646 BELL SYSTEM TECHNICAL JOURNAL 



With the tube under test operating at zero grid resistance, the attenu- 

 ator is adjusted to give a convenient deflection of the microammeter 

 (due to noise in the tube under test). Grid resistance is now added 

 until this deflection is exactly doubled, thus making En"^ equal to Er^. 

 This value of input resistance, designated by Rq, is a measure of the 

 inherent noise of the tube. Substituting Rg in equation (13) the 

 tube noise is calculated from the relation 



F/ = A^kTRo = 1.64 X IQ-^'Rg volt^, (14) 



where Rg is expressed in ohms and T is 300° K. (approximate room 

 temperature). 



Noise in Certain Vacuum Tubes ^' 



Quantitative measurements of tube noise were made on four different 

 types of standard Western Electric vacuum tubes, namely: Nos. 102G, 

 264B, 262A and 259B. These tubes have as low a noise as any tube 

 obtainable at the present time. 



In order to obtain the best signal to noise ratios it was found that 

 operating conditions different from those normally recommended must 

 be used. In general, the cathode must be operated at as high a 

 temperature as possible without impairing the life of the tube, the 

 negative bias of the control grid must be reduced to as near zero as 

 possible without causing excessive grid current, and the plate voltage 

 must be reduced below the value normally recommended. In all 

 the measurements described here the tube under test was coupled to 

 the first amplifier unit through a 50,000-ohm load resistance. It was 

 found that the signal-to-noise ratio could be improved a fraction of a 

 db by increasing the load resistance (in accordance with equation (4)) ; 

 this, however, necessitated a large plate voltage which was incon- 

 venient. Six tubes of each type were tested and the noise data given 

 below were obtained by averaging the six measurements for each 

 type. Individual tubes may differ from these average values by as 

 much as ± 1 db. 



No. 102G Tube 



This is a three-element, filament-type tube. Its long life, exception- 

 ally high stability of operation, and good temperature saturation 

 make it a desirable tube to use in the input stage of certain high-gain 

 amplifiers. This tube also has a comparatively small microphonic 

 response to mechanical and acoustical shock although it is not as 

 good as the No. 262A and the No. 264B tubes in this respect. '^^ 



" Noise in other types of vacuum tubes has been reported by G. F. Metcalf and 

 T. M. Dickinson, Physics, 3, 11 (1932); E. A. Johnson and C. Neitzert, Rev. Sc. Inst., 

 5, 196 (1934); E. B. Moullin and H. D. M. ElHs, /. E. E. Jour., 74, 323 (1934); W. 

 Brentzinger and H. Viehmann, Arch. f. Hochfr. und Elektroauk, 39, 199 (1932). 



'■' The microphonic response of several types of Western Electric vacuum tubes to 

 mechanical agitation is reported by D. B. Penick in this issue of the Bell Sys. Tech. Jour. 



