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Technical and scientific discussion of amps, cables and other topics.

Idea for Prolonging Tube Life

My understanding of the way tubes wear out is that the few billion molecules of gas left in tubes get positively ionized by the bombardment of electrons passing from cathode to plate. A few of these massive (relative to an electron, anyway) objects make it past the protective cloud of electrons protecting the cathode and strike the cathode. These impacts physically abrade off the oxides that emit electrons. After a while (determined by evacuation of the tube, quality of the electron cloud, voltage the plate and grid are operated at, and probably other factors) enough of the oxide has been removed so that the emission of the tube is meaningfully decreased. Residual gas is the bad guy.

I've never seen an analysis of the gases left in tubes. My hypothesis is that since as a good approximation hydrogen leaks through anything, that at least a disproportionate amount of the gas in tubes is hydrogen that has leaked in through seals and even straight through the metal parts such as the leads. Helium is right behind hydrogen in leakiness, so there cold be a little helium in there too.

So the idea, at least for signal tubes, is to surround the glass envelope with a second glass envelope leaving a small gap between the two. A nipple would be fitted to the outer envelope and the signal tubes in a device would be connected to a a common continuously running vacuum pump. This should keep down leakage into the tube of light gasses by limiting the amount of gas in contact with the inner envelope. The vacuum pump wouldn't have to be very good to have a significant effect on tube life, if my conjecture is correct. Quietness of the pump would be important, though.

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Topic - Idea for Prolonging Tube Life - Lee of Omaha 08:37:05 09/01/11 (11)


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