There are three ways the electron stream within the tube can be modulated. One is through the use of a magnetic field; this is demonstrated in the picture tube. The second method is electrostatic ; this is the most commonly used in controlling all vacuum tubes. The third method, seldom used but just as valid, is to change the positions of the elements while holding the voltages constant. This is what occurs when a tube responds to mechanical vibration and reproduces the waveform of the stimulating energy as an AC component in its plate current. This third method is also the cause of microphonics.
Since the moving element within the tube may be an entire structure (the cathode assembly) or only part of a structure (a grid side-rod or a single grid turn), the frequency at which the element vibrates may cover a very wide spectrum. It them becomes almost impossible to measure a specific tube in terms of its susceptibility to microphonics. Some effort has been made to analyze the spectrum response when a vacuum tube is stimulated by a sharp impact. Although there is a correlation between tubes that are very tight and those that have very little output at any audio frequency, the fact also remains that many tubes having relatively high outputs at certain specific frequencies do not appear to be microphonic. Apparently the mechanical stimulation they encounter does not fall into that portion of the spectrum.
There are certain limited ways of dealing with microphonics. All forms are affected by the amount of mechanical stimulation delivered to the tube elements. Therefore, any technique which lengthens or raises the impedance of the path by which the stimulation will be fed back to the tube will reduce its susceptibility to microphonics. Thus, sensitive sockets should be positioned as far from mechanical vibrators as possible. Some vibrators, such as speakers, convey some of their energy directly to the tube via the air space between them. In such instances, it is desirable not to place the socket where the maximum sound pressure can be developed against the tube envelope.
Most mechanical energy is fed to the tube elements by means of the common supporting member, such as the chassis. Standing waves are often present which effectively couple a high percentage of the energy directly to the tube elements. Two methods are very effective in breaking up the standing-wave pattern and hence in reducing the feedback loop. One of these is to add a weighted shield over the tube, thereby changing its vibration period and eliminating the mechanical resonant circuit. Another method is to isolate the socket from the chassis by using rubber grommets.
In a few cases, low microphonic tubes are available for some of the more critical applications. These tubes attempt to solve the problem through the use of tube structures that are known to be less susceptible to microphonics. These techniques cannot be applied to very many tubes because they will alter the performance of these tubes too greatly. Therefore, they are appropriate for only a limited number of tubes.
In general, the technique is to shorten the unsupported distance between the top and bottom micas as much as possible, thereby reducing bending of elements. Heavier micas are also used; they are sometimes forced into the envelope under tension in order to stiffen the entire mount. These tubes often are effective in reducing microphonism, although depending upon them to always solve the problem is not recommended because some of them are inferior to their prototypes or to other, more efficient tubes.
This excerpt is taken from ‘Getting the Most Out of Vacuum Tubes’ by Robert B. Tomer published in 1960.