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
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