SB200 RF choke

[wa3dsp]

I posted this on QRZ but thought I would see if anyone here had any input.

One of my winter projects will be rebuilding and updating a Heath SB-200 amplifier. Unfortunately the plate RF choke is physically broken and not repairable. I have not been able to find a commercial choke that would be physically and electrically compatible. I suppose I could wind a new choke. I have a suitable piece of Teflon. The original choke was close wound, no breaks, and measured about 50uh. I am sure Heath did some engineering of this choke but other similar amps seem to have multi-segmented chokes with much higher inductances , 200 uh is common. I don't intend to use the amp above 20 meters.

Any ideas would be appreciated? If anyone has replaced this choke with a commercial value or rewound it let me know your experience.

Doug, WA3DSP

[Pete, WA2CWA]

How about this one for $20: http://www.ameritron.com/Product.php?productid=10-15197

[VE3LYX]

Because of the difficulty finding plate chokes or often the cost of shipping them here is many times the price of the choke I now always make my own. While I am often only working on a homebrew single ended pa or linear I have learned a bit about it. I prefer a ceramic core or some heat resistant core. I have unwound or buffed off the windings from 10 or 20 watt wirewound resistors and wound a rf plate choke there. Mistakes I have made was not enuf turns, (probably no such thing as too many) and not heavy enuf wire. Hardest time for you homemade choke is no signal or is with the PAs I have built. I would suggest you duplicate what you have as best you can following these guidleines ad then give it a smoke test with you right there every minute. Worst case senerio is it will get warm which wil be obvious BUT if you more or less copy what you have there is no good reason why anything bad will happen. Best case senerio is what if it works? I have now  made about 10 I think I will never ever buy another Rf plate choke again I think. When you figure out why they are there it is not hard to come up with a workable reliable solution. When you make stuff there is a bit of fear for sure, that is just normal healthy caution. There are a thousand folks who will try and discourage you because they would never try it. I grew up in the home of a father who was an aircraft radio designer for Collins. He used to say to me this. "One thing for sure that we know Don is if you dont try it, it wont work." As a kid that used to make me angry but when I grew up and worked in my own career although a different field the number of times that thought passed through my headis uncountable. It has served me well. I was talking to dad last week. He is now 89. He was asking about my ham radio stuff and I was relating some of this very stuff you are asking about. (homemade Rf chokes ,screen grid modulation etc ) He asked me did I remember what he used to tell me as a kid. Same saying. I said yes and thanked him for the encouragement. And for all those who would never try I can only offer a big rasberry . You dont know what you are missing.  Go for it. It will work fine. It isnt magic , just plain old science.
Don VE3LYX

[KA0HCP]

http://www.rfparts.com/choke.html#highrfsingle

[wa3dsp]

The RF parts RFC3 $24.95 (same as Ameritron 10-15197 $20) is too large to fit in the SB200. The original choke was 50uh and about 3.5 inches tall. The RFC3 is close to 6 inches tall and larger in diameter. I guess it would have to be as it is more than four times the inductance. I did find a guy that mods the Heath linears for six meters and he has used original chokes for $35 post paid. I am not sure if any of the other chokes RF parts offers would work. I sent them a note asking them what they might recommend. I suspect I may just end up buying the used choke.

I don't know how to determine the current rating for a choke. Are the numbers given an operating current or an absolute maximum. In other words if a choke says 500ma then could I run 500ma through it all day with no problem and exceed that for shorter durations.

[KD6VXI]

I did find a guy that mods the Heath linears for six meters and he has used original chokes for $35 post paid.

If it's Carl or Lou, I've gotten things from both of them, and they where as new.  And priced well below what I thought I'd have to spend.

--Shane
KD6VXI

[KM1H]

Im all out and the used ones at $35 are a rip off; I sold at $15.

At 50-60uH just wind your own on a 1/2" ceramic or Teflon form. The reason its so low a value is that any resonances are above 10M , no engineering involved.

Did you check Fleabay using choke and coil as keywords? Ive found all sorts of good stuff there when sellers dont know what it is.

And Shane, where the heck you been hiding all these months....you OK?

Carl

[wa3dsp]

I wonder if anyone has used iron core material in RF chokes? This article -

http://kk5dr.com/chokes.htm

Seems to indicate that it works well but I am sure there is another side to the story. If it works as well as he claims I am sure you would see them in commercial products.

There is some discussion out there disputing KK5DR's claims but it appears they tried to use ferrite which he points out NOT to use in his article. 

73 Doug

[VE3LYX]

I am sure you would see them in commercial products.


Not an assumption I would ever make now after a lifetime of trying stuff.
It has to physically fit the spot. It has to be robust enuf to stand full plate current plus a safety factor (of say 50%) It has to be NOT reasonant on any portion of HF band or near. In your case I know you would like it to look like original as well. I suspect a lot of the funny winding methods are in fact huey-goey and while they look exotic probably in real life make no real difference. There may be exceptions with difficult cases or where one is cheating on the number of turns or final inductance. Seems to me and I may be wrong here that amp does not do 160m so hence the plate choke doesnt have the higher uh to cover that lower freq area. Probably should have anyway.
There are instructions for making them in a couple of ARRL handbooks I have.  One it seems to me is even about revamping that exact amp of memory serves me. If what you make will handle the current , fit in the space, is non resonant at any freq available in the amp it will keep Rf out of the power supply which is all it is there for.
Don VE3LYX
here is respectively the first Plate choke I ever made (for a 45 tube TNT)
A small one I made for the RF PA section (1930s pentode) of a two tube low power transceiver and an encased one used to feed the plate of a 6293 tube (very HD version of 6146B) des[ite its strange appearance it handles high current for long periods with no ills and works as close to perfect as one could ever want.

[W1RKW]

I wound my own on a 3/4" ceramic form about 5" long for the 813 rig. It's for 75m only but it works. I'm sure one could  find a multi section homebrew type from another project and duplicate it. Unfortunