Amps
[Top] [All Lists]

[AMPS] Re:

To: <amps@contesting.com>
Subject: [AMPS] Re:
From: km1h@juno.com (km1h@juno.com)
Date: Tue, 19 May 1998 10:43:06 EDT
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

_____________________________________________________________________
You don't need to buy Internet access to use free Internet e-mail.
Get completely free e-mail from Juno at http://www.juno.com
Or call Juno at (800) 654-JUNO [654-5866]

--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm

<Prev in Thread] Current Thread [Next in Thread>