Abstract :
Any engineer who has been faced with designing a piece of electronic equipment for industrial or military applications, where reliability of operation is of paramount importance, is probably familiar with the common failings of its essential element, the vacuum tube. Standard receiving types are made by the millions for radio and television and are expected to be replaced when they fail in the normal course of events. This same failure, however, in industrial applications may have serious consequences, either of a financial nature or even to the extent of endangering personnel. In the case of the commercial airlines, it became evident several years ago that even with all possible preventive maintenance, the percentage of unpredictable failures occurring in flight could not be tolerated. This was increasingly true as more electronic gear was added, such as glide-slope receivers, omni-range directional indicators, and automatic pilots, as well as the vital communication equipment.
