Must be getting senile, time to join the 160M SSB crowd!. Anyway this was
meant for the list...not just Tom. I keep forgetting you guys...no wise
cracks!
Cheers...Carl KM1H
-------- Begin forwarded message ----------
From: km1h
To: w8jitom@postoffice.worldnet.att.net
Subject: Re: [AMPS] suppression
To: <amps@contesting.com>
Date: Thu, 22 May 1997 12:11:32 PST
Message-ID: <19970522.121500.9671.6.km1h@juno.com>
References: <r2X5HDAVwagzEwSI@ifwtech.demon.co.uk>
On Thu, 22 May 1997 09:52:17 +0000 w8jitom@postoffice.worldnet.att.net
writes:
>
>Hi Bill,
>
>No, that's not exactly correct. Let me take a stab at "un-confusing"
>this issue.
>We all know adding turns to the suppressor makes most PA's more
>stable at VHF. The bigger the coil, the better the stability but
>the bigger the resistor also has to be. A bigger suppressor has
>higher Rp.
Please explain the relationship between the resistor value and the coil
Tom. In commercial amps there appears to be no set pattern; but yet I
assume that real engineers were involved.
>The reason a higher Rp value in series with the anode DECREASES VHF
>gain more than lower Rp value is simple to explain. The parasitic
>suppressor is not the only component there. (A more correct term
>would be Zp, since it contains reactance as well as resistance.) The
>anode to chassis path has finite impedance (it also has an equivalent
>Zp or Zs value). If the Rp (Zp) of the suppressor does NOT dominate
>or come close to dominating the entire anode to chassis impedance,
>the suppressor does little good. The suppressor makes little or no
>overall change in the SYSTEM, unless its Rp value approaches or
>exceeds the normal anode to chassis path impedance.
I 100% agree.
>That's why layout is so critical. That's why we all know short wide
>direct leads from the anode to the tuning capacitor are very
>important to stability, and why external anode tubes are generally
>more stable than tunes with long anode leads.
>
>That's also why we add turns to a suppressor, or add turns and
>increase the resistance value. Empirically, we are finding a
>combination that allows the lossy suppressor resistance to dominate
>the anode system at VHF, and NOT at HF where the resistor
>would burn up on ten meters. We generally want high Rp in the
>suppressor, so the resistance of the suppressor is dominant.
>
>Series resistive losses (such as winding the coil from a lossy
>material) in the coil have greater effect as the frequency is
>lowered, while parallel resistances (the kind most people use) have
>less effect at low frequencies.
There I must disagree. Pure resistive losses due to skin effect become
more pronounced as frequency is raised, not lowered. The same reasoning
that is used in wire sizing for the tank coil should follow thru from
anode to the output connector.
>
>When the entire anode SYSTEM is considered, a certain optimum value
>of suppressor impedance will be ideal. Either higher or lower Rp
>(Zp) can be less effective, with lower values of Rp is
>series with the anode decreasing stability and higher values
>increasing heating at HF.
>
>If you put all the anecdotal chatter and personal comments aside,
It comes with the territory IMO.
and
>look carefully on paper the equivalent circuit of a anode system,
>you'll see distributed shunt capacitance and series inductance
>dominates the path between the anode and the chassis. Lowering the Q
>makes this path look more like a resistance.
>
>In order to de-Q this system, the equivalent parallel impedance
>(suppressor Rp, or better yet Zp) added in series with the anode
>SYSTEM must approach or exceed the value of the anode path's
>impedance. If we add a suppressor system having too low
>an Rp value, it has little effect on the anode system. It's just
>too small a change to a larger impedance.
Agree.
>If we add a suppressor with too much Rp, it might get too hot on ten
>or 15 meters.
As I pointed out several days ago.
>This stuff really isn't that complex, it's just no one has ever
>written anything detailed in amateur publications. It would
>take at least three or four pages of text to explain the anode
>system alone in a way that everyone could follow.
>
>The lack of articles is probably why engineers (like Ian, N7WS Wes,
>Gary Coffman, and so on), who learn this stuff formally and use it
>every day, often get perplexed or irritated when someone
>appears to "twist it all up".
So why are there no articles? Even "RF Design" in its heyday didnt
broach the subject. Or any tube manufacturer has surfaced with a
definitive paper or article.
>Engineers and experienced designers may think misuse of the terms is
>intentional (I know I sometimes think people must surely know better,
>but then I remember people confusing Rs and Rp have no formal
>engineering background,
It seems that even those with formal training can confuse and cloud the
issue. If one looks at it in the "black box" context your argument makes
sense. If you climb into the box then Rs becomes a real part and is
frequency dependent. One cannot continue to explain everything as a
theoretical lossless circuit at RF; it is not as easy as basic 60Hz AC
Theory. Even us uneducated types can understand that Tom.
You should also remember that if all of these super engineer types were
so good there would be no need for good techs to actually get their
circuits to work. Granted modern CAD software has removed a lot of the
guess work and may even obsolete the circuit engineer sometime in the
future.
and probably do not use the terms in everyday
>life). That may explain why it sometimes sounds like an argument,
>because people get frustrated when simple terms are misused or
>misunderstood.
>
>Meanwhile, the poor people who are REALLY trying to learn something
>and understand something about amplifiers probably get confused
>by all the misunderstood terms and turned off by the personal
>flaming.
I really have not seen any personal flaming Tom. Maybe if someone uses
your name in the same paragraph that he disagrees with you, you consider
that a personal attack.
What I do see here is a healthy and spirited discussion. What I also see
here is at least one person that ever refuses to admit even the thought
of a mistake. What I do see is one person that appears to not want to
answer many pointed questions. If this person wants to set himself up as
the appointed expert here it would seem that these continuing
oversights/lapses are deliberate.
Meanwhile I will continue to use my lossy material to stop them pesky
parasitic critters in their tracks as I have for 30 years. I suspect Rich
will continue with his method also since no one has yet made any
meaningful attempt to prove us wrong....or right as the case may be.
Lots of talk, yes, but zero hard proof.
I also notice that none of those "experts" have followed up on my offer
for a full spec sheet on the material.....maybe they already know
everything about everything.
73...Carl KM1H and the beat goes on
>73, Tom W8JI
>
>--
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>Problems: owner-amps@contesting.com
>
--------- End forwarded message ----------
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