1114 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



tube (or amplifier) is replaced. Now, for a going system, with 120 tubes, 

 and assuming an abnormally short life of say 1200 hours, a tube will fail 

 every ten hours on the average unless preventative testing is used. Even if 

 very frequent testing be done in order to replace 90% of the potential 

 failures before they occur, one circuit interruption every 100 hours may 

 be expected. 



It is evident that a life many times greater than that assumed in this 

 illustration is imperative if reliable service is to be obtained and costly 

 maintenance avoided. Laboratory life tests predict that a tube life of at 

 least 15,000 hours may be expected in the L3 system. The actual results 

 will depend on the extent to which the operating conditions are closely 

 controlled, the severity of the field rejection limits, and the ability of the 

 tube factory to control the processing. 



2.7 Interchangeability 



The objective is to make the characteristics of the tubes sufficiently 

 uniform so that tubes may be replaced at will without circuit adjustments 

 being needed. In the L3 amplifiers, the circuits have been designed so that 

 a relatively wide range of characteristics can be accepted for individual 

 tubes. However, it is essential that the average characteristics be held in 

 close control from one manufacturing lot to another. This has been pro- 

 vided for by setting up distribution requirements which will be discussed 

 further in a later section. 



2.8 Cost 



As will be seen from the description which follows, it has been possible 

 to meet the L3 requirements with tube designs which do not require too 

 great departures from the manufacturing methods employed for conven- 

 tional telephone tubes. With a reasonable demand, it is accordingly ex- 

 pected that the tube costs should be moderate. 



3. Design Description and Characteristics 



3.1 Mechanical Description and Mechanical Problems 



Figure 8 shows the three L3 tubes along with some of the earlier high 

 figure of merit tubes. The W.E. 386A (left hand side) was designed to be 

 soldered directly into the circuits and had its input lead at the stem end 

 while the output lead came out through the top of the bulb. The flexible 

 leads used for soldering purposes, and the double-ended lead construction, 



