OK, so negative screen current is caused by secondary emission from the screen
grid. According to Terman, "With surfaces treated in such a way as to enhance
secondary emission, as many as ten secondary electrons may be produced for each
primary electron, while surfaces prepared to resist secondary emission will on
average have only one secondary electron for perhaps five or ten primary
electrons".
So far, so good. But it means that the secondary emission exceeds the number of
electrons captured by the screen grid - in a way reminiscent of the 931A photo
multiplier tube or the pre WW2 Philips EE50 video amplifier tube (not to be
confused with the EF50 pentode) which both used multiple secondary emissions to
get high gain and bandwidth. But from what Terman says, suitable treatment can
reduce the tendency to secondary emission, and so the question becomes 'Are
such treatments used and if not, why not?'
73
Peter G3RZP
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