Microphonic Amps?

tubes can become microphonic. usually means it is defective and should be replaced. good tubes should not be microphonic.

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.

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
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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/