The Measurement and Reduction of Microphonic Noise in Vacuum Tubes

T h e microphonic response uf different t y p e s of vacuum tulx*s to t h e s a m e mechanical a g i t a t i o n c o v e n a 70 db range of levels. T u b e s of t h e s a m e typo, on t h e a v e r a g e , cover a range of a b o u t 30 d b . T h e s e response levels a r c too sensitive to m i n u t e v a r i a t i o n s in testing conditions to be measurable with a n y great precision, b u t values which a r e reproducible to within 5 db a r e obtainable with a l a b o r a t o r y test set comprising a v i b r a t i n g h a m m e r a g i t a t o r , a calibrated amplifier, a n d a t h e r m o c o u p l e g a l v a n o m e t e r indicator. S p u t t e r noise is made measurable by f r e q u e n c y discrimination method*. M i n i m u m microphonic d i s t u r b a n c e under given service conditions is a t t a i n e d by using the lew microphonic t y p e s of t u b e s which are available, by selecting t h e q u i e t e r t u b e s of a given t y p e for use in positions sensitive to mechanical disturbance, a n d b y protecting t h e t u b e s from mechanical a n d acoustic vibration. Examples of quiet triodes a r e t h e Western Electric N o . 264 B (filament) a n d N o . 262A (indirectly h e a t e d cathodc). Indircctly h e a t e d c a t h o d e t y p e t u b e s a r e intrinsically less microphonic t h a n f i l a m e n t a r y types. F u r t h e r microphonic i m p r o v e m e n t in t h e t u b e s themselves is m a d e difficult by r e q u i r e m e n t s for favorable electrical characteristics.