What causes a tube to be microphonic?
Microphony is caused by instability or play in the internal structures of the tube. Since the various elements inside a tube have various voltages applied to them, and since (like any two pieces of metal which are in close proximity) there are various inter-element capacitances involved, when mechanical excitation is applied to the elements these capacitances change, causing a signal to be developed on the tube’s plate when, as a consequence of the changing capacitances, the voltages change. The more play there is in the internal structure, the more pronounced this effect will be. It is exaggerated in smaller tubes by the fact that the elements are in closer proximity and therefore smaller motions will cause larger signals to be developed. (This is why it’s more common to find microphonic preamp tubes than microphonic power tubes.) HOWEVER – and there is always a however – some of the best-sounding preamp tubes out there are microphonic. Microphony actually plays a positive role (when
