The measurement and reduction of microphonic noise in vacuum tubes

The microphonic response of different types of vacuum tubes to the same mechanical agitation covers a 70 db range of levels. Tubes of the same type, on the average, cover a range of about 30 db. These response levels are too sensitive to minute variations in testing conditions to be measurable with any great precision, but values which are reproducible to within 5 db are obtainable with a laboratory test set comprising a vibrating hammer agitator, a calibrated amplifier, and a thermocouple galvanometer indicator. Sputter noise is made measurable by frequency discrimination methods. Minimum microphonic disturbance under given service conditions is attained by using the less microphonic types of tubes which are available, by selecting the quieter tubes of a given type for use in positions sensitive to mechanical disturbance, and by protecting the tubes from mechanical and acoustic vibration. Examples of quiet triodes are the Western Electric No. 264B (filament) and No. 262A (indirectly heated cathode). Indirectly heated cathode type tubes are intrinsically less microphonic than filamentary types. Further microphonic improvement in the tubes themselves is made difficult by requirements for favorable electrical characteristics. Well designed cushion sockets can reduce microphonic levels by as much as 30 db, and other methods of cushioning, more expensive and less compact, can extend the reduction even farther. Sputter noise can be eliminated almost entirely in most types of tubes by commonly applied design features and manufacturing methods.