[Amps] 3CX800A7 Grid Current Problem

[Carl]

Paul, it sounds like the fixed loading padder is overheating and drifting.

Carl
KM1H


----- Original Message ----- 
From: "Paul Elliott" <paab at valornet.com>
To: <amps at contesting.com>
Sent: Wednesday, January 25, 2012 12:15 PM
Subject: [Amps] 3CX800A7 Grid Current Problem


>I have a Command Technologies HF-1250 amplifier (single 3CX800A7 tube). 
>The
> amplifier does not have grid over current trip out protection.  On all 
> bands
> except 160 m It is working fine (700 to 800 watts  output, grid current
> about 20 ma) with tuning and load capacitor settings very close to the
> settings of those on the factory tuning chart specific to this amplifier.
>
>
>
> On 160 m, starting with the factory settings, tuning is normal at very low
> drive.  The tuning and loafing capacitors are both close to the factory
> settings.  I increase drive slowly (I do not have experience with such a
> high gain and fragile grid tube as the 3CX800A7).  As I get to 250-300 
> watts
> output the fun starts.  If I hold the key down for any time at all (either
> into an antenna on a dead band or a dummy load-it makes no difference) the
> grid current begins to increase and increases more rapidly with time. 
> This
> increase can be stopped by either decreasing the drive or increasing the
> loading.  By increasing the loading drastically I can get to 500 watts out
> with a plate current about 600 ma and grid current 20 ma. At this output 
> the
> tuning capacitor setting is very close to the factory tuning sheet number,
> but the loading capacitance is only about 60% of the factory setting.  If 
> I
> start to decrease the loading, the output starts to rise rapidly, but the
> grid current starts rising even more rapidly. At no time is the SWR higher
> than 1.3:1 (using an LP100A).
>
>
>
> Could someone please tell me what is happening?
>
>
>
> 73 Paul W5DM
>
>
>
>
>
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