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[Amps] Driving Impedance

To: <amps@contesting.com>
Subject: [Amps] Driving Impedance
From: k7fm@teleport.com (COLIN LAMB)
Date: Tue, 11 Jun 2002 21:41:5 -0700
Now, wait a minute.  The answer that you cannot run the 4CX800A tube in
grounded grid is a simplistic answer that is only partly true.  You can
cathode drive the tube, with normal grid and screen dc voltages, while
bypassing the grid for rf.  Grid current will be controlled by the negative
grid bias.  Collins does this on the 30S-1.  However, the grid cannot be
grounded as you do with a 4-400 because very little current will destroy
the tube.  You will need a normal bias supply and separate the control grid
from the screen grid and use normal negative bias.  The grid can be
grounded for rf purposes, however.  

The amplifier will be more complex than the standard grid driven 4CX800A
amplifier, though, because you will need all of the voltages required by
the tube in grid driven configuration.  In fact, it will be more complex,
because you will need a tuned cathode circuit for each band, additional
band switching wafers and coupling.  The advantages probably will be a 6 to
8 decrease in distortion products.  However, you could pick up a few db in
the grid configuration by simply using a cathode degeneration resistor.  

So, you are adding a lot of complexity for just a few db of potential
improvement.  The 4CX800A tube, with cathode degeneration, is probably
cleaner than most transceivers, so the complexity is probably not
worthwhile - unless you want to do it just to see if it can be done.  

And, if you like doing things strange, you can ground the grid and run all
the other voltages off ground - just to say you are running a 4CX800A tube
in grounded grid.   

To answer your question as to what is the impedance of the cathode input? 
I do not know.  

73,  Colin  K7FM

> [Original Message]
> From: WA5JCI <wa5jci@flash.net>
> To: <amps@contesting.com>
To: <amps@contesting.com>
 > Date: 6/11/02 5:29:04 PM
> Subject: [Amps] Driving Impedance
>
> Greetings,
> 
> What's the input impedance for a GG 4CX800A?
> 
> Thanks,
> 
> 
> es 73 de Pete WA5JCI   EM21jv
> 
> 6 Mtr VUCC#361/782 Confirmed--2 Mtr VUCC#346/190 Confirmed
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> 
> 
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--- COLIN LAMB
--- k7fm@teleport.com
--- EarthLink: The #1 provider of the Real Internet.



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