6FM7 tubes use a size "30" damping instrument. I recommend an UltraSonic Rx-30 for each of these tubes which run fairly hot.
Inducing a microphonic response by tapping on a tube of itself does not indicate any problem with the tube and is just a natural response, just as if you tap on a capacitor or other sensitive electronic part. There's no reason to tap on a tube. Tube microphonics occur in the music as fuzziness, glare, linearity and tonal irregularities, loss of imaging focus, and other anomalies. Distortions may be extremely subtle or quite profound but have little or no relation to what you get by tapping on the tube.
Input/driver, power and rectifier tubes all have generally equivalent susceptibility to microphonics. Degree of microphonics will vary from one tube to another and from one component to another. Small-signal, and especially tubes in a phono stage, amplify smaller signals so oftentimes appear to be more microphonic as they are amplifying proportionally more of the low-level and electronic "noise floor" information. With some components, however, damping the power tubes achieves the most significant improvement, so there's no one-size-fits-all rule that fits every audio system. Damping the rectifier tubes is often the most significant improvement made with an amplifier because the rectifier tube typically affects all the other tubes in the circuit--if you have a fuzzy rectified current, you'll have fuzzy amplified music along with other distortions.
Though all vacuum tubes are susceptible to some degree or another to microphonics and micro-vibrational distortion, it usually makes sense to start out with damping just the source tubes and amplifier input/driver tubes if not going all-out to properly and adequately address all the tubes.
Steve Herbelin
Herbie's Audio Lab
