50 



BELL SYSTEM TECHNICAL JOURNAL 



surface produced by space charge. An appreciation of this fact is 

 essential to a clear understanding of the characteristics of multi- 

 electrode tubes. 



Screen-Grid Tetrodes 



Utilizing the simple theory of triodes, which has been shown to be 

 applicable to multi-electrode tubes also, the characteristics of a multi- 

 electrode tube will be analyzed next. For this purpose the screen-grid 

 tube is chosen, since it admirably illustrates the type of characteristics 

 found in several types of multi-electrode tubes. 



The objective in this case is to reduce the direct capacitance between 

 the plate and control grid through which energy is fed back from the 

 plate to the input circuit. This is accomplished by inserting an 



CATHODE 

 CONTROL GRID 

 SCREEN GRID 

 PLATE 

 OUTER SCREEN 



Fig. 1 — Schematic diagram showing the arrangement of electrodes in a screen-grid 



tetrode. 



electrostatic shield between the plate and control grid of what other- 

 wise would be a three-electrode tube. The condition that such a 

 screen must allow an electron stream to flow through it to the plate, is 

 satisfied by making it of fine mesh or in the form of a finely wound grid 

 structure. To be effecti\'e, it must be maintained at some constant 

 potential with respect to ground. 



The arrangement of the electrodes in such a tube is shown diagram- 

 matically in Fig. 1 . The screen structure outside the plate is added for 



