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/

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.