A.C. Plate Voltage and D.C. Grid Voltage

Author: J.B. Hoag

Fig. 19 C. Applying alternating potentials to the grid and plate of a gas-filled tube

We shall next examine some of the interesting possibilities of applying alternating voltages to both the grid and plate circuits of gas-filled tubes, as in Fig 19 C.

Fig. 19 D. A thyratron operated with a.c. on the plate and d.c. on the grid. The plate voltage is ep, the grid voltage is eg, the striking potential is dotted

Figure 19 D shows what will happen when the plate voltage alternates and a steady grid voltage is progressively raised from a negative value, well below the striking-potential, to a value above that potential. At first, as at the top of the figure, there is no current flow in the plate circuit at any time. When, however, the grid voltage is equal to the maximum striking-voltage (bottom of the dotted curve), for the peak value of the plate voltage, then the plate current is turned on and continues to flow for the remainder of the positive half-cycle. In other words, the plate current flows during one-quarter of each cycle, as indicated by the shaded areas. The strength of this plate current is proportional to the plate voltage.

If the d.c. voltage on the grid is made still less negative, then the plate voltage will be turned on at an earlier time in the positive half-cycle and will continue to flow, in an amount proportional to the plate voltage, until that voltage has become zero. For zero grid voltage the plate current flows during the entire positive half-cycle, i.e., for one-half the total time. Thus, by a slight change in the grid voltage, it is possible to vary the plate current from zero up to a value equal to the average of a half-wave rectified current.