http://www.duncanamps.com/technical/microphonic.html

On Mon, 04 May 1998 09:44:57 -0800, czempel@ns.net (Chip Zempel) wrote:

>I was reading some of the amp reviews at Harmony Central, and saw several
>mentions of people having trouble with tubes that were (doing? exhibiting?
>experiencing? picking up?) microphonics. What is this? What what causes it
>and what are the symptoms? Is it fixed by replacing the tube?

Chip,

The tube is made up of a number of metal parts assembled together inside the glass envelope. If shocked mechanically, these metal parts can move, and in doing so will alter their capacitance and position such that a variation in voltage is produced.

Much the same way as a microphone. Mechanical impulses are turned into voltage. Trouble is, some tubes don't do it very much, and some do it a lot! The ones that do it a lot fit the description of "microphonic" you saw at Harmony Central. In extreme cases, you can get feedback from the amp.

If the tubes are reasonably easy to access on your amp, you could try tapping them (gently) with a pencil, and listen to them act as a microphone!

ps: not all the tubes will make the same amount of noise, a loud one may not be the most microphonic, it may just be placed where in a high gain stage where it is easier to pick up the dink-dink from the glass...

Best Regards,
Duncan

At 08:40 PM 5/5/98 +0100, Kevin wrote:
Great expanation. I just want to add a point to the original poster
since on occasion guys get freaked when any tube makes a noise when
tapped on. In some positions any tube you put in will make a noise if
you tap it. It's normal. True microphony will rear it's ugly head and
make itself known with the most obnoxious distortion you can imagine.
A smidge of microphony can (at least in the hi-fi world) add a sense of
"air". That's why some guys buy tube dampers and find it sounds better
without them. Sometimes.


http://www.stereophile.com/showarchives.cgi?351:3

Of the three pairs of 2A3s, the AVVT mesh plates were the most distinctive, visually and sonically. These big, macho tubes are considerably taller than the other 2A3s, and had by far the most dynamic presentation. It was an exciting sound, and the soundstage seemed even deeper than usual. In fact, I began to wonder if the sound of the recordings was being enhanced in some way, almost like one of those analog reverb devices used by studios before digital.
I'd heard rumors about AVVT 2A3 mesh plates being microphonic, so I did a bit of informal testing. Tapping the tube gently, or even tapping the amplifier chassis, resulted in a clear bong coming through the speakers. (The response to tapping the amplifier chassis was greater when the amplifiers were not supported by the Aurios.) With the other tubes (2A3 or 45), the same sort of tap produced hardly any sound.

Then, in an even more literal test of microphonics, I got within a few inches of the tube and sang at it, as if it were a microphone. I was going to have my wife listen to the speaker to check if my voice was being amplified, but I didn't have to: when I stopped singing, I could hear the echo of my voice coming from the speaker! The sound-pressure level produced by a person singing a few inches from the tube is undoubtedly much louder than the sound that would normally reach the tube from the system itself, but this level of microphonic response can't be good for unvarnished sonic fidelity.

I think the gist of the string here was TIM in solid state amps verses tube amps, and possibly using MLS analysis to evaluate it. That is not the same thing as micorphnonic tubes, although they can (like capacitors!) exhibit it. Bottom line, don't set your tube amp (or solid state one for that matter) on top of your speakers

Great post though!


[This message has been edited by soundhound (edited January 21, 2003).]

With the talk here about single ended triode (SET) amps, some of you may not be familiar with them. Even if you can't read schemetics, looking at the attached schematic of one channel of my SET amp, you can see the very straightforward and simple nature of the circuit compared to a typical solid state amplifier. This simplicity is one reason they sound so good. Notice that there is no global negative feedback whatsoever.