1218 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1953 



life of a tube is, therefore, terminated long before more apparent failures 

 occur in the tube structure. Other points of weakness are heater and 

 filament leads. 



As may be deduced from the above, microphonism is a frequent cause 

 of complaint when tubes are subjected to low but repetitive shocks or 

 transient vibrations. To illustrate the influence of such disturbances on 

 tube performances, the results of recent investigations of tube micro- 

 phonism found in a certain equipment will be cited. 



In this case, field reports indicated that certain tubes exhibited exces- 

 sive microphonism in the equipment, although the tubes were found to 

 be within limits when judged by standard factory tests. It was apparent, 

 therefore, that these tests did not simulate actual conditions. Accelera- 

 tion measurements made at the equipment base and at tube sockets 

 revealed that the steep, short duration impact of a blow delivered to 

 the outer case excited resonant vibrations of the chassis on which the 

 tubes were mounted. The magnitudes of these vibrations were only in 

 the order of O.lg, but their lowest frequency (approximately 550 cps) 

 was close to the mount resonances of some of the tubes. Further tests 

 also showed that those tubes having pronounced response, i.e., low damp- 

 ing, to vibratory motion in this frequency range also proved to be micro- 

 phonic in the equipment. (The vibration spectrum of one of the tubes is 

 reproduced in Fig. 9.) It was found that the various modes of vibration 

 of the tube mount produced the high peaks in the range between 500 

 and 1000 cps. Unfortunately, present factory tests do not include a 

 complete evaluation of tube response over a wide enough frequency 

 spectrum. 



Observations made on several equipments indicate that structural 

 changes in the chassis or re-positioning of tubes would, in some instances, 

 reduce the effect of mechanical disturbances on tubes. An occasional 

 source of trouble is introduced by equipment motor vibrations, especially 

 after mechanical wear has increased play in the moving parts. The ac- 

 celerations involved in these vibrations are usually very small, but if 

 their frequencies coincide with structural resonances in the tubes, unsatis- 

 factory operation of the equipment may result. The influence of such 

 disturbances on tube operation is not always recognized by equipment 

 designers. 



Several programs are currently pursued by both military and com- 

 mercial agencies to increase tube reliability. Since the necessary requi- 

 sites that make a tube reliable depend on the type of service to be per- 

 formed and environmental conditions, the requirements stressed in the 

 various programs differ in many respects. Some of these requirements 

 are still in a state of flux, as actual needs are as yet not clearly known. 



