Pentodes

Author: J.B. Hoag

A pentode contains five electrodes: a filament, a plate, and three grids, as shown in Fig. 15 C.

Fig. 15 C. A pentode. (From E. & N. P.)

This is constructed like the screen-grid tube, with the addition of a so-called suppressor grid located in the path of the electrons between the screen grid and the plate. The suppressor grid is fastened to the cathode and serves to repel or suppress secondary electrons, driving them back into the plate from which they were ejected by the primary electrons. As a result, the " negative resistance " region AB of Fig. 15 B is largely eliminated, the current rising more smoothly from zero up to its saturation value as the plate voltage is increased. The tube may then be used for greater power outputs for a given input grid voltage.

The plate resistance of various commercial pentodes in normal operation is fairly high, ranging from 22,000 to 2,000,000 ohms. The mutual conductance is high, ranging from 400 to 6,000 micro-mhos, and the amplification constant is also large, ranging from 70 to 1,500.

In some of the commercial pentodes the third grid is brought out of the tube socket instead of being connected inside of the tube to the filament or cathode. Many circuit combinations are then possible.