Hi Bill,
> I think I see a basic error in the above. The R introduced by the
> nichrome-wire suppressor isn't Rp, it's Rs - so it's in series with
> the suppressor inductance, not in parallel. Right?
>
> 73, Bill W7TI
Hi Bill,
No, that's not exactly correct. Let me take a stab at "un-confusing"
this issue.
Every combination of components has a two terminal impedance at
any given frequency. This "Rp value" allows engineers to consider the
effects of that parts combination on the rest of the system. We
can freely translate the "outside appearance" of any complex
component combination to either a series or parallel equivalent
network.
It is unimportant how things are connected or what is inside
the "black box" of components we are discussing, The only thing that
matters is what impedance the rest of the circuitry "sees" the
circuit as being at every frequency. That's why Rp varies with
frequency, and it's still Rp even if the components inside the black
box are in series!
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.
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.
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.
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, 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.
If we add a suppressor with too much Rp, it might get too hot on ten
or 15 meters.
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".
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, 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.
73, Tom W8JI
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