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[AMPS] Grid Swamping resistor for 4CX250B

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Subject: [AMPS] Grid Swamping resistor for 4CX250B
From: dail@CEBAF.GOV (Walter Dail)
Date: Tue, 30 Sep 1997 10:39:46 -0400
Shawn,
Thanks for your comments.

The 4CX1000 on 2m that I saw in the 1990 ARRL Handbook pg. 31-57 uses
a 680 ohm 2W resistor for swamping in the grid bias (R1).

I have no other choice (for mechanical reasons) than to go this route.(using
a swamping resistor as opposed to neutralization). I have had fair luck
using a swamping resistor in the past, just I dunno on 432...

Thanks,
Walter


At 06:55 AM 9/30/97 +0700, you wrote:
>AT this frequency you might be better off trying to use the screen to give
you the isolation from plate to grid.  
>Several broadcasters do this with big tetrodes on the FM band.  There is a
design using a 4CX1000 on 2M in 
>one of the handbooks that uses this approach.  It complicates the metering
and power supply setup but it can 
>work.  
>
>The swamping resistor doesn't always work.  We had a devil of a time
getting a 4-1000 to settle down at 6M.  
>Turns out the problem was a resonance from the plat tune cap thru the plate
to the grid.  The resistor actually 
>helped it, at 110Mhz.  We fixed this one by using a stainless steel strp
from a hose clamp, the slots added a 
>small amount of inductance that swamped the 110Mhz energy real nice.  Grid
resistors are more for matching 
>than helping stability, at least in my experience.  They do help but they
are not the end all fix.
>
>
>
>On Tue, 30 Sep 1997 09:36:55 -0400, Walter Dail wrote:
>
>>Hello All,
>>  I have a question regarding using a grid swamping resistor
>>on a tetrode amp. I have been building a twin cavity amp for 
>>432 Mhz using a 4CX250B tube. I understand that instead of 
>>neutralizing a grid driven amp a grid swamping resistor may be used. 
>>These are found in all the handbooks, etc. showing 2m amp
>>schematics. However, in the 1985 ARRL Handbook, page 32-8,
>>"A Tetrode UHF Amplifier" it does not show such a resistor
>>in the input compartment. They are using 10W drive also.
>>But I noticed in the grid PS they have a 100 ohm 2W resistor in the 
>>line that feeds the grid bias. Is this what they are using? What gives?
>>Is the Handbook wrong and a grid swamping resistor should be added?
>>
>>Comments please,
>>Walter Dail -KT4JA
>>
>>
>>
>>--
>>FAQ on WWW:               http://www.contesting.com/ampfaq.html
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>>
>
>
>de Shawn
> N7LQ (formerly WB7TDI)
>
>DM09dn & DN01fa
>
>
>


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