1104 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



relay system and in the TD2 radio relay system. The W.E. 435A, W.E. 

 436A and W.E. 437A tubes are the latest types to come out of this long 

 range program. 



It will be seen in what follows that the key to continued development 

 along these lines has been improvements in the techniques of grid making 

 to meet the basic objective of providing a grid which can be spaced very 

 close to the cathode and which, in effect, acts as a uniform potential plane 

 controlling the current drawn from the cathode without offering any physi- 

 cal obstruction. This objective is approached by using many turns of very 

 small diameter wire for the grid winding. The reason for the close grid- 

 cathode spacing is that the transconductance or sensitivity depends on this 

 factor. Although the increase in input capacitance which results is a dis- 

 advantage because of its effects on the interstage circuits, this disadvantage 

 is more than compensated for by the higher transconductance obtained. 



2. Principles of Design 



2.1 Requirements 



The overall requirements for the L3 system, and the manner in which 

 they are related to the tube parameters, are very complex. However, in 

 its simplest terms, the objective for the L3 system is to provide on one 

 coaxial p'pe a facility suitable for the simultaneous transmission over a 

 40(X)-mile circuit of a television signal and 600 one-way telephone channels 

 or, alternatively, 1800 one-way telephone channels when no television 

 channel is required. The transmission band being provided is from approxi- 

 mately 0.3 MC to approximately 8 MC. The amplifier needed to compen- 

 sate for the cable attenuation must meet very exacting requirements with 

 respect to gain-frequency characteristics, stability, noise, and linearity. 



The design features necessary to provide suitable electron tubes for use 

 in the L3 amplifiers are closely related to the requirements mentioned above 

 for the amplifiers. In general terms, the tube design objectives are: (1) 

 high transconductance-capacitance ratio (figure of merit), (2) minimum 

 excess phase shift or phase delay, (3) low noise, (4) well controlled modu- 

 lation, (5) long life, (6) interchangeability, and (7) lowest cost consistent 

 with the first six objectives. In the material which follows, each of these 

 objectives will be discussed in detail and its relationship to the system 

 objectives brought out. 



2.2 Figure of Merit 



Figure of merit is of particular importance. It is a direct measure of the 

 bandwidth over which the required amplification can be obtained. In gen- 



