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Re: [Amps]  negative screen current

Subject: Re: [Amps]  negative screen current
From: peter chadwick <>
Date: Tue, 31 Dec 2013 13:15:06 +0100
List-post: <">>
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 

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?'


Peter G3RZP
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