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

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Subject: Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback
From: "Tom Cathey" <>
Date: Fri, 16 Dec 2005 16:25:17 -0500
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That's more valuable info, Marv - Tnx again!

A few follow up questions:

If I tap off the FT-1000D 10mW low level point, can I run a long  ~ 20' coax 
cable to the amplifier CA2XX module's input, or will this cause problems? 
This is for 75M only. Maybe there is a way to do this.

I looked at the FT-1000D's circuit that puts out 10mW, just before it goes 
into the power amp board. I'm trying to figure if it's possibly as clean 
as -55db there. I don't have a spec analyzer.  It uses all pnp transistors - 
a 2SC2026 base driven, driving another 2CS2026 in emitter follower, driving 
a 2CS1973 in emitter follower at 10mW out. There's much more stuff involved, 
like the balanced modulator, etc.  Is this enough info to make a guess from 
your experience of what kind of IMD we are dealing with at this point?  This 
will have a big effect on what direction I take, of course.  Or maybe I 
could sample it into a receiver and get an idea using the same relative IMD 
procedures I use for a big amp.

OK on the sample amplifier using a 6146 input running reduced voltages and 
1W, low power for cleanliness. Guess NFB will not help there.

Last question:  Let's say I did put two 4CX-350's in cascade, so had lots of 
gain to work with. What is the practical limit for conventional negative 
feedback?  Is it a matter of running into instability, perhaps?  And I take 
it from your comments, that you would run feedback from the final to the 
predriver, [two stages at a time only] and then where would you run the 
second loop to cover the pre-pre driver 4CX-350 and the input 6146, for 

Tom, K1JJ


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 

  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 

  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 

73 & Good morning,
  Marv WC6W

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