Dick, give me a mail address, I'll send you a complete SB-220 tank coil
and switch....I'll pick one with only a minor arc!
73 Carl KM1H
On Mon, 18 May 1998 18:13:27 -0600 "Richard W. Ehrhorn" <w4eto@rmii.com>
writes:
>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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