High-power valves: construction, testing, and operation

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-metal seals 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 1000 amperes are described; these support the whole cathode system, the evolution of which into a freehanging multiple construction is outlined. Heat transfer between anode and cooling liquid is discussed, and also forced-air 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 to variable dimensions. Methods are given for extrapolating low-power space-current readings into the operating region, with allowance for division between anode and grid, and the ?tail? of the anode-current 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 experien- ce 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 a 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.