Beam-Power Tubes

Author: J.B. Hoag

This class of tubes is useful for the same purposes as the pentodes described in the preceding section. Actually they contain only four electrodes: a filament, a plate, and two grids, and hence should be properly classified as tetrodes. But, by proper design of the internal structures, as shown in Fig. 15 D, the electrons build up a potential which takes the place of the suppressor grid of the pentodes.

Fig. 15 D. A beam-power tube. (Courtesy R.C.A. Review)

The electrons en route to the plate are concentrated by beam action in the region between the screen grid and the plate, and serve to repel secondary electrons back into the plate, removing the undesirable kink (AB, Fig. 15 B) from the characteristic curve.

In the construction of the tubes, the screen-grid wires are placed in the " electrical shadow " of the control-grid wires. Beam-forming plates, connected to the cathode, are used to direct the electrons in two directions, as shown in Fig. 15 D. The screen and plate are widely separated so that there will be a large space charge in this region. The potential minimum which is thus created and serves to force secondary-electrons back into the plate is shown in Fig. 15 E.

Fig. 15 E. Potential distribution in a beam-power tube