The analysis of residual gas in electron tubes

A method has been devised for analysis of the residual gas content of small tetrodes or pentodes. In contrast to other methods which have been reported, the gas content of the tube being studied is not changed markedly by the analysis; analyses may be made repeatedly as the tube is operated, and moderately low pressures (1 × 10^-10mm Hg) and small quantities (10^-9cc mm) can be detected. A 90-degree sector magnetic analyzer with a 3-cm radius of curvature and 30-mil slits (resolution 1:20) is sealed to the tube to be analyzed prior to pumping. The analyzer is made of Corning 0120 glass with a conductive lining. The tube being tested serves as an ion source, avoiding the complications of a second hot filament. The device has been calibrated by comparison with known gases on a vacuum station. This method has been applied to study the gas content of the M1941 experimental pentode intended for applications which require extreme life and reliability. The normal gas pressure in this tube, after trolley pumping and 100 hours aging, is about 1 × 10^-9mm Hg, predominately He, H2, and CO. When no getters are used, the partial pressures of H2and CO are higher, perhaps 1 × 10^-8mm Hg. When titanium-alloy anodes are used in tubes without getters, the partial pressure of hydrogen is much higher, up to 1 × 10^-6mm Hg. A number of these tubes have been followed for over 10,000 hours. Concurrent studies of the cathode emission have shown it to be markedly influenced by small partial pressures of hydrogen when pure nickel cathode cores are used. Hydrogen is very effective in raising the emission, but the emission falls quickly as the hydrogen pressure is reduced. At least part of the improved cathode performance observed in tubes with titanium anodes must be attributed to the increased hydrogen pressure which accompanies the use of titanium anodes. These data on hydrogen with pure nickel should not be extrapolated arbitrarily to active cathodes.