1220 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1953 



many of these machines in terms of acceleration and frequency content, 

 etc. The response of tube structures may be vastly influenced by shocks 

 of the same nominal acceleration but differing acceleration wave forms. 

 Shock tests are also performed on tubes during their initial development 

 to obtain the degree of cushioning required for safe shipping and handling 

 purposes. 



(h) Vibration tests. These are low acceleration, fixed frequency tests. 

 They are made on machines with sinusoidal displacement output. Short 

 time duration 25-cycle — 2.5g, or 50-cycle — lOg tests are specified for 

 tubes that are generally not exposed to constant vibrations. Usually no 

 voltages are applied to the tubes under test since their prime purpose is 

 to check for sound tube structures. Electrical post vibration perform- 

 ances are the criteria of tube quality. In this category also fall the long 

 time duration vibration tests made on tubes intended for use in equip- 

 ment that is kno^vn to be subjected to continual vibration, such as 

 shipboard or mobile equipment. Present specifications call for 96-hour 

 tests at 2.5g — 25 cycles. This, therefore, is a fatigue test which deter- 

 mines the capability of tube structures, under prolonged cyclic stresses. 



(c) Microphonic vibration test. A 25-cycle — ^ 2.5 g test performed with 

 specified voltages on the tubes under test to investigate the influence of 

 low acceleration vibration on the output of tubes. This test is specified 

 on certain tubes, especially those used for audio applications. The per- 

 missible magnitude of spurious signals excited by vibration is limited on 

 the respective tube specifications. 



(d) Tap tests. These tests are performed for two purposes. One is for 

 the detection of defects such as foreign particles between close spaced 

 adjacent elements, damaged elements, or poor welds. Open and short 

 testers of given sensitivity are used to indicate these defects. The second 

 purpose for tap testing is the investigation of microphonic response of 

 tubes to mechanical disturbances. For these tests the tubes are made to 

 work in Class A amplifier circuits. Acoustic feedback may be used, so 

 that the tube is not only subjected to the mechanical tap, but also to 

 the sound of the tap excited microphonic signal which is reproduced 

 through a loud speaker spaced at a given distance from the tube. Sus- 

 tained microphonism can be produced by these means under certain 

 conditions. The purpose of this type of test is to simulate conditions to 

 which tubes are subjected in some equipments, especially those closely 

 coupled to audio output components. 



Equipment 



The following is a brief description of the machines used for the per- 

 formance of the above tests and their output characteristics. 



