Design Factors of the Bell Telephone Laboratories 1553 Triode

N D E V E L O P I N G microwave relay systems for frequencies around 4000 megacycles, one of the major problems is to provide an amplifier tube which will meet the requirements on gain, power output, and distortion over very wide bands. As the number of repeaters is increased to extend the relay to greater distances, the requirements on individual amplifiers for the system become increasingly severe. A tube developed for this service is the microwave triode B.T.L. 1553, the physical and electrical characteristics of which were briefly described in a previous article. 1 In the development of such a tube, both theoretical and experimental factors are involved; illustration of these factors in some detail is the purpose of the present paper. Given the application, a number of questions arise at the outset. What determines the tube type--why pick a triode for development, rather than a velocity variation tube, or perhaps a tetrode? What electrode spacings are necessary in such a tube, and what current must it draw? How is its performance rated, and how does it compare with other tubes? To what extent can the performance be estimated in advance? What experimental tests can give more precise information? Some answers to these questions were obtained by the use of figures of merit, which led up to the choice of a triode as most promising for development, and which also led to the subsequent method of optimizing the design for the particular system application of microwave amplifiers and modulators.