Pulse distortion effects of gas in vacuum receiving tubes

Pulse distortion as manifested by droop and rise in plate and grid currents has been observed over a portion of the positive grid region for certain samples of standard receiving type vacuum tubes when tracing their characteristics by a succession of pulses using the curve tracer described by the writer in Electronics, April, 1951. It appears that some of these effects fit in with the work of K.G. Hernqvist on “Space-charge and ion trapping effects in tetrodes”, (PROC. I.R.E.; December, 1951) in which it is shown that the secondary electron current from the plate to screen grid is affected by residual gas ionization and results in a transient response corresponding to the period of ionization buildup. Tests made on triode receiving tubes at positive grid potentials appear to confirm this theory. Other behavior observed is found to be affected by gas, but does not appear to depend upon secondary emission. The effects are illustrated by oscillograms taken of positive-grid characteristics and pulse wave forms for commonly used tubes, such as the 6SN7 and 6SL7 twin triodes. It is shown that distortion is noticeable for grid gas current readings well below the maximum specification value.