MICROPHONIC NOISE IN VACUUM TUBES 



627 



the cathode rigidly without conducting away large amounts of heat 

 along the supports. The diameter, length, and spacing of the grid 

 lateral wires are fixed within relatively narrow limits when the desired 

 values of amplification factor and internal impedance are fixed, pre- 

 cluding any important increase in stiffness here ; and where high mutual 

 conductance is desired, it is necessary to use relatively close spacings 

 between the elements, under which condition a given amplitude of 

 vibration produces a relatively large per cent change in spacing, and 

 therefore a high microphonic noise level. 



The Western Electric No. 264B Vacuum Tube is an example of 

 what has been done in working for a stiff, compact structure. The 

 plate support wires, which also support the whole top of the structure, 

 are short, straight, and as heavy as is practicable, and an extra wire 

 from the press braces the glass bead. One of the most important 

 features of this tube, however, is its filament. In most filament type 

 tubes the vibration of the filament is the chief source of microphonic 

 noise. In the No. 264B Tube, therefore, the filament is made com- 

 paratively short and heavy and is mounted in the form of a broad, 

 inverted V to whose apex considerable tension is applied by means of a 

 cantilever spring. The effectiveness of this treatment may be seen 

 from Table III which lists the mean noise levels for a number of types 

 of Western Electric small tubes, and the maximum and minimum 



TABLE III* 



* The microphonic properties of the No. 259A Tube given in this table are identical 

 with those of the 259B discussed by Pearson.^ 



