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Author Topic: SB-200 Parasitic Supressor Kits?  (Read 21640 times)
KB1LKR
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Posts: 1898




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« Reply #30 on: June 07, 2011, 07:14:23 PM »

"You DO know, don't you, that parasitic suppressors have to be wound the same direction as the PA tank inductor?"

but isn't this only true in the Northern hemisphere: when operating "down under" aren't they oppositely wound (unless the tubes are mounted socket up)?

Of course when using a VHF amp to feed a Circularly polarized antenna all bets are off.

Seriously though: Tom, what year, roughly, did the ARRL Handbook review/printing deadline issue occur?
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W0BTU
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« Reply #31 on: June 07, 2011, 07:28:40 PM »

"You DO know, don't you, that parasitic suppressors have to be wound the same direction as the PA tank inductor?"

but isn't this only true in the Northern hemisphere: when operating "down under" aren't they oppositely wound (unless the tubes are mounted socket up)?

Of course when using a VHF amp to feed a Circularly polarized antenna all bets are off.

I'll let you in on a little-known secret: the plate tank coil (and any antenna tuner inductor) has to be wound the same direction as the inductors in the receiver front end of the other station.  There's a 3.1416 dB advantage if you pay attention to that important detail! Grin
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W8JI
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« Reply #32 on: June 09, 2011, 06:10:18 AM »

"You DO know, don't you, that parasitic suppressors have to be wound the same direction as the PA tank inductor?"

but isn't this only true in the Northern hemisphere: when operating "down under" aren't they oppositely wound (unless the tubes are mounted socket up)?

Of course when using a VHF amp to feed a Circularly polarized antenna all bets are off.

Seriously though: Tom, what year, roughly, did the ARRL Handbook review/printing deadline issue occur?

I don't know, it was a long time ago. I would say in the early 1990's. I'm not sure if I still have that paperwork or not, or what shape it would be in after being in a storage barn for years.

It actually was a pretty sad deal for everyone. The ARRL got in a panic because they were on a deadline, the original author never should have been put in the embarrassing position he was put in, it doesn't look good when people appear to gang up on one person. The time for peer review is when the thing is first progressing, not when it is ready to be printed or after (in the case of a QST article) after printing.

Surely there were enough advanced alarm bells that almost everything was over the top.
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K8AXW
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« Reply #33 on: June 09, 2011, 09:15:45 AM »

So Tom, as the original "poster" of this question, let me ask:  Bottom line, are the suppressors and circuit designed by Heath (used by Heath might be more accurate) sufficient for parasitic suppression?

If so, do you have a source of anode parasitic suppressor resistors or can you recommend a size/type of resistor?

I'm sure a lot of the 500+ viewers of this thread would like this answer.  Thank you.

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W1QJ
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« Reply #34 on: June 10, 2011, 08:38:32 AM »

If you don't mind paying $2.50 each for a carbon comp 47 ohm 2 watt resistor to make a new set of standard suppressors, you can buy them and wind a coil yourself around it with some #14 copper wire and you will be good to go for another 30 years.  And yes, a standard type suppressor is just fine.  Unfortunately the resistor is a perishable item in that amp just like the tubes.  Read the post above about the guy who bought a set from me.  Operating on 15 and 10 meters heats up the resistor and more so if you run RTTY, little by little the resistor changes value and becomes less effective.  Change the resistor and you are back in business again.
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KB1LKR
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« Reply #35 on: June 10, 2011, 08:41:18 PM »

Any reason in this day and age *not* to buy a 3W *metal film* from Mouser or DigiKey for $0.60 and wind a copper coil around it, instead of around a carbon Comp?
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W0BTU
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« Reply #36 on: June 10, 2011, 09:18:27 PM »

As long as it's non-inductive. Most resistors are inductive, because they adjust the resistance by grinding a spiral grove in the resistance material before the outer coating is applied.

