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

Subject: Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback
From: "" <>
Date: Fri, 16 Dec 2005 19:20:53 GMT
List-post: <>
Hi Tom,
   That '5106 is probably for Cable TV use and likely cuts off around 40 MHz 
(on the low end!).

   If I recall correctly, some of the general purpose units that go down to a 
MHz use numbers from CA28XX group.  They come in both single ended and 
push-pull versions.
   I've seen a few of the 350J's for sale on the web over the years.  I don't 
believe the demand is very high for those as they have a 26V heater and as such 
can't be dropped into a 4CX250B socket.  There is also the 4CX600J/JA/JB.

  The 6146 shown on the Hughes schematic is run deep into Class A.  Note the 
low screen & plate voltages.  I bet the numbers were just fine running all of a 
watt output. 

  For feed forward, an error amp with perhaps another 4CX350FJ would be 
necessary.  It is not a difficult scheme to implement but, it would double the 
parts count for the project.  If you read about the technique on the web, keep 
in mind that "they" are typically working with transistor amps that start with 
distortion numbers 20dB worse than tubes.  Therefore, in this case, less 
correction power will be required, the output combiner ratio will optimally be 
a bit higher, and power lost from the main amp will be lower.

  The subject line said you wanted "the Cleanest Linear".  To avoid 
degrading the system, the driver should exhibit distortion specs at least 10dB 
better than the amplifier.   Your FT-1K will still be useful for the receiver.

73 & Good morning,
  Marv WC6W

-- "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.

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 

  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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