18C) THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



6.8 



10,000 15,000 20,000 



LIFE IN HOURS AND YEARS 



30,000 



Fig. 13 — Behaviour of a group of 92 type 6P12 tubes over a period of three 

 years. 



In the early stages of the test there were eight mechanical failures. 

 The cause in all instances was identified and corrected in subsequent 

 production before the start of the T.A.T. project. 



Mechanical Characteristics 



The chief mechanical characteristics of the 6P12 have been mentioned 

 before in that they are, as explained, very similar to those of the 6P10. 

 A photograph of the interior of the tube is shown in Fig. 14. 



Tube Selection Techniques 



Not all the tubes, found after production to be potentially suitable for 

 the British T.A.T. amplifier, remained equally suitable after the life test 

 period of about 4,000 hours. A brief account of how the best were se- 

 lected is given below. 



The fact that every tube had to pass conventional static specification 

 limits needs little emphasis here. This test was, however, supplemented 

 by three additional types of specification. First, every tube was tested 

 in a functional circuit, simulating that stage of the amplifier for which 

 the tube was ultimately intended. Here measurements were made of shot 

 noise (appropriate to first stage usage) and harmonic generation (ap- 

 propriate to the output stage) in addition to the usual measurements of 

 transconductance, anode impedance and working point. 



