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Author Topic: Basic L/R VHF Parasitic Oscillation Suppressor Design  (Read 31432 times)
W8JI
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« Reply #120 on: October 18, 2011, 01:29:05 PM »

However, I have to commend him on how may Ni Cr suppressors he has sold to SB-220 owners and others.  I'll say this, he is a good marketing man.  Lou


A good marketing man has the one simple universal fix for every problem, portrays himself as the only one in the world that understands the problem, and makes everyone else look like they are money grabbers or conspiring against public good.

There is nothing wrong with that, until they start denigrating those who interfear with their marketing.
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G3RZP
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« Reply #121 on: October 18, 2011, 02:04:44 PM »

If one is such an expert, one should be able to say as a consultant, how many professional companies one has as customers.

I understand W8JI can so do.....as can G3RZP
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N2EY
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« Reply #122 on: October 18, 2011, 02:32:01 PM »

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.

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.

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.

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. 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.

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.  

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. 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.

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. 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. 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.

It should be obvious to anyone who understands the basics of AC circuit theory that there is absolutely no advantage to using a coil of low Q, nor is there any possible advantage to using an inductive resistor. What's needed are parts which are as close to ideal as possible.

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.

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. 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.

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.

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.

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.

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







« Last Edit: October 18, 2011, 02:34:07 PM by N2EY » Logged
AG6K
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« Reply #123 on: October 18, 2011, 02:35:44 PM »

If one is such an expert, one should be able to say as a consultant, how many professional companies one has as customers.

I understand W8JI can so do.....as can G3RZP

  chortle. Peter joins the club.
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G3RZP
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« Reply #124 on: October 19, 2011, 02:42:16 AM »

Buit can AG6K list how many professional customers he has?
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G3RZP
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« Reply #125 on: October 19, 2011, 02:48:57 AM »

We also should not forget some of Rich's statements earlier on this thread that are completely and demonstrably wrong.
 
e.g. The Q of a parallel L-R circuit can be decreased by increasing R

e.g. The gain of an amplifier is gm/RL

These are such basic errors in AC theory and electronics that it is like the clock that strikes 13 - you can't believe anything else it strikes, either.
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AB5Q
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« Reply #126 on: October 19, 2011, 05:47:07 AM »

N2EY - Well put, my thoughts exactly....

73,
John -AB5Q
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AG6K
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« Reply #127 on: October 19, 2011, 07:09:38 AM »

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.
[/quote]

  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.  

Quote
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.

Quote
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.

  Well put.

Quote
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.

Quote
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.

Quote
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.  

Quote
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).

  correctamundo Jim

Quote
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.

Quote
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 ?

Quote
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.

Quote
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.

Quote
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,
Quote

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.

Quote
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.
Quote

  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.  

Quote
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.  

Quote
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" ?

Quote
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....)

  chortle.  

Quote
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.  


cheers
Rich, ag6k







[/quote]
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AG6K
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« Reply #128 on: October 19, 2011, 07:18:55 AM »

We also should not forget some of Rich's statements earlier on this thread that are completely and demonstrably wrong.
 
e.g. The Q of a parallel L-R circuit can be decreased by increasing R

e.g. The gain of an amplifier is gm/RL

  So if an amplifier has X amount of amplification with RL=2000-ohms, it will have 1000X as much amplification with RL=2-ohms and 2000X as much amplification with RL=1-ohm ?

Rich, ag6k
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AG6K
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« Reply #129 on: October 19, 2011, 07:38:51 AM »

AXW, This is about the 4th time around for me in the last 20 years following the parasitic thread with Tom and Rich.  Needless to say I was as frustrated as you on which ring to toss my hat into.  I'm no engineer in this field by any stretch of the imagination, I am basically a seat of the pants guy and when I can't ascertain for myself just what the "truth" is I simply use logic and my seat of the pants experience.  I made my decision several years ago on which ring to throw my hat in with.  Tom always seems to back up what he says by actually showing screen shots of different test equipment on his website, not only that, this is his business, people pay him the big bucks to design all sorts of amplifiers both ham and commercial alike.  I figure if anyone is in the market for any sort of amplifier they have done their homework on who they want to design their equipment.  To my knowledge I haven't heard of Rich designing anything but the "UGLY AMPLIFIER".  However, I have to commend him on how may Ni Cr suppressors he has sold to SB-220 owners and others.  I'll say this, he is a good marketing man.  Lou