An Ohmite type OY (2 watts) is non-inductive, and should work.
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G3RZP
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« Reply #37 on: June 11, 2011, 02:00:46 AM »

How 'inductive' or 'non-inductive' is required? In other words, how many nanohenries can one stand in the resistor?
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W0BTU
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« Reply #38 on: June 11, 2011, 03:37:50 AM »

Technically speaking, all resistors have inductance, just like every straight piece of wire has inductance. A resistor either has an internal spiral structure (inductive), or it doesn't (non-inductive). The spiral structures I've seen in resistors that I've broken open or ground into had many more turns (and therefore more inductance) than the copper wire coil in parallel with the resistor in the suppressor.
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W8JI
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« Reply #39 on: June 11, 2011, 05:02:56 AM »

So Tom, as the original "poster" of this question, let me ask:  Bottom line, are the suppressors and circuit designed by Heath (used by Heath might be more accurate) sufficient for parasitic suppression?

If so, do you have a source of anode parasitic suppressor resistors or can you recommend a size/type of resistor?

I'm sure a lot of the 500+ viewers of this thread would like this answer.  Thank you.




Sorry for the delay, I don't check this forum often.

The Heath suppressors are fine.

The correct resistor has to be non-inductive, it cannot be a spiral wound metal film or carbon. See this link:

http://www.w8ji.com/images/Carbon_Metal_resistors.jpg

and this

http://www.w8ji.com/vhf_stability.htm

I would use an Ohmite OX or OY resistor. They are superior replacements for carbon comp resistors.

73 Tom

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K8AXW
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Posts: 3827




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« Reply #40 on: June 11, 2011, 09:06:35 AM »

Tom:  Continuing on with this discussion.....I read most of the information on this link:

http://www.w8ji.com/vhf_stability.htm

While it explains a great deal it will require 2 or more readings to digest the most important information.

Now, since most amplifiers use either 2 X 3-500Z or 2 X 572B tubes, give us a good design for anode parasitic suppressors, including number of coil turns and resistor values.  Please.

I'm quite sure this information will be welcomed. 

So far we have learned that Ohmite XO or XY resistors are ideal for parasitic suppressors.  Please finish the design.

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N4UE
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Posts: 292




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« Reply #41 on: June 11, 2011, 03:48:06 PM »

VERY interesting discussion. Tom and Lou, what would you suggest as a suppressor for a dedicated 6 Meter only amp, using a pair of 3-500Zs?

thank you!

ron
N4UE
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KE7KLY
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Posts: 65




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« Reply #42 on: June 11, 2011, 05:44:49 PM »

This is all very interesting stuff, --- mostly wayyy over my head.
I have an SB-200 that I have never used,  I would like to 'tune' it up to run with my 830s.  It had been my intention to do the Harbach mods (new caps, soft start, and soft key) -- but now it looks like I should be replacing these suppressor resistors also. 
If anyone can send me to a link, website, or discussion that would help me know everything I should do to 'tune' this baby up before I use it,  I would be much appreciative.
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W8JI
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« Reply #43 on: June 12, 2011, 06:44:17 AM »

Tom:  Continuing on with this discussion.....I read most of the information on this link:

http://www.w8ji.com/vhf_stability.htm

While it explains a great deal it will require 2 or more readings to digest the most important information.

Now, since most amplifiers use either 2 X 3-500Z or 2 X 572B tubes, give us a good design for anode parasitic suppressors, including number of coil turns and resistor values.  Please.

I'm quite sure this information will be welcomed. 

So far we have learned that Ohmite OX or XY resistors are ideal for parasitic suppressors.  Please finish the design.



I cannot finish the design without making up a big tall tale.

Anyone telling people they have a universal suppressor design for a certain tube type, that works in all amps using that tube type optimally, is feeding people pure nonsense.

The suppressor is part of a SYSTEM that involves the anode system in the tube and all the leads and components from the anode to the chassis, AND back through the chassis to the cathode and grids of the tube.

The likely frequency of oscillation, if oscillation occurs,  depends mostly on the control grid path characteristics in grounded grid triodes. To oscillate at VHF, the grid has to become divorced from the chassis at a frequency where the ***internal*** elements of the tube are still producing gain. So even grid path impedance, from inside the tube itself to the chassis, will play a role in determining if and when a parasitic suppressor is necessary and what the optimum impedance of the suppressor would be.

