All vacuum tubes exhibit microphony to some extent, the generation of spurious sounds in response to mainly airborne vibration. This is usually low in level but mixes with the music signal to add subtle, pervasive “grundge”, muddiness and harshness. These effects worsen as the sound pressure level increases. Of course, all electronic components have this general problem to some extent, where platforms, special feet, damping and weighing down the chassis are the curative measures. With tube microphonics, the active devices themselves, the vacuum tubes, need to be separately damped.

Many audiophile accessory tube damping products are being marketed, but most are a kind of negative tradeoff where there is some improvement in apparent clarity, but longer term listening reveals that there is also an unacceptable loss of life and transparency of the overall sound.

Other negative effects of various commercial tube damping devices can include a metallic “ringing” quality and loss of dynamics. The most likely theoretical explanation is that the high frequency resonances of the tubes glass envelope are indeed being damped and other vibrations are being reduced, but the fields surrounding the tube's plate, grid and cathode are also being affected. Introduction of material with poor dielectric properties or metals with remnant magnetic fields and inherent eddy current effects are indirectly affecting the electron flow and therefore the music signal.

We have found only two devices that have almost entirely positive sonic effects in reducing tube microphonics. These are special types of “o”-ring (FEP Teflon-coated silicone) which can be obtained in the right size to fit onto miniature (12 AX-type) tubes, and plain silicone o-rings which much improve other types such as 2A3, 300B and 6SNT. Though certainly not designed for this purpose, these o-rings make amazingly effective damping devices.

Next best, but introducing noticeable muddiness and loss of midrange information, are the 3M elastomeric damping rings.

Sources: FEP Teflon coated silicone o-rings, size 116, fits miniature (12AX7-size) tubes, silicone o-rings, size 133, fits 2A3 an 300B types: McMaster-Carr (562) 692-5911, catalog numbers 9319K26 (#116), 9396K154 (#133); 3M Tube damping rings: Michael Percy, 530-470-8650 (www.percyaudio.com).