Rich & interested others...
Observations, opinions, caveats, and a question...
1) Ran into Tom at Dayton and after reading all the flak here was surprised
to find that he's still a pretty decent guy who seems to know amps well.
It's so disillusioning!
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.
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(!)
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.
5. Rich: I never had an SB-220, but if you'll post the approximate
dimensions of its 80M tank L (conductor dia., coil I.D., # of turns,
overall length) I'd like to make a similar coil, tweak it to ~10 uH, and
measure its impedance at 110 MHz on a good network analyzer. My guess is
that, at 110 MHz, distributed capacitance of the coil, mostly, makes it
look quite different from "2pi f L" = 6910 ohms reactive. 'Course it may
look quite different even from what's measured on the bench when it's
located "in situ," in the amp.
What's the point? IMHO speculation about what an HF tank looks like at VHF
tends to ignore distributed capacitance of the coil winding and its tap
leads, stray (or "parasitic"!) inductances in variable capacitor and other
rf structures, miscellaneous other little reactances and losses that may
become significant at VHF/UHF, and the plethora of resonances - series and
parallel - that the whole ensemble creates to snare the unwary. Sort of a
manifestation of the principle of unintended consequences ("PUC").
6. Another caveat related to the "PUC," which I haven't seen mentioned in
references to paralleling multiple capacitors "to get rid of troublesome
resonances," more or less: If you acknowledge that every capacitor has some
parasitic (sorry!) inductance as well as various other distributed
reactances, it's apparent that multiple caps in parallel have more series
and parallel resonances collectively than they do individually. WAY more.
If you don't check them out and yet don't get bitten by them, consider
yourself lucky.
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. BTW, this isn't an academic exercise - just
another lesson earned the hard way and pretty obvious after-the-fact.
Pretty obvious a priori, too, if we consider a simple cap equivalent
circuit as consisting of the nominal C in series with a small parasitic L,
the pair then paralleled by a small parasitic C. Put a couple of those in
parallel and it quickly gets complicated to calculate all the series and
parallel resonances. Bottom line? If you're lucky, paralleling 2 or more
caps may avoid resonance problems and effectively just give you C1 + C2 +
... + Cn overall. If you're NOT lucky, it may give you a nasty composite
resonance right where it hurts.
7. Finally, I think Carl's reference to resonances created in or by shorted
turns of bandswitched tank coils is highly relevant. Extremely high-Q
resonances often are created and can result in very high rf voltages, and
unintended coupling among circuit elements to boot. These tend to occur at
unexpected frequencies (e.g., shorted-out 160 & 80/75M sections of a pi
coil may exhibit a bodaciously hi-Q resonance around 14 or 21 MHz, along
with rf voltage that can create a half-inch or longer rf arc.) This can be
absolutely mystifying until you look for the resonance with a simple GDO.
Yep - bet most of us learn this "the hard way," too.
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. Result: many of us, by
sheer luck, never operate our amps close enough to these unintended
resonances to trigger the big arc or its counterpart, the big heat created
by excessive circulating current. It's sort of like the fact that we never
know how close we came to being hit by a log falling off a truck if it
doesn't actually quite break loose.
73, Dick W0ID
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