The Design of High Power Vacuum Tubes for Industrial Heating Applications

Analysis of tube failures from industrial application has pointed up three principal sources of failure: (1) copper-glass seals are not strong enough to withstand handling by industrial maintenance people; (2) filament structures of the spring-tensioned type are not too reliable; (3) thin-walled copper anodes will not stand occasional overloads. Tubes have been designed which greatly increase tube life by eliminating failures of the above types. The use of thick kovar metal in glass seals will withstand about ten times the torsion of the conventional copper seals commonly used in transmitter tubes. A self-supporting filament structure not only eliminates the failures caused by filaments sticking in guides and annealing of the spring material, but maintains uniform tube characteristics throughout life. Data on a type ML-5658 20-kw tube shows less than 2 per cent change in static data after 10,000 hours of life. The use of copper anodes from four to eight times thicker than customary permits nearly 100 per cent increase in plate dissipation ratings without additional water flow. For use at normal plate dissipation, the anode temperature runs lower and scaling of the anode, with resultant anode puncture, is eliminated. Failure of tubes due to excessive cathode temperature cannot be reduced since evaporation of the cathode material is a physical reality which cannot be compromised. Instead of running a tube above ratings, a larger tube with greater cathode area must be used.