[AMPS] parasitic suppressors

[Ian White, G3SEK]

Rich Measures wrote:

>>From: Ian White, G3SEK [SMTP:G3SEK@ifwtech.demon.co.uk]
>>
>>I used to think that the Q of a parasitic suppressor had some meaning.
>>having looked at the problem in detail, I don't believe that any more.
>>
>€  Has standard AC Circuit Analysis been obviated?  

On the contrary... but it has been used on the whole circuit, not just
stopping at the suppressor.

>Has Dick looked at 
>the numbers in Wes' measurements?  

The original was from me - Dick was the messenger.

>>The parasitic suppressor operates only on #3 and #1. But in fact the big
>>unknown is #2, because it depends on the individual design and
>>construction. 
>
>€    Fortunately, #2 can be found on the mfg's technical specification 
>sheets.  It is the tube's internal feedback C from the output element to 
>the input element.  
>
If that one parameter was truly was all that mattered, you'd be able to
design a suppressor by theory alone.

Other unknowns that affect feedback include: the series impedance that
prevents the common element (grid or cathode) from being completely
grounded; the impedance of the input side, as seen by the tube at the
frequency of oscillation; the effects of direct inductive or capacitive
feedback around the tube; feedback through not-quite bypassed wiring;
etc, etc...


>> That's why you always have to develop the suppressor by
>>cut-and-try methods.
>>
>€  If R-supp burns out on 10m, the vhf suppressor is out of business.  

Ironically, the dissipation at 10m is one of the few things you *can*
estimate accurately using theory.


>>Finally, the characteristics of the suppressor consist of TWO numbers, R
>>and X (both of which will vary with frequency... yes, even R). To work
>>out what the suppressor will do for an amplifier, you need BOTH of those
>>numbers separately. If you roll the two numbers together and talk only
>>about a single value of Q, you have just thrown away any chance of truly
>>understanding what is going on!
>>
>€   Calculating Q at the anode resonance freq. is throwing away any 
>chance of truly understanding? 
>   Q is inexorably related to Rp and Rp is 
>a major determiner of vhf gain. 
>
If you take "Standard AC Circuit Analysis" ALL THE WAY, you need to know
both R and X. Their ratio Q doesn't tell you enough. 

>//  To me, Dick's dire proscription to 
>ignore Q sounds like what prestidigitators call "misdirection". 
>
Misdirection is making the audience look at only one thing (let's call
it Q), while the real action is somewhere else. What I said was exactly
the opposite: look at EVERYTHING that matters.

>  
>- In the Wizard of Oz, wasn't there a proscription to ignore the man 
>behind the curtain? 

Indeed there was... but this isn't Oz.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.demon.co.uk/g3sek

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