500 BELL SYSTEM TECHNICAL JOURNAL 



complex, having several grids instead of one. A number of modifications 

 of klystron operation were considered, but all looked more complex me- 

 chanically and more speculative theoretically than a triode. 



In a triode there is also an upper limit to the transconductance that 

 can be achieved by spacing cathode and grid more closely. This limit 

 would be reached if the spacing were so close that the velocity produced 

 by the grid voltage were of the same order as the average thermal velocity 

 of cathode emission. The triode limit of some 11,000 micromhos per 

 milliampere is, however, many times greater than that for ordinary 

 klystrons. What is still more important is the fact that previous micro- 

 wave triodes were still a factor of twenty to twenty-five below this limit, 

 leaving considerable room for improvement. Thus, if mechanical methods 

 could be devised for decreasing the cathode-grid spacing and at the same 

 time maintaining parallelism between cathode and grid, it seemed highly 

 probable that great improvements would be available from a new triode. 



The choice to develop a triode for this application was therefore taken 

 not merely on the basis of simplicity, but also with the expectation that 

 performance improvements would be not only larger but also more cer- 

 tainly obtainable than by use of a modified klystron. Moreover, the 

 possibilities of using the triode over a wide frequency range in other 

 ways — as a low noise amplifier, modulator and oscillator — lent additional 

 weight to its choice. By translating the known requirements on gain, 

 bandwidth and power output into triode dimensions as discussed below, 

 it was found that the input spacings of existing commercial tubes would 

 have to be reduced by a factor of about five. In addition, cathode emis- 

 sion current densities would have to be increased about three times. A 

 design was evolved in which the required close spacings could be produced 

 to close tolerances by methods consistent with quantity production re- 

 quirements. The B.T.L. 1553 tube was the result (Fig. 1). Many of its design 

 features were adopted for use in the Western Electric 416A tube, which is 

 an outgrowth of this investigation. 



Description of B.T.L. 1553 Triode* 



The electrode spacings of this tube and of a 2C40 microwave triode are 

 shown in Fig. 2. In the 1553, the cathode-oxide coating is .0005" thick, the 

 cathode grid spacing is .0006", the grid wires are .0003" in diameter, 

 wound at 1000 turns per inch, and the plate-grid spacing is .012". It is 

 interesting to note that the whole ini)ut region of the 1553 including the 

 grid is well within the coating thickness of the older triode. 



The arrangement of the major active elements of the tube is shown in 



* This section is repeated from reference 1 for completeness. 



