[Amps] Designing the Cleanest Linear with RF Negative Feedback

[Tom Cathey]

Hi Marv,

Well, you've given me a lot to think about.  I'll look for  cleaner drivers 
than the 6146 and 4-400A. You make a good point about not giving up IMD in 
that lower level area, if there is an easy choice.

Think I will start by building up the 10mw to 1W module and get that super 
clean before drilling and blasting the chassis for the larger tubes. I'm 
already working on three other rigs for the shack right now, including a big 
6M amp, a 10-40M HF linear and a plate modulated AM rig. Winter projects. 
But this 75M super clean linear will be given priority.

I liked the many helpful sources you have on you webpage. That site will be 
something when finished.

One last thing I have looked at and do not know if it is worth pursuing... 
It's a linear using three hi mu GG triodes in a chain with NFB as shown in 
the URL below. Maybe like a 3-500Z driving a pair of 3-500Z's driving a 
3CX-3000A7 with a negative feedback loop back to the beginning. Each tube in 
GG will help IMD too. This design obviously requires power to drive each 
stage and may not be as clean as the tetrode chain we haved talked about... 
If you feel not, then I'll start on the 1 watt module for the tetrode chain 
and see what tubes I can find out there in the marketplace...

See:
http://i19.photobucket.com/albums/b153/throbbo/feedback3.jpg

73,
Tom, K1JJ







Hi Tom,
   First, let me reiterate that to end up with -55dB you'll need to budget 
your distortion carefully.

   Consider your project has 3 stages:  The FT-1K,  the preamp (10mW to a 
watt or so), & the main amp.

   IMD does not add linearly so, for best results each "stage" should 
exhibit perhaps 10dB better numbers than the following one.

   I would verify your FT-1K numbers (looking for -75db) with a very low 
distortion two-tone source -- perhaps from a PC sound card?  Or..., not that 
you need another project at this point, I describe a stand alone unit, with 
enough info to duplicate, at:

http://wc6w.50webs.com/wc6wtest/index.html?fr20.html

  If you can't run down a good hybrid brick, then maybe an IC-tube combo 
would be an innovative approach to the preamp.  The TI op-amp I suggested 
could drive a tube grid directly with a (10dB?) negative feedback loop 
around these two components.  As for the choice of a tube, I'd search around 
a few audiophile websites and see if you can find the lowest distortion 50W 
tube ever designed.  There just has to be something better than the 1950's 
vintage 6146 out there!  You'll want this stage to output better than -65dB 
distortion.  When were you going to acquire a spectrum analyzer?  :-)

  The 4-400A is not a good substitute for the '350FJ.  It probably starts 
with 10 dB or so worse numbers and even though it will be run even lighter 
than the '350, it probably won't catch up.  This is not the place to toss 
away hard to come by distortion dB.  Note: Also the 4-400A is directly 
heated so, you would have to include a big filament choke to provide a spot 
to couple in the feedback.

  I don't know the numbers on the 4CX3000A, but, assuming it lives somewhere 
around -30dB (3rd Order) at full power, running it at 1/3 power (1500 watts) 
should get it down to perhaps -45 dB  (Note: A pair of 4CX1500B's will do 
this for sure.  Time to buy an old Harris RF-110A?).   Figure in the 10-12dB 
negative feedback around the "main amp" and you make your target spec.

  I want to hear this contraption on the air when it's done!

73 & Good morning,
  Marv WC6W

P.S. -- Season's greetings to the folk at the NSA who have to read all these 
messages!





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