A Power Amplifier for Ultra-High Frequencies * 



By A. L. SAMUEL and N. E. SOWERS 



A consideration of the special problems encountered at ultra- 

 high frequencies has led to the design of a push-pull power pentode, 

 useful as an amplifier, frequency multiplier, and modulator at fre- 

 quencies of 300 megacycles per second and below. Unusual con- 

 struction features include the mounting of two pentodes in the 

 same envelope with interconnected screen and suppressor grids, 

 complete shielding between the input and output circuits with no 

 common leads, and provision for cooling all grids while maintaining 

 extremely small inter-electrode spacings. The electrical char- 

 acteristics depart from the conventional mainly in the low value 

 of lead inductances and the high value of the grid input resistance 

 at ultra-high frequencies. 



The second part of the paper describes a single stage amplifier 

 unit built for testing the tube at frequencies between eighty and 

 300 megacycles, and the associated apparatus for measuring input 

 impedance, gain, and harmonic distortion. The results given 

 indicate that by using this new tube it is possible to construct stable 

 amplifiers at ultra-high frequencies up to 300 megacycles, having 

 gains of twelve to twenty-five decibels per stage and delivering 

 several watts of useful power. Stability and distortion compare 

 favorably with those obtained from conventional tubes at much 

 lower frequencies. 



PART I— THE VACUUM TUBE 

 By A. L. SAMUEL 



WE ARE witnessing a rapid expansion and extension in the use 

 of radio communication. A corresponding extension in the 

 usable portion of the radio-frequency spectrum is highly desirable. 

 With this in mind, special forms of vacuum tubes have already been 

 developed for use as oscillators at frequencies above 100 megacycles.^- ^ 

 Except at low power levels,^ amplifier tubes have not been available. 



* Presented at Institute of Radio Engineers meeting, New York City, October 7, 

 1936. Published in Proceedings I.R.E., November, 1936. 



1 M. J. Kelly and A. L. Samuel, " Vacuum Tubes as High-Frequency Oscillators," 

 Elec. Eng., vol. 53, pp. 1504-1517, November, 1934; Bell Sys. Tech. Jour., vol. 14, 

 pp. 97-134, January, 1935. 



2 C. E. Fay and A. L. Samuel, "Vacuum Tubes for Generating Frequencies Above 

 One Hundred Megacycles," Proc. I.R.E., vol. 23, pp. 199-212, March, 1935. 



' B. J. Thompson and G. M. Rose, "Vacuum Tubes of Small Dimensions for Use 

 at Extremely High Frequencies," Proc. I.R.E., vol. 21, pp. 1707-1721, December, 

 1933. 



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