996 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



Table IV — Characteristics of Vacuum Tubes Studied 



435A and 436A 



Cathode current (h) 

 Grid-plate capacitance {Cg^^p) 

 Plate-cathode, screen grid cap. (Cp-kg^) 

 Grid-heater cap. (Cg^^h) 

 Heater-cathode, screen grid cap. (Ch-kg^) 

 Heater-plate cap. (Ch-p) 



437A 



Plate current (lb) 

 Plate resistance (Rp) 

 Heater-cathode cap. (Ch-k) 

 Heater-grid cap. {Ch_g) 

 Heater-plate cap. {Ch-p) 

 Grid-cathode cap. {Cg-k) 

 Plate-cathode cap. {Cp-k) 



tion are subject to the requirements of the Records method previously 

 described. 



In addition to the characteristics mentioned above, control charts 

 were maintained on a number of other electrical characteristics at final 

 test in order to determine process capabilities and to aid in setting final 

 test specification limits. Among the characteristics that were studied 

 were those given in Table IV. It was recognized at the beginning of 

 production of these vacuum tubes that control of the test characteristics 

 could not be achieved without similar control of the critical piece parts 

 and processes going into the assembly of the tubes. Accordingly, control 

 charts were started on the parts and processes, given in Table V, and 

 have been maintained throughout the manufacture of the product. 



Production of vacuum tubes represents a complex interlinking of many 

 separate processes, each of which is essentially a batch type of manu- 

 facture. These batches vary considerably in size depending on the process 

 or part involved. No part or process can usually be singled out as the 

 controlling effect which would isolate one group of tubes from another. 

 For example, a given lot of 100 tubes would probably be made from 

 cathode blanks taken from a supplier's production run of 5 to 10 thou- 

 sand parts. The cathode coating would be applied in batches of 50 to 

 200 parts. The control grid side rods used in making the grid frames 

 would have come from a production run of perhaps 5000 parts and the 



Table V — Parts and Processes on Which Controls Charts 



Were Used 



Cathode blank Outside diameter 



Coated cathode Outside diameter 



Control grid support rod Outside diameter 



Control grid wire diameter Unplated 



Gold thickness Control grid wire 



Control grid Minimum lateral resonance frequency 



Screen grid Minor axis, outside diameter 



Plate Inside diameter 



