Some Characteristics and Applications of Four-Electrode Tubes

Four-electrode tubes may be classified by their designs and uses as "screen-grid" tubes, "space-charge-grid" tubes, and "double function" tubes. In the screen-grid tube the inner grid is the control electrode and the outer or screen-grid is kept at a fixed potential. The capacity between plate and control-grid is thereby reduced to an almost negligible value. A second result of the screen-grid is a large increase in amplification factor and plate resistance without reduction of mutual conductance. This permits high amplification in connection with high impedance coupling circuits, without undesired regeneration or oscillation. The screen-grid principle has been applied to transmitting tubes as well as receiving tubes. Characteristics of these tubes are given in detail. In the space-charge-grid tube the outer grid is the control electrode and the inner grid is maintained at a fixed potential. The purpose of the inner grid is to reduce the effect of the space-charge around the filament and thereby reduce the plate resistance of the tube. The space-charge-grid tube performs the same functions as ordinary three-electrode tubes, but in general has higher mutual conductance than a three-electrode tube of similar design. Several double function tubes and circuits are described in which both grids act as control-electrodes or in which one grid acts as control electrode and the other as a combination space-charge-grid and control or output electrode. These circuits are sometimes useful but are subject to certain definite limitations.