Whose bucket is the fullest, I ain't got a clue.
I'll toss in my own bucket a few lines down. I'm no expert, in fact I'm pretty much a disinterested party, except that I want to see accurate information, not myths, not urban legends. To me it does not matter who says something; what matters is what the facts and logic are to back it up.
Indeed . History tells us that accurate may not be entirely popular - especially when a popular "expert" gets his feathers ruffled. .
Also consider SOME merit to each other's arguments.
Considering merit is one thing; granting is another.
If one person starts a discussion by saying two plus two equals five, and another replies that two plus two equals four, would anyone who knows anything about arithmetic say they both have merit and consider the answer to be four and a half?
I didn't think so.
Nor do I. During the Grate Parasitics Debate, I was approached by the opposition to negotiate a behind the scenes settlement much like the four and a half example. It seemed to me that the opposition assumed this was a popularity contest instead of a debate about a common design flaw in HF amplifiers. I found their face-saving proposition to be moderately amusing.
For what it's worth, here's my understanding of the technology. I will purposely avoid math because what really matters is the concepts. Once you have the concepts the math follows readily. If the concepts are wrong the math doesn't matter.
The first step is to understand what "parasitics" are, and where they come from. This discussion is specifically about VHF-region parasitics; low-frequency parasitics (typically below the AM BC band) are another discussion.
In tube-type RF power amplifiers that amateurs typically use for HF/MF, the input and output circuits are usually tuned circuits of some kind. Well-designed HF/MF amplifiers are stable at and around the operating frequency by means of shielding, neutralization and/or other methods.
The problem with neutralization is that it neutralizes feedback at the operating frequency, but not for the freq. of the parasitic VHF resonance that is present in the anode circuit of all MF and HF amplifiers.
A problem frequently encountered in such amplifiers is the existence of VHF resonances and feedback paths that can result in VHF oscillation. The resonances and feedback paths are the result of unneeded but unavoidable inductances and capacitances in the amplifier itself, which cannot be eliminated by the shielding, neutralization and other methods which work at HF/MF. These "parasitic" resonances and feedback paths are the result of things such as internal tube capacitances, lead inductances, etc.
For example, consider a typical grounded-grid amplifier with pi-network output circuit. The HF resonance is established primarily by the plate and loading variable capacitors and the pi-net coil. Other effects (RF choke reactance, tube capacitance, lead inductance) are involved but are not the dominant factors.
However, at VHF the circuit looks very different. The inductance of the leads from the tube plate(s) to the plate variable capacitor, plus the capacitor itself, may form a resonant circuit.
It's not a may, it's a sound wager in HF/MF amplifiers.
The pi-net coil, OTOH, has so much reactance is out of the picture; it looks like an RF choke. Even if the wiring is made of heavy strap (which is often a good idea), the total inductance is may not be negligible at VHF.
Another was of saying this is: the Pi-network output circuit is a low-pass filter so VHF energy can not safely pass through to the load.
Similar things happen on the input side. And since all tubes have at least some internal capacitances between elements, there is a feedback path. A fraction of a pF that is negligible at HF may be more than enough at VHF.
Example: The TL-922 has an anode circuit parasitic resonance c. 120MHz. 2, 3-500Zs have a feedback path of c. –j4200-ohms at this freq.
The end result is unwanted VHF oscillation from these "parasitic" resonances and feedback paths - hence the name "parasitic oscillations" or just "parasitics".
An amplifier can only oscillate at a particular frequency if its gain and feedback are of sufficient amplitude and phase at that frequency. (Yes, there are other conditions, but those are the biggies).
Without adequate gain and feedback at a frequency, oscillation can't happen at that frequency. So what is needed to stop VHF parasitic oscillations is to reduce the feedback and/or gain (at VHF) enough that oscillation at that frequency can't start.
Reducing the feedback consists mostly of using good layout, good parts choices and other well-established practices. Unfortunately, even when all these practices are done as well as possible there is often still enough feedback to sustain oscillation at VHF even though HF is perfectly stable.
There is no way of making the feedback path inside the amplifying device disappear.
So the rest of the job is done by reducing the gain at VHF.
One way to reduce the gain is to introduce resistive loading into the amplifier circuit. With enough resistive loading, the parasitic oscillations will not be able to start - they will be "suppressed" - and the amplifier will be stable. Unfortunately, such resistive loading will also reduce the gain at the operating frequency, and use up considerable amounts of desirable RF in the process.
How considerable is 0.1db on the S-meter at the Rx end ?
So what is needed is a form of frequency-selective resistive loading. Ideally, the loading would be very high at VHF and very low at HF. Usually the loading is done in the plate lead of the amplifier, right at the plate connection(s), because that's usually the best place. In some grid-driven amplifiers, the loading may be done at the grid terminal, or both plate-and-grid terminals.
This requirement is usually met by connecting a small (at HF) coil of very high conductivity (very high Q coil) in parallel with a resistor of very low inductance (very low Q). The result is a "parasitic suppressor" which provides VHF resistive loading without affecting HF too much.
At parasitic frequencies the coil has quite high XL and very low R. Not much RF current can pass through it because of the XL, so most of the parasitic RF current has to go through the resistor - which is so lossy a parasitic can't start.
... and the reason the parasitic can't start is that VHF gain was reduced by the L/R suppressor. The lower the VHF-Q of the suppressor, the lower the VHF amplification.
VHF problem solved.
At the operating frequency the coil has very low XL and very low R. RF currents can pass right through it with very low loss.
