I run a pair of those on 1296 and 2304 and have noticed a bit of drift also
and it takes quite a bit before power drops. Try it with less drive and
lighter output loading. Use the minimum drive to maintain the output.
Back bombardment is a likely reason for negative current, its rather common
in some tetrodes anyway.
Carl
KM1H
----- Original Message -----
From: "Gary Patterson" <gpatterson53@hotmail.com>
To: <amps@contesting.com>
Sent: Saturday, March 05, 2011 9:22 AM
Subject: [Amps] weird grid current
>
>
> I have a 23cm four tube 7289 grounded grid cavity amp. The four tubes are
> commonly biased. It is generating good output. The strange thing is that
> on key down for a couple of seconds, the grid current starts at a
> reasonable value then goes to zero. At zero, I still have full output.
> I realize its thermal changes that are varying the grid current, but dont
> understand full output with no grid current. Also, if the amp is miss
> tuned, the grid current will go "negative" (grid meter reads pinned
> below zero the wrong way). How can this be??
>
> W4AF
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