[Amps] Secondary emission in Screen grids

[John T. M. Lyles]

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.

73
John
K5PRO