Tube Microphonics and Micro-vibrations
What causes tube microphonics?
Microphonic vibrations are most often caused by internal component electronics like transformers, relays, motors, and external vibrations like sound waves. These external vibrations create mechanical impulses inside vacuum tubes, inducing small parts to move, altering voltage and capacitance, resulting in amplified distortions. Internal tube vibrations are caused by alternating opposing charges between plate and filament, which cause those parts to vibrate.
Are certain tubes more susceptible to microphonics than others?
Virtually all vacuum tubes are susceptible to tube microphonics to one degree or another. Symptoms of tube microphonics can be fuzziness throughout the frequency spectrum, high-pitch "glare," blending of similar frequencies, acoustical feedback, mushiness in the soundstage, and false emphasis of certain frequencies, among other things. Properly handling tube microphonics/micro-vibrations helps virtually all tubes, allowing for a more faithful rendering of the sound that isn't distorted by the unintended effects of tube microphonics.
Small-signal preamp tubes are especially problematic, as any interference caused by tube vibrations at this stage will be amplified later on down the signal path.
How do I handle tube microphonics?
There are many products out there that supposedly help deal with tube microphonics, most of which come with some kind of sonic tradeoff such as false emphasis or attenuation of certain frequencies, “coloring” the musical signal. Some cause overheating of the tube, diminishing tube life, and still more degrade rapidly over time, becoming brittle and ineffective.
Herbie's vacuum tube damping instruments significantly reduce all forms of microphonic distortion, without the unwanted effects associated with many other types of tube dampers. A single UltraSonic SS or Rx damping instrument placed on each tube will significantly damp tube microphonics, allowing each tube to render the music more faithfully.
Will Herbie’s dampers make inexpensive tubes sound as good as rare NOS tubes?
Oftentimes, the main difference between premium-quality and ordinary tubes is their ability to deal with microphonics. By reducing the microphonics factor, Herbie's damping instruments allow a greater variety of tubes to compete on a more level playing field. Then again, inherently better tubes will usually sound even better with damping instruments. In general, the more a tube suffers from microphonic distortion, the more it will be improved with damping instruments. We have seen significant improvements in the performance of most every kind of tube, including some of the world's best NOS tubes like Amperex, Siemens, Mullard, Telefunken, and others. For specific examples, Siemens CCa tubes present an extremely linear, uncolored musical result, but tend to have a slight bit of microphonic "fuzz" on the dynamic edges; Herbie's damping instruments eliminate that artificial fuzz while maintaining the same uncolored integrity. Amperex white label 6922 PQ'a are amongst the finest tubes ever. With PQ's, Herbie's damping instruments reveal just a little more of the inner, intricate nuance present in the music by lifting a thin microphonic veil.
Tubes that are most revealing of subtle, sonic nuance are often equally sensitive to microphonic distortion. Damping instruments really bring out the best in these tubes. Different tubes each have their own unique character, so determining whether one sounds better than another is usually just a matter of personal preference. Almost all, however, do a better job with Herbie's damping instruments.
Which tubes do you recommend treating first, pre or output?
As to which tubes will benefit more than others, this will vary from one system to another, among components, and will differ among individual tubes. Whether damping pre or output tubes will achieve the greater benefit will vary from one system or component to another. If you "roll" or swap tubes much, you might have an idea which tubes or tube locations are most sensitive to subtle differences. Ideally, you'll have damping instruments on both pre and output tubes--if you damp just the preamp tubes, the output tubes will re-introduce microphonics and distortions, or if damping just the output tubes you'll be amplifying the distortion coming from the preamp tubes.
All in all though, with all things considered equally, damping the preamp tubes first usually makes the most sense. Oftentimes, damping just the preamp tubes achieves a degree of improvement sufficient to decide to just leave the power tubes alone (just the opposite is sometimes the case, as well).
Herbie's Tube Dampers:
UltraSonic SS: Recommended for all cool-running small-signal tubes.
UltraSonic Rx: Recommended for most rectifier and output tubes.
Guitar Amp UltraSonic: Recommended to minimize microphonics, oscillation, and tube rattle in severe vibrational environments as found in a combo guitar amp. Works superbly with home audio systems as well.
HAL-O III: Recommended for rectifier tubes and for all extremely hot-running tubes.