Editing Vacuum tube (section)

====Other failures====
Vacuum tubes may develop defects in operation that make an individual tube unsuitable in a given device, although it may perform satisfactorily in another application. ''[[Microphonics]]'' refers to internal vibrations of tube elements which modulate the tube's signal in an undesirable way; sound or vibration pick-up may affect the signals, or even cause uncontrolled howling if a feedback path (with greater than unity gain) develops between a microphonic tube and, for example, a loudspeaker. Leakage current between AC heaters and the cathode may couple into the circuit, or electrons emitted directly from the ends of the heater may also inject [[Mains hum|hum]] into the signal. Leakage current due to internal contamination may also inject noise.<ref>Tomer, R. B. (1960). [https://archive.org/details/Getting_the_Most_Out_of_Vacuum_Tubes_-_Tomer_1960 pp. 30–33]</ref> Some of these effects make tubes unsuitable for small-signal audio use, although unobjectionable for other purposes. Selecting the best of a batch of nominally identical tubes for critical applications can produce better results.

Tube pins can develop non-conducting or high resistance surface films due to heat or dirt. Pins can be cleaned to restore conductance.