>This may seem like a real basic question but keep in mind -- I write software
>for a living.
We will forgive you. :-)
>
>As I increase the drive to my SB-221 (2x3-500Z), there is a fairly linear
>relationship between input drive power and output power UNTIL I reach a magic
>point where the output doesn't increase as fast (gain decreases) and the grid
>current rises dramatically. Prior to this point, the grid current only goes
>up slightly with increased RF drive. My question is, why? What causes the
>grid current to go up and what is this telling me? The amplifier has a gain
>(<500 watts out) of about 15 at 40 & 80 meters. At more than 800 watts, the
>gain decreases and it is difficult to get more output power.
It sounds like to me that the amp is being driven into compression too
early. Once you've begun to compress the gain significantly, you'll have
a nasty signal on the air. The grid current is going up because you are
not getting a linear relationship between input and output. That's the
best way I can easily describe it. Perhaps someone else can put it into
better words. But if your input power is increasing and your output
isn't, the added input power has to go somewhere (the grid).
What is your input power when the gain begins to compress? I remember
reading somewhere that the SB-221 can be over driven by 100 watts.
The SB-221 should put out around 1200 watts if I remember correctly. My
guess is one of two things: Your anode voltage is off (too low) or you
have a tube that is not functioning properly. If you are getting 800
watts out with normal drive when you should be getting 1200, I would bet
that the tube might be bad.
How old are the tubes, etc?
It's been about 10 years since I've used an SB-221 so I am not the most
help. You certainly have some sort of problem and I would drive the amp
into compression very far. You might damage something.
Perhaps someone else can provide more help. Just thought I'd add my 2
cents worth.
73,
Jon
KE9NA
--------------------------------------------------------------------------
Jon Ogden
jono@webspun.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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