92 BELL SYSTEM TECHNICAL JOURNAL 



For the calculation of tul)i' noise several formulas have been pro- 

 posed, either entirely empirical - or with some basis on theory.'-'- -*• -'' 

 These formulas sent-rally fail in the prediction of noise in tubes for 

 the reason that the greater part of the noise in practical tubes is caused 

 by things that ha\e not been included in theory and that are still in a 

 state of flux so far as manufacturing is concerned. It is best, there- 

 fore, to rely only on actual measurements of the noise in specific types 

 of tubes. With modern tubes, the noise level of a given type of tube 

 can be represented reasonably well by measurements made on a small 

 number of samples. 



Published data on noise of tubes are rather meager. The best 

 series of measurements is that of Pearson,'" which covers four Western 

 Electric tubes at different frequency bands. These tubes are known 

 commercially as types 102G, 262^, 2645, and 2595. The General 

 Electric tube type P/-11, designed specially for work at low fre- 

 quencies, was studied by Metcalf and Dickinson.-* They also give 

 data, for the low frequency region, on the tubes known commercially 

 as types 222, 240, 201, and 112. Johnson and Neitzert ^^ have given 

 data for the PJ-\\ and the type '38 tube. Certain British tubes were 

 studied by Moullin and Ellis,^^ and of these the type AC/IHL tube 

 was found to have the lowest noise level. Brintzinger and Viehmann ^' 

 studied a few German tubes. Of these the type RE-084l appears to 

 have the lowest noise rating, but the data cannot be reduced to absolute 

 measure. 



In many of these studies the tubes were operated at voltages different 

 from those usually employed. For these, the original papers should 

 be consulted. In general for the best triodes Rg has a value of a 

 few thousand ohms, while for screen tubes it has a value of a few 

 tens of thousands. At the lowest voice frequencies the values may be 

 somewhat greater. 



Other Sources of Noise 



While the more fundamental sources of noise have been discussed, 

 it may be well to add some remarks on a few types of disturbance that 

 often can be eliminated. 



Noise From A-C Cathode Heating "'^~^^ 

 The indirectly heated cathode may be operated on alternating 

 current when the tube is employed in radio frequency circuits. In 

 audio amplifiers with gains in e.xcess of 50 db, additional precautions 

 must be taken to reduce the effects of the electric and magnetic fields 

 of the heater and of coupling impedance between the heater and the 

 other electrodes. Even under the best conditions, however, the hum 



