The Development of Electron Tubes for a New Coaxial 

 Transmission System 



By G. T. FORD and E. J. WALSH 



(Manuscript Received July 27, 1951) 



1. Introduction 



As THE demand for long distance telephone circuits has increased, 

 -^ ^ new transmission systems capable of handling more channels per con- 

 ductor have been developed. Also the advent of television has created a 

 demand for broad band channels for network facilities. One of the latest 

 developments now nearing completion is the L3 Coaxial System. 



Three new tubes have been developed specifically to meet the exacting 

 requirements of this system: two tetrodes, the W.E. 435 A and W.E. 436A, 

 and a triode, the W.E. 437A. All three types are used in the line and office 

 amplifiers. The new tubes make possible a substantially higher level of 

 broad band amplifier performance compared to their predecessors. They 

 represent the result of improvements made by applying well known basic 

 principles through new tube-making techniques. These techniques have 

 been developed largely within the framework of existing conventional tel- 

 ephone tube manufacturing methods. 



The development of special, small, low power vacuum tubes for high fre- 

 quency application in the Bell System began in 1934. The tube program 

 was instituted originally as part of a research project in the field of radio 

 communications. When the development of the LI Coaxial System began 

 it was recognized that similar tubes would be needed. Part of the tube 

 development effort was therefore directed toward the coaxial requirements. 

 Work on the W.E. 384A and W.E. 386A tubes used in the LI system was 

 completed in 1939 as an outgrowth of this program. 



The demand for amplification over wider frequency bands resulted in 

 further development work along the same lines. During World War II this 

 effort was applied to the development of the 6AK5 tube which became 

 available early in 1943 and was used widely in IF amplifiers in radar equip- 

 ment. Shortly after the war the W.E. 408A tube was developed for tele- 

 phone repeater uses. This is a long life version of the 6AK5 tube having 

 the same electrical characteristics except for the heater voltage and current. 

 The W.E. 404A tube appeared in telephone circuits in 1949. This tube, 

 having a higher figure of merit than the W.E. 408A, provided improved 

 performance in the IF amplifiers used in the New York to Boston radio 



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