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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
Date: Wed, 14 Dec 2005 17:34:48 EST
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Tom, if you are operating AM as in steady carrier, the resistor bias is  
probably ok. If you plan to operate SSB or cw then the bias voltage will be  
changing from zero with no drive to a maximum value with maximum drive. You  
have the problem of cutting the tube(s) off when in standby.
Gerald K5GW
In a message dated 12/14/2005 4:28:17 P.M. Central Standard Time, writes:

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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