Beam transmission in electron guns having a single-apertured control grid

The ratio of target current to cathode current (beam transmission) has been studied for a number of vidicons and scan converters. These are high-resolution electron tubes that characteristically have a control grid with a single aperture. The results, as anticipated, show that beam transmission is affected much more than total cathode emission by tube aging processes. The transmission was determined by direct beam current measurement and the cathode condition recorded by the cathode imaging technique. Because of the centralized area in which the loss of emission occurred, beam transmission was reduced much more than total cathode current. In vidicons, the decreasing beam transmission was correlated with increasing residual signal after erasure and with saturation of the gamma transfer characteristics at lower light levels. In practice, beam current rather than cathode current, should be monitored.