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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: Wed, 21 Dec 2005 11:52:49 -0500
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That's the info I needed, Steve!  -30 db IMD at 300+ watts.

Yes, as you said, the 8560's [4CX-250's] will be used as class A voltage 
drivers putting out barely a watt or two, and 30 volts of swing. This will 
help bring them up into the -50 db+ IMD range, I'm  hoping.

I ordered some OPA-695 RF op amps yesterday for the 5mW to 250mW 
amplification, so I'm on the way. Newark has them in stock for about $2.88 

For the general list members:

In the meantime, I was thinking of the possibility of a grounded grid chain 
for my other project, a 10M-40M amplifier. Since grid driven is a little 
more squirrelly and GG is inherently more stable, I was thinking of a mostly 
all GG amp.  Say the same 250mW op amp drove a class A 6550 [ grid driven] 
that put out 2 watts. This drove a class A  GG 3-500Z that put out only 35W. 
This drove a GG 8877 that put out ~1500W.    Question...  what numbers do 
you feel the  6550 to 3-500Z chain would do for IMD considering it is 
running at about 10X LESS than it's rated output?  But, the 8877 final is 
running at about 60% of it's ratings...  If I put negative feedback around 
it, could it be brought up to a reasonable IMD match to the driver tubes if 
I add more low level  gain to work with?   It would appear the effort would 
be lost on the 8877 final... Or maybe I need a 3CX-3000A7 in the final to be 
able to derate it too.

My point is, can a GG DRIVER chain running at 10 times less than rated power 
compete with a conventional grid driven DRIVER chain with NFB, giving -50db 
to -55 db+  IMD figures?

Another question: If I put 10db of negative feedback around the GG final and 
GG driver, I can make up for that gain loss by increasing the gain 10 db in 
the low level op amp stages I can still get 1500W out, right?   Just want to 
be double sure.

Tom, K1JJ


>> Does anyone know what the IMD figures are for 8560A's or how this tube
>> compares against the standard 4CX-350 for IMD?  These tubes were once
>> popular in commercial quarter kilowatt FM rigs.
> The STC data for 4CX250B gives typical values for class AB1:
> 2kV anode, 350V screen, 100mA Iq. 2 tone Po 325W pep, IMD3 -30dBc, IMD5
> -50dBc.
> Same IMD at 1500V anode, 225W pep out.
> The power levels mean it's not that useful as a guide to what they will
> do at much lower level. -30dBc IMD3 is probably somewhere around 1dB
> gain compression, and the IMD might be somewhat higher than you would
> extrapolate from a compression point prediction. Back the power off 10dB
> and you will probably see more than 20dB reduction in the IMD relative
> to the tones.
> If you can find data where someone has used them in a class A TV tx, you
> will get better figures for what they will do as drivers/predrivers in
> your system.
> Steve

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