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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 17:45:14 -0500
List-post: <>
Yes, Gerald, I thought about the DC bias variation too... the fact it would 
be swinging under ssb modulation... and thought that was not a good thing 
unless the tube was run in class A. This is class B amplifier, of course.

I saw that Tom/W8JI recommended it back in 2000 or so, so figured it was a 
good trick. He said "unbypassed".
Maybe I misunderstood something.

I also saw guys talking about using an RF choke across this resistor. THAT 
would cure the DC bias variation problem, and maybe be transparent to RF as 


> 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  still
> have the problem of cutting the tube(s) off when in standby.
> 73,
> 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... :-)
> 73,
> Tom, K1JJ
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