[Amps] 3CX800A7 Grid Current Problem

Gary Schafer garyschafer at comcast.net
Thu Feb 16 09:42:17 PST 2012

Often the plate choke is designed with less than optimum inductance on 160
and sometimes 80 meters. This is done to avoid choke resonance on the higher
With the smaller choke it must be resonant on 160 and that is accomplished
by using part of the plate tune capacitance to resonate the choke. This adds
more current in the choke bypass capacitor. If that capacitor is too small
or gets warm it will drift and you will get the results that you observed.

Gary  K4FMX

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]
> On Behalf Of Paul Elliott
> Sent: Monday, February 13, 2012 11:10 PM
> To: amps at contesting.com
> Subject: Re: [Amps] 3CX800A7 Grid Current Problem
> 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
> do).
> 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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