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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, 14 Dec 2005 17:27:30 -0500
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Thanks for all the replies!  cheezz... I got close to 15 private ones and 
they're still coming in - really appreciate the ideas, guys. Please excuse 
me if I don't answer them all...

K5GZ [good whiskey] that's an good idea about determining how clean the 
FT-1000D 200W module is at 90w or so, FIRST, to possibly eliminate a few 
linear stages. I've actually idled the finals a little higher in the past, 
but hit the pot's limit for now.

Also, the fellas who emailed me about the Harris and Colins amps - Worth 
looking at.

I was looking through the Amps archives and came across talk about using a 
cathode resistor for conventional negative feedback. I tried a 10 ohm 
resistor in my GG triode amp  today and found it idled the amp at about 1/2 
it's normal level. I use diodes normally that get switched in and out to 
idle it at various levels... so just switched out a few to get it back to 
normal idle.  Does anyone know how to determine the optimum value for this 
cathode resistor and about how many db of IMD/negative feedback improvement 
you can expect with an optimum value?

I could actually eliminate the diodes completely and increase the resistor 
to 20 ohms - so that all the idling bias is supplied by the resistor.  Is 
this a good limit?

BTW, After adding the cathode resistor, I didn't see much difference in the 
amplifier's drive requirements, so it makes me wonder how little negative 
feedback it was adding. ie, if it was a measly 3db, I'd need TWICE the 
drive, right?  No way I  saw anything near that.

Another idea I received -   For a 75M mono-band amplifier: How about 
building a single hi-mu GG triode final and a smaller hi-mu GG driver .... 
How would you connect an addtional feedback loop around them? If a capacitor 
coupled RF sample came off the final's plate, where would it be fed back 
into the driver?  The driver's grid is usually grounded, so that's out 
unless another config is used, and I'm not sure of the phase to feed back to 
the driver's cathode...  What would be a proper hookup?

There's more ideas, but that's enough for now... :-)

Tom, K1JJ 

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