Very low only in Fantasyland. In the real world there is no such thing as a 300mpg carburetor. An additional loss of c. 2% at 28MHz seems to be the price of an amplifier with no surprises.
Only a tiny bit of the desired RF current has to go through the resistor and be dissipated, because the XL and R of the coil are very low.
Now of course we can't get ideal parts. But we can come close by using large copper wire or strap for the coil, and the lowest inductance resistors available. Which is what is done by those in the know.
It's true that there is no advantage of using a low-Q coil if low-L resistors of any desired wattage were available off the shelf. However, IME a R-supp for a 1500w amp needs to have <12nH of L in order to work well and it has to be able to dissipate c. 30w at 29MHz. Since there aren't any off the shelf resistors that qualify, a workaround is to use resistance wire to construct L-supp. This shifts some of the dissipative burden from R-supp to L-supp. This technique is hardly revolutionary Jim, so why does it raise eyebrows in 2011:
“The combination of both resistance and inductance is very effective in limiting parasitic oscillations to a negligible value of current.”
- - F. E. Handy, W1BDI 1926 Ed. The Radio Amateur's Handbook, p. 72,
Remember that if a parasitic suppressor works, the parasitic never starts. All the heat dissipated in a parasitic suppressor is either DC or desired RF. Therefore it makes sense for the suppressor to have as low loss at the desired frequency as possible.
Everything has a trade-off, and any device which does an adequate job of reducing 30 - 300 MHz gain is quite likely going to have some loss at 28MHz. IME the trade-off with a successful VHF suppressor is c. 0.1db at 28MHz and 0.02db at 14MHz.
The practical challenge is to determine the suppressor coil and resistor values such that parasitics can't start *and* the desired-RF losses aren't too bad. The usual method is empirical, that is, an educated form of cut-and-try.
That's really all there is to it. Nothing magic, and nothing new; such suppression techniques and concepts go back over half a century. The ARRL and RSGB handbooks of the 1950s through 1970s cover all this.
I never read anything in either the ARRL's or the RSGB's handbook that explained that a VHF oscillation suppressor functions by reducing VHF gain.
So why all the fuss today? Couple of reasons:
1) Some amplifiers weren't designed all that well back-in-the-day. The well-known problems of the 30L-1 are an example. The loopy bias scheme of the L-1000A is another. Such amplifiers often can be improved by specific, well-thought-out modifications. (If I ever built a 2x813 GG amp, I sure wouldn't use the L-1000A bias scheme!) However, that doesn't mean ALL amplifiers need to be modified!
2) Because amplifiers tend to be expensive items, and their basic design hasn't really changed all that much, a lot of old amps are still in service 2, 3, 4 or more decades after they were built. In all that time, things wear out, parts change values, etc., particularly when stressed by heat and such. Why should anyone be surprised if 2 watt carbon-comp resistors used in a suppressor changed their value over a half century of use, and the amp doesn't quite work right today? Such an amp doesn't necessarily need a modification; it just needs repair.
That's often the case but when the shiny new resistor changes value from 100-ohms to 410-ohms after a big bang, odds are good that old-age was Not the problem.
3) Sometimes "interchangeable" parts aren't. For example, I have seen parts that look brand new which are older than I am and way off their specs. Others look like trash and are dead-on. Some new tubes from some suppliers look great yet are useless right out of the box, or are OK in one application but fail miserably in another. (Classic example is horizontal mounting of RCA and non RCA 811-A).
4) Sometimes hams abuse their rigs. Can't tell you how many times I've seen/heard hams expound a "tune everything for max" philosophy, who think that taking a couple of minutes to tune up is fine, who think "if some is good, more is better", etc. Or who reduce power in an SSB GG amp by retuning the output instead of reducing the drive.
Some of us seem to think that it's stupid to turn down the mic. gain to reduce power - although the opposite is true.
5) Sometimes voodoo, urban legends and myths are taken as gospel. A thing works once, without a real understanding of why, so it becomes the Universal Cure even though what actually happened is completely unrelated.
Were N7WS/Wes' parasitic suppressor Z and Y measurements made with HP's 4191A "voodoo" ?
Superstition goes back a long way with humans, this should be no surprise. (Ever since I've started wearing my lucky ARRL shirt during contests I haven't had equipment failure....)
Some folks may ask why the Ancient Ones didn't have parasitic suppressors in *their* rigs. One need only look at QST or other mag from the 1920s-30s and see high-power transmitters using big glass bottles with nary a suppressor in sight. Nor any shielding, and not much bypassing. What's the deal? (my favorite is the one from 1932 where a pair of 852s were run at 1000 watts input with 4500 volts on the plate, no antenna coupling condenser (full B+ and RF on the antenna, thank you) and open-wood-frame construction. That ham's guardian angel never got ANY rest.)
At least two things were going on back then:
1) Some of those old tubes, circuits, layouts and parts accidentally performed the suppressor function without it being obvious. For example, a tube with a "molded mud" base and socket could be so lossy at VHF that the only way to get it to oscillate up there was to remove the base and not use a socket, yet for its intended use (audio and low-end HF) it was perfectly fine. ("Debasing" of tubes to get them to work at VHF was actually done - I'm not making it up). Etc.
2) Some, probably many, of those old rigs had parasitics galore. They just weren't identified because almost nobody was using the parasitic frequencies at the time.
73 de Jim, N2EY
Epilog: When NHTSA proposed that motor vehicle manufacturers equip their vehicles with air-bags, said manufacturers wailed and moaned loudly.