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Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback

To: Tom Cathey <>
Subject: Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback
From: Gary Schafer <>
Date: Fri, 16 Dec 2005 11:26:21 -0500
List-post: <>

Also look at the schematics of the Collins KWM2 or 32S1 and the Collins 
KWS1. They all use negative feedback and show real values. The collins 
uses what they call a "feedback neutralization" capacitor that is the 
last adjustment which balances for minimum grid current at exactly the 
point of minimum plate current. This is done after the normal 
neutralization of the final and driver are accomplished. This sets up 
the proper phase relationship of the feedback signal.

Gary  K4FMX

Tom Cathey wrote:
> Hi Marv!
> Now THAT'S  the kind of info I've been looking for, OM.
> OK on using the 'J' tube versions and the TRW  class A chips. I imagine the 
> J's are harder to find used.
> I went to a site that has a CA5106  TRW RF Broadband stick... is this the 
> right one?
> Here's the link - they have lots of interesting low level RF chips:
> Your website schemtic of the amplifier with negative feedback is quite 
> helpful. I can see where they neutralize each stage, grid driven, and see 
> the simple neg feedback from the final to the driver using a 20pf cap from 
> plate to the driver cathode.  Though, I didn't see any feedback for the pre 
> driver, the 6146. I guess this amplifier was is just for an example since 
> the 6146 w/o feedback is not that clean, I understand.  I don't think this 
> will be too hard to pull off using other tubes and these basic techniques.
> I just did some reading about adaptive feed forward systems that you 
> mentioned can approach -80db 3rd order IMD.  They talk about cancellors. I'm 
> not sure if I should persue that - might be too complex for the job. Then, 
> as you said, the FT-1000D even at the low level 10mW tap, would not be a 
> good driver for this super clean system..  The homebrew ssb generator is 
> intriguing. I do have experience constructing SS stuff, but wonder how much 
> of the FT-1000D I would retain as a unit with this outboard stuff.  I think 
> the 10mW FT-1000D output with simple negative feedback in the linear might 
> be all I need to grab another -15 to-20db IMD over the 3-500Z amps.  That's 
> what I'm looking for.
> Thanks again, Marv.
> 73,
> Tom, K1JJ
> Hi Tom,
>   Well, you have a certainly started a string here.
>   A couple of suggestions for you.
>   Use the 4CX350FJ.  May as well start with the lowest distortion.
>   Don't try to employ RF feedback across more than two stages.
>   Use one of the old TRW - Motorola - M/ACom class A bricks for the first 
> stage.
>   Since you are operating narrowband, consider the feed forward 
> configuration.  If you start with a -50dB distortion amp, you should be able 
> to slice another 30dB off that figure for a -80dB system.
>   I doubt if your exciter, at any level, is clean enough to drive the above 
> FF system.
>   Fortunately, a clean 80M SSB generator is easy to construct compared to 
> the balance of this undertaking.
> 73 & Good afternoon,
>   Marv WC6W
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