Anyone ever telling people they can sell a "slap in" design based on tube types is deluding everyone. Some amps, in particular using tubes with grid flanges like the 3CX800A7 and 8877, can be unconditionally stable with no suppressor at all.


As a matter of fact I'm working on an AL800H amp right now, and I pulled the suppressors totally and it remains 100% stable. When I built the very first AL800H amp prototypes, they were 100% stable with 3500 volts on the anode and NO suppressors at all. I put suppressors in because Hams give people some much crap about needing suppressors, it was easier long term to add unnecessary parts than to face questions about why something expected to be there is not there. So obviously I could sell anything as a suppressor for that amp and it would work (like I could an AL1500) because they are 100% stable, even if anode voltage is nearly doubled, under all tuning conditions. This is because the grids have a VERY short low-impedance connection to the chassis.

The same is not true for a 572B or an 811 tube, which have long skinny grid leads. Even with the grids directly grounded they can be unstable, as can be a 3-500Z and 3CX1200D7 or A7.

A 3CX1200Z7 is unconditionally stable because of the conical grid support and flange. Its sister tubes, the D7 and A7, are not.

As a general rule this is how it works:

1.) Longer thinner grid leads move the tendency for oscillation lower in frequency. This means the suppressor needs more INDUCTANCE.

2.) Longer thinner anode leads, and longer "thinner" paths through the chassis back to the tube, require more suppressor impedance to have an effect.

3.) Some resistance value will maximize damping. This is generally where the resistor dominates the impedance at the parallel resonant frequency of the grid (where the grid is divorced from ground). At that frequency, a higher anode to chassis path impedance  would require a higher resistance resistor. A lower impedance path would require a lower resistance for optimum damping.

Heath, if the engineers were any good, went all through this when they did they did the SB200. Any other manufacturer, if they are any good at design, would do the same. They would have also checked stability at maybe a 30-100% overrun of plate voltage with the tube biased at various voltages on all bands with all control settings to check for unconditional stability.

It is unlikely some 5-minute Ham design would improve on something like that.

As a matter of fact if you do a stability test on an AL80B and substitute a certain west coast nichrome kit, you will find it becomes substantially LESS stable. In either case it is OK at normal voltages, but when run through the same test of over-voltage and control settings it will take off like a buzz saw with the nichrome kit. This is because the inductance of the nichrome kit is too low, so the resistor does not load the circuit enough at upper VHF where the tube oscillates.

The SB220 Heath requires even more inductance than the AL80B does, because of socketing and grid leads. So an optimum suppressor in the AL80B has half the inductance of one for the SB220. The same nichrome kit that has too little inductance for the AL80 amp is even further off in the SB220, and the tube types are the same. The only difference is in socketing and impedances of paths from the grid to chassis and anode to tube circuit in each of them.

If I told you I could give you one optimum design for a certain tube type, it would clearly show I did not really understand how amplifiers work.

My advice to anyone would be if the amp was properly engineered, duplicate what they had. If the resistor is bad, replace it. Carbon resistors age fast, especially when hot. Use an OX or OY metal composition of the same value and use the same size inductor as the original suppressor.

73 Tom

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N4NYY
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« Reply #44 on: June 12, 2011, 07:51:07 AM »

Quote

Tom,

I am working on a HW-101 and also have a SB-200 for a winter project. I have little to no experience with tube rigs. I replaced the suppressor resistors in the HW-101 with the metal oxide (like upper right in pic, except I think they were pink in color). The replacements were 3W, when the originals were 2W. The originals were nasty and bubbly looking.

I went to 3W for a reason. It appeared they originals were underrated, but that was just a visual observation. Should I go back to 2W? I have not gotten to the point of firing up the rig, yet.

Also, I have run into problems with the same wattage Metal Oxides being physically much smaller, and the leads don't reach. Having little expertise in high power amps, I'd figure I ask you.

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