High-power valves: construction, testing, and operation

Abstract
In the Introduction the authors discuss how the development of high-power valves is influenced by the standard of reliability demanded by such services as broadcasting.The principles governing the constructional design of each main part of the valve are outlined; differential expansion frequently appears as a controlling factor.The glass-work is of lead-potash-soda glass, the anodes of thick copper tubing; for glass-to-metalseals nickel-iron alloy is preferred, but copper is sometimes used.Insulating members have been eliminated from the active part of the valve. Grids are not cross-braced, grid seals include a large ring section let into the bulb, and there is an 8-lead multiple seal for pentodes.Cathode seals for 1 000 amperes are described; these support the whole cathode system, the evolution of which into a freehanging multiple construction is outlined. Heat transferbetween anode and cooling liquid is discussed, and also forcedair cooling.Modern evacuation technique proves to be governed by two effects; the lower readings of grid current in the “gas test” are shown to be due to photo-electric electron emission caused by X-rays from the anode and not directly to gas, and the clean-up capability of the valve is found to be very large.The method is described for determining the operating filament voltage for a standard emission by extrapolation to full emission from a reduced-emission test. Examination of the statistics of emission-test data shows that former variations are to be ascribed to variable thermal emissivity and not variable dimensions.Methods are given for extrapolating low-power spacecurrent readings into the operating region, with allowance for division between anode and grid, and the “tail” of the anodecurrent characteristic is discussed. The control of the secondary-emission component of the grid current and the effect of the magnetic field of the filament are described.An account is given of later experiments and of recent experience with flash-arc breakdowns (Rocky Point effect).Recommendations are made for switching-on filaments and anode potential in operation, and for purity of cooling water.Curves are shown of the distribution of evaporation wastage in various types of filament. Typical valve-life data from normal operation are illustrated by survivor curves for constant voltage and for constant emission during life, and Table is given showing the recent performance of the largest valves at three stations.A table of ratings for the various types of valve covered by the paper is also included.

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