[AMPS] Re: Parasitics

[Tom Rauch]

> Date:          Tue, 19 May 1998 10:04:39 -0500
> From:          Jon Ogden <jono@webspun.com>

Hi Jon,

> >Can you explain how lower Rp in the suppressor produces less VHF 
> >gain?
> >If lower Rp means LESS gain, why use a suppressor Jon? What is the Rp 
> >of a dead short?
> 
> Arrgh!  I thought we worked through all that math!  Gain is directly 
> proportional to Rp. 

I assume you are talking about voltage gain, and not power gain. 
When the tube is delivering power and consuming power from 
the driver, more plate load resistance can mean either more or less 
power gain. 

> Rp is different from the R of the supressor 
> resistor.  Did you not just read Ian's notes? 

No, I deleted most of the messages that built up over three days. 
Ian almost never needs checked. By the way, it would be better if we 
avoided jargon like Rp. Rp means different things in different 
applications, and use of excessive jargon can confuse everyone.

 >Anyhow, if gain is proportional to Rp doesn't it makes sense you 
>want to lower it?  

If you want to decrease voltage gain, that is true. The problem 
with that statement, perhaps Ian addressed this point, is suppressor 
"Rp" or impedance is one part of system impedance "Rp" seen by the 
anode. The suppressor has little effect on the SYSTEM unless the 
impedance of the suppressor approaches or exceeds the impedance 
presented to the tube while "looking into" the tank.  

That is why a direct short, with a parallel equivalent resistance of 
zero ohms, will not improve stability in an unstable PA. Lower 
suppressor impedance is not always better, as a matter of fact it is 
almost always worse! It's easy to see a suppressor's impedance 
MUST be a large fraction or more of the impedance of the system it is 
installed in, or it does nothing at all.

Now imagine all the many layouts using different tubes that all 
require about the same suppressor method and values. Does it 
sound likely that one idea is the ONLY sure-fire cure, works in 
every case, is better than any other metod, and the cure would always 
be a suppressor of "R? + jX?" ohms impedance?

Another thing to consider, and to keep in mind, is the suppressor 
tested was not even the design marketed! The unit tested was of 
totally different construction, with more inductance. My puzzlement 
is how anyone can sell something claiming it is a cure or improvement 
with having any data about any impedances, either in the equipment or 
in the replacement part.   

> You cricizie the "hairpins."  What difference does it make what kind of 
> inductor one uses?  It still has inductance?

The hairpin has less inductance than the suppressor tested. IMO, if 
one is claiming results and misusing the data, he should at least use 
a component similar to the one being marketed.
 
> Please explain why lower Rp is not always good.

Rp where? What Rp? Plate load impedance or suppressor impedance or 
tank impedance or what? What Rp do you mean?

This is like calling a pi-net input cap C1. C1 can be any capacitor. 
In many amps, it winds up being a bypass cap on the power line.

I think I explained what you mean above.

73, Tom W8JI
w8ji.tom@MCIONE.com

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