236 BELL SYSTEM TECHNICAL JOURNAL 



the development of water cooled tubes, and show.ed promise of providing a 

 very convenient means for construction of resonant circuits in which a 

 part of the circuit was external to the vacuum envelope and part was internal. 



Numerous forms of focussed beam-disc seal velocity modulation tubes 

 were constructed for examination of the various factors affecting the per- 

 formance of such tubes. Sizes and shapes of gaps, length of drift space, 

 voltage and beam current, and other factors were adjusted to optimize the 

 performance. Both magnetic and electrostatic focussing of the beam were 

 explored. Triple gap tubes, from which gains of more than 30 db were ob- 

 tained, were experimented with. 



The end result of this work was the development of a four-stage amplifier 

 employing velocity modulation tubes of the disc-seal type (Fig. V-14) 

 designed especially for this application.* The electron gun is at the left, 

 and the collector is at the right. These tubes employ external circuits in 

 the form of resonant cavities assembled around the tube. The electron 



'Y"'^'' f ' 



Fig. V-14. — Disc seal velocity modulation tube. 



beams are accelerated by 1500 volts and focussed magnetically by means of 

 small alnico permanent magnets placed around the cavity structure as shown 

 in Fig. V-15. This shows a single stage of coaxial-coupled amplifier with 

 half of each cavity removed to show internal details. The cavities are 

 tunable over approximately a 250-megacycle range by means of metal screw 

 plugs located around their periphery. Two different sets of cavities are 

 sufficient to cover the entire frequency range of the radio repeater. 



A single amplifier stage of the type shown in Fig. V-15 will exhibit a 

 single-tuned type of gain characteristic which is only 5 megacycles wide at 

 the 3 db points when operated under matched input and output impedance 

 conditions. It was therefore necessary to widen the band by using double- 

 tuned circuits throughout. In fact such a design would be difficult to avoid 

 because each tube has its own resonant cavities. By running a short length 

 of waveguide or coaxial transmission line from the output cavity of one stage 

 to the input cavity of the next, a double-tuned structure results auto- 

 matically. The coupling from cavity to transmission line is made by means 



* These tubes were developed by A. L, Samuel and J. W. Clark. 



