The system formed by the cathode, control grid and screen grid of a tetrode valve is considered from an electron-optical point of view. It is shown that the ratio of the screen current to the anode current, and its variation with grid bias, can be accounted for over the whole of the characteristic. The formulae developed yield the data required to obtain optimum performance under any given conditions. Attention is also given to the application of electron-optical considerations to the problem of noise in the so-called silent screen-grid valves, and also to an interesting filamentary-beam tetrode in which the screen current is kept low by the application of electron-optical principles different from those used in other aligned valves.
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