632 



BELL SYSTEM TECHNICAL JOURNAL 



Zi 60 



(r 



o 



ui 



O 50 



> 40 



50 55 60 65 70 75 80 85 90 



SPUTTER NOISE LEVEL IN DECIBELS BELOW I VOLT 



Fig. 10 — Effect of filament loosenees on sputter noise. 



groups of tubes identical in every respect except that one group has 

 spring filament supports while the other has the older rigid supports. 

 In 80 per cent of the tubes, the improvement in the sputter noise is 

 from 20 to 25 db when the spring hook is used. 



The source of sputter noise most difficult to control in present day 

 tubes is insulation leaks. These are commonly due to very thin films 

 of conducting material which have been deposited on the surface of the 

 insulating members by sputtering or evaporation during the exhaust 

 or operation of the tube. Experience has shown the conductivity of 

 these films to be intrinsically unstable and discontinuously variable. 

 This condition alone can and does produce sputter noise, but to make 

 matters worse, the metal support wires of the tube are often in only 

 loose contact with the insulating parts and the conducting films cover- 

 ing them so that mechanical agitation breaks and makes the contact 

 and increases the intensity of the noise. The reduction of these insula- 

 tion leaks is largely a matter of choice of materials, of manufacturing 

 technique to reduce the evaporation of conducting material during 

 exhaust, and of mechanical design to shield important surfaces from 

 contamination during the normal operation of the tube. Great prog- 



