[Amps] Secondary emission in Screen grids

[R.Measures]

On Sep 17, 2004, at 9:31 AM, John T. M. Lyles wrote:

> A few weeks ago we had this discussion, and the question was 
> asked,"why do the older glass tetrodes like 4-400A, 4-1000A appear to 
> be free of negative screen current while small ceramic metal tubes 
> like 4CX250B, 4CX1000A, etc have significant effects."
>
> I got an email directly from a former Eimac employee, who is a ham and 
> who reads postings here. I am ashamed for not thinking of this, and 
> perhaps it was mentioned by others who commented. The significant 
> cause for the secondary emission and subsequent reverse current is 
> that the ceramic metal tubes have oxide cathodes, while the big old 
> glass tubes used thoriated tungsten filaments. The TT tubes are much 
> 'cleaner' inside, as far as deposition of heater material onto the 
> grids.
>
So why is it possible to have reverse screen current during tune-up 
with an 8170?

> 73
> John
> K5PRO
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org