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Re: [Amps] 3CX800A7 Grid Current Problem

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Subject: Re: [Amps] 3CX800A7 Grid Current Problem
From: "Paul Elliott" <>
Date: Tue, 14 Feb 2012 04:09:31 -0000
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I really didn't know what was the cause of the grid current changing,
creeping or rising rapidly, depending on loading, only on 160 m, but it did
seem that something was heating up and changing with time when the key was
held down.  There was no visual sign of anything wrong in the amplifier RF
output tank section.  I decided to change the plate blocking capacitors and
the B+ bypass cap at the cold end of the plate choke. (relatively easy to


I replaced the two paralleled 2500 pf plate blocking caps.  Nothing changed.
I then replaced the 2500 pf B+ bypass capacitor with a 4700 pf (thanks,
Carl) capacitor.  Now, on 160 m, the amplifier tunes smoothly, and there is
no change in grid current with a key down 600 watts for three minutes (into
a dummy load).


I still don't know exactly what was going on-but am glad the amp is now
doing well on all bands.


My thanks to all who responded.  Your kindness and knowledge are both
appreciated very much.


73 Paul W5DM 


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