Microphonism in a Subminiature Triode

The simple theory of the symmetrical plane triode has been applied to the calculation of the change in plate current as a function of motion of the grid and cathode. For perfect symmetry, the first-order approximation gives zero change in current. The second-order expressions are evaluated and show that an output signal may be expected having twice the vibration frequency. A description is given of equipment for vibrating a tube for test, measuring the amplitude of vibration, detecting very small resonances of tube parts, and studying the wave form of the electrical output. Experimental studies have been made on tubes to determine the mechanical origin of several different forms of electrical output. In some cases, individual tubes which exhibited particular forms of microphonism have been opened up, the structure altered, and the tube operated under vibration in an evacuated bell jar. In this way, the mechanical origin of particular forms of microphonism were determined quite definitely. A few general comments are given regarding tests for microphonism, and the design of tubes.