994 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



electrical uniformity required by the end product. By means of special 

 circuits and testing techniques developed for electrically measuring the 

 characteristics of this transformer it has been found possible to indicate 

 microscopic variations in secondary mechanical dimensions not directly 

 under control. 



The problem of bringing this transformer under control is not com- 

 pletely solved. Progress to date in the analysis of the data obtained indi- 

 cates that the mechanical variations of the parts have complex inter-rela- 

 tions to the electrical requirements; however, manufacturing variations 

 in the parasitic impedances mentioned above are being controlled to an 

 order of magnitude better than on any transformer previously produced. 



3.6 VACUUM TUBES 



3.61 General 



Three new vacuum tubes, the 435A, 436A and 437A have been de- 

 veloped for use in the L3 amplifiers. These tubes were described in detail 

 in an article appearing in The Bell System Technical Journal of October, 

 1951.^ Two of them, the 435A and 436A are high transconductance 

 tetrodes and the third tube, the 437A is a high transconductance triode. 

 By applying the latest advances in design and in manufacturing tech- 

 niques to tubes of conventional basic type, substantially higher levels of 

 broadband amplifier performance have been realized. 



The key to improvement in broadband amplification lies in an increase 

 in figure of merit or transconductance to capacitance ratio. Figure of 

 merit is a direct measure of the bandwidth over which the required level 

 of amplification can be obtained. In general a given increase in figure of 

 merit can be directly reflected in a wider transmission band which will 

 provide more communication channels. 



The higher transconductances and higher figures of merit obtained 

 with the 435A, 436A and 437A over earlier broadband amplifying tubes 

 are a direct result of advances in the art of manufacturing fine pitch grids 

 of sufficient accuracy and rigidity to permit extremely close grid to 

 cathode spacing. The basic objective is to provide a grid which can be 

 placed very close to the cathode to act as a uniform potential plane con- 

 trolling the cathode current without offering any physical obstruction 

 to the passage of the current. This objective is approached by winding 

 the grid with many turns of very small diameter wire. 



The conventional method of grid manufacture consists of winding the 

 grid lateral wire in a spiral around two side rods, usually of nickel. A 

 groove is cut in the side rods at each point where the lateral crosses it 