  When was the last time you saw an advertisement for low VHF-Q parasitic suppressors?
Rich, ag6k
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W8JI
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« Reply #130 on: October 19, 2011, 07:48:51 AM »

It's easy to analyze the suppressor system, or to measure it with proper equipment. There is no debate, and there is no need for all the diversions and personal BS people like to use to either justify bad science or obscure good science.

Diversion of the issue to things like air bags or telling us what other people have said is a common tactic of those who don't have anything to support their junk science.
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G3RZP
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« Reply #131 on: October 19, 2011, 08:38:30 AM »

>So if an amplifier has X amount of amplification with RL=2000-ohms, it will have 1000X as much amplification with RL=2-ohms and 2000X as much amplification with RL=1-ohm ?<

That is exactly what your statement that the gain is gm/RL means.

You said it.......
« Last Edit: October 19, 2011, 08:43:13 PM by K5TR » Logged
K8AXW
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« Reply #132 on: October 19, 2011, 09:42:11 AM »

RZP-JI-6K-QJ -  While all of this discussion is very interesting and some of it is understood, I'd like for you guys to come back to planet Earth for a moment.  We have a guy in Cyprus about to replicate a set of parasitic suppressors for his recently acquired SB-200.

I'm quite sure he is in the process of obtaining the OY resistors needed to do this but since they come in a power rating lower than the originals, the plan is to parallel two 100 ohm resistors with the 3 to 3.5 turn coil.

My question for the moment, in his behalf, is how does he do the paralleling?  Two resistors side-by-side with the coil wrapped around one resistor or does he wire the two resistors in parallel and then connect the coil 'outside' of the two resistor combination? Spacing between components?

Or if neither is correct, what would be the proper way of creating this combination.

Tom, I understand (or at least I think I do) that the amplifier should be "tested" to determine if these values are suitable but a starting point needs to be established.  This starting point would be the correct assembly.
« Last Edit: October 19, 2011, 09:46:54 AM by K8AXW » Logged
W8JI
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« Reply #133 on: October 19, 2011, 09:50:32 AM »

RZP-JI-6K-QJ -  While all of this discussion is very interesting and some of it is understood, I'd like for you guys to come back to planet Earth for a moment.  We have a guy in Cyprus about to replicate a set of parasitic suppressors for his recently acquired SB-200.

I'm quite sure he is in the process of obtaining the OY resistors needed to do this but since they come in a power rating lower than the originals, the plan is to parallel two 100 ohm resistors with the 3 to 3.5 turn coil.

My question for the moment, in his behalf, is how does he do the paralleling?  Two resistors side-by-side with the coil wrapped around one resistor or does he wire the two resistors in parallel and then connect the coil 'outside' of the two resistor combination? Spacing between components?

Or if neither is correct, what would be the proper way of creating this combination.

Tom, I understand (or at least I think I do) that the amplifier should be "tested" to determine if these values are suitable but a starting point needs to be established.  This starting point would be the correct assembly.

The only ill effect of too much inductance is needless ten meter resistor heat.  I would start with more than 3 turns. It is easier to remove turns or spread turns to reduce ten meter resistor heating than to wind more turns to increase stability.
Even if three originally worked, I find very few 572 amps stable with such low inductance.   I wouldn't worry much about a composition resistor being inside the coil. A slight change in turns would have the same effect as moving it outside.
« Last Edit: October 19, 2011, 09:54:11 AM by W8JI » Logged
G3RZP
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« Reply #134 on: October 19, 2011, 09:53:32 AM »

Putting three resistors in parallel with the coil around all of them worked well in a 572B amp made in the hundreds in the UK.

Can't remember how many turns.
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