the power amp section being 4 large Svetlana EL34 valves:
Well, you'll get no answer to your question that everyone will agree on. For small tubes in my amps I have found it easiest, and most effective, to buy tested low noise tubes in the first place on which I place those inexpensive rubber damping rings. Then I try to reduce exposure from floorborne vibrations by using a combination (sandwich) of materiels with different resonance frequencies to absorb the vibrations (a DIY project). Some times I use cones (or not) to couple the component depending on the effect it makes. I do not try to couple the rack itself to the floor (except for stability) as I feel the combo of materiels used under the component are sufficient, besides I have wood floors and I think coupling might just enhance floorborne vibrations effects on the components. JMHO. Good luck getting any consensus.