Hi Peter,
This link is to a section of the "care and feeding book" that is on the CPI
site.
http://www.cpii.com/docs/related/22/C&F2Web.pdf
Starting at page 8 it goes into some of the different materials used for
grids and why. Some are more prone to secondary (and primary) emissions than
others. Depending on the type of tube and its construction and screen
voltages determines the material used for the grids.
73
Gary K4FMX
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of peter
> chadwick
> Sent: Tuesday, December 31, 2013 6:15 AM
> To: amps@contesting.com
> Subject: Re: [Amps] negative screen current
>
> 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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