THEORY OF MULT I- ELECT RODE VACUUM- TUBES 55 



on the combined effects of space charge, intensity, and distribution of 

 the field at the surface of the screen wires. 



Normal operating conditions for the screen-grid tube the character- 

 istics of which are shown in Fig. 2 are: Ep = 180 volts, Eg = 75 volts, 

 and Eg ^ — 1.5 volts. At this operating point P in Fig. 2, the plate 

 current is 5.5 ma and the plate resistance is 400,000 ohms. The 

 operating range is confined to the flat portion of the characteristics. If 

 the tube is operated with plate-voltage swings sufficiently large that the 

 instantaneous values of the plate potential extend into the region of 

 rapidly falling plate current, serious distortion of the output results. 

 This is a serious limitation in screen-grid tetrodes, because it requires 

 that the operating plate potential be much higher than the screen 

 potential. How this limitation may be removed by the introduction 

 of an additional electrode in the tube will be shown later. 



Curves showing the variation of plate resistance with plate voltage 

 are shown in Fig. 2. The plate resistance decreases with plate voltage, 

 falling off very rapidly as the plate voltage approaches the screen 

 voltage. The ordinates of the plate-resistance curves give a measure of 

 the flatness of the plate-current curves. That the latter are not as flat 

 as the total-space-current curves, thus resulting in values of plate 

 resistance approaching infinity, is attributed to two factors: secondary 

 electron emission from the screen, and an increasing ratio of plate 

 current to screen current with increasing plate voltage. The increasing 

 percentage of the primary space current drawn to the plate with 

 increasing plate voltage is an involved and undetermined function of 

 several factors including : the ratio of the openings in the screen grid to 

 the total conducting area subtended by it, the intensity and distribu- 

 tion of the field at the screen grid, the velocity and directional distribu- 

 tion of the electrons arriving at the screen grid, and space-charge effects 

 in its vicinity. 



Thus, there is the interesting situation in screen-grid tubes that, for 

 any given set of operating voltages, the magnitudes of the plate 

 resistance and amplification factor are determined largely by factors 

 not directly determined by the geometry and design of the tube. This 

 is quite different from triodes in which the plate resistance and 

 amplification factor both are determined directly by geometrical 

 dimensions and the arrangement of the electrodes. 



Obviously, the number of electrical parameters in vacuum tubes 

 increases rapidly with the number of electrodes. The curves of Fig. 2, 

 which correspond to the usual plate current-plate voltage characteris- 

 tics for a triode, were obtained with the screen maintained at a constant 

 potential of 75 volts as a fixed parameter. To obtain a complete 



