Tube Discussion Microphonics?


I have read in quite a few places where someone complained of this tube or that being microphonic. I understand what is meant in a discussion of matching output parameters but I have heard people say that a pair, or quad, of tubes were matched but still microphonic.

What does it mean for a tube to microphonic? How does this translate into audible identification? I mean how do you tell by listening whether a tube is microphonic or is just not a good sounding tube or not good sounding with a particular component?
motdathird
The first few pages of the Cunningham artcle are hard to read but after that it gets easier. As discussed microphonics are caused by (unwanted)changes in the spacings of the internal structure (electrodes) of the tube. Usually by some vibration. The strength of the electric field between the tube's electrodes is closely dependant on the distance between them. If this distance changes (due to movement caused by vibratioin) it is like getting a voltage change. Various parts of the tube, depending on how they are mounted, may vibrate and have resonate frequencies that can cause trouble in the audio range. This is especially a problem with some bass. The bass that shakes your pant leg may be having the same effect on the tube if you put your amp in the wrong place.

Tapping a tube (generally not a good idea) can sometimes sound the same as if you tap a microphone. Ive seen tubes that will pick up speech..that is you can talk into them and hear yourself. Evans says that even matched pairs can vary in this regard and need to be tested separately for M too. Some parts of the tube can move by internal force (the filament of directly heated tubes).

* J. Evans: Valve Microphonics: http://www.thevalvepage.com/valvetek/microph/microph.htm

* T.M. Cunningham: Practical Consideration in the design of low Microphonic Tubes http://www.triodeel.com/micro1.html

Cheers
I remain,
A little microphonics can be a good thing. Older vintage tubes are more likely to be a little microphonic. But these are also the ones audiophiles will pay a premium for. Is there a connection?

A little microphonics can add some "air" to the sound, or slightly emphasize the highs, which might be a good thing in your system? Trial and error as usual.

A tube with absolutely zero microphonics will very likely sound kind of dull in many systems. This is true of many new modern tubes.
Generally speaking, I can't agree with anything Sugarbrie said, but hey, that's audio.

Microphonics is not frequency dependant and it is the last way you want to adjust highs in a tube amp.

Tube amps without micrphonic tubes sound very good and generally better than those with them. If your tube amp sounds "dull" the last thing you want to look for is a microphonic tube to add to the mix. Change amps and use a different tube, change or tweak the circuit, make a better match with your speakers, blah, blah, blah... but I would not suggest shopping for microphonic tubes.

Sincerely,
I remain
Sugarbrie...OK...If fate gives you lemons, make lemonaid! Now please tell me why turntable rumble is a good thing.
I'm with the Clueless one here. Yikes, talk about an unpredictable tone control/ reverb pedal! Microphonics be bad, very bad!