Evaluation of Amplification Factors in Multi-Grid Tubes by an Electrolytic Tank Method

Abstract

The technique of determining the amplification factor of a triode by bridge measurement on an electrolytic tank model of the tube is well known. This method does not appear to have been used for multi-grid tube structures hitherto, probably owing to the difficulty involved in a simultaneous adjustment of the various electrode potentials in the proportion required to produce the current cut-off condition on the tank. It is shown in this paper that this difficulty may be overcome by a technique of grounding the electrodes selectively. In a tetrode, for instance, the anode will be grounded in measuring the screen-grid to control-grid amplification factor, and the screen-grid will be grounded in measuring the anode to control-grid amplification factor. The authors have successfully used this method to study typical screen-grid tube and pentode configurations, and also to verify analytical expressions describing the ‘inselbildung’ effect encountered in beam power tubes. In the latter case, a probe is placed in the close vicinity of the cathode, to facilitate observation of the off-cathode gradient of potential. The minimum setting of the probe potential corresponds to the cut-off condition. Moving the probe parallel to the cathode permits an experimental evaluation of the ‘Durchgriff’ (penetration) factor in a given tube structure. This study has shown how with simple tube models and by electrical measurements an important parameter of complex tube forms may be directly determined.