After having reviewed briefly the reliability of vacuum tubes on spacecraft, it is shown that the operation in space of a vacuum tube with an open structure is possible provided adequate measures are taken. The tube considered here is a multielectrode electron gun emitting electrons into space, in order to control the potential of a satellite. To avoid the failure of the most sensitive element of the gun, namely the impregnated tungsten cathode, the gun design includes such characteristics as a slow warming up of the heater element, a reactivation program used in case of cathode contamination, and a clean opening system which is not contaminant. Similarities with communication tubes are considered and the advantages of open tubes are mentioned.
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