[AMPS] Re:

[Jon Ogden]

Dick,

GREAT posting.  Really good!

>
> 2) Never met Ian, but the gut feeling grows that anyone who challenges him 
>in the area of network analysis or any fundamental EE stuff has a real good 
>chance of losing. Very respectable stuff, Ian - I appreciate it.

He's very brilliant.  And has the patience to teach some of us 
knuckleheads (like me) a thing or two!
>
>3) Too many guys are talking about output circuit "Q" without specifying 
>"loaded Q" or "UNloaded Q." They're very different things and the ambiguity 
>can cause confusion. I absolutely agree with Tom, Pete and others that 
>inadequate loading of a class AB, B or C amplifier, whether the output 
>network is "old-fashioned" parallel resonant, a pi, or a pi-L, results in 
>excessive "LOADED Q" of the tank circuit which can AND DOES often create 
>peak rf voltages several times the DC plate voltage. And it's not magic or 
>very hard to understand. I defer to Ian, however, for the rigorous proof, 
>because it's easy to see that he won't have to work as hard as I would to 
>dig it out(!)

Agreed.
>
>4) Maybe I've missed someone else's similar description, but I believe the 
>logical, conventional, and easiest-to-understand explanation of how a 
>common parasitic suppressor works is that [a], it does not ABSORB VHF/UHF 
>parasitic power or energy, but PREVENTS (or "suppresses") the parasitic 
>oscillation from occurring in the first place; [b] it does so by lowering 
>the loaded "Q" of the existing parasitic resonance(s?) in the anode circuit 
>to the point where feedback loop gain at the parasitic resonant frequency 
>(-ies) is too low to support oscillation. The trick, if you want to call it 
>that, is to introduce enough loss (resistance) into the parasitic resonant 
>circuit to do the job without absorbing so much of the 
>fundamental-frequency power as to either overheat itself or unduly reduce 
>amp efficiency/output.

Again, agreed.  This was one of my original conclusions after reading 
everybodys stuff.
>

>Try paralleling 3 or 4 common "850 series" doorknob caps, say by bolting 
>them between flat plates of copper to minimize stray inductance. Really 
>clean, right? But then investigate with a GDO, or better yet, a vector Z 
>meter or a network analyzer.

My DC blocking cap network consists of 3 doorknobs paralleled and held 
together by two brass plates.  It has a nice resonance around 60 MHz 
which I originally thought was the anode circuit resonant point.  Another 
very good point, Dick!

>
>Suspect the only reason the home-brew amps I built years ago (and maybe 
>yours) didn't/don't self-destruct from these sorts of things is that the 
>unintended resonances are typically quite high-Q and have far more NON-ham 
>spectrum than ham frequencies to inhabit by chance.

Well, it's always easy to get ONE RF circuit functioning in the lab.  
It's quite another to put them into mass production.  Big, big, big 
difference.  Try mass producing most homebrews and watch the troubles 
pile up!

Great post again.  

73,

Jon
KE9NA


-------------------------------------
Jon Ogden
KE9NA

http://www.qsl.net/ke9na


"A life lived in fear is a life half lived."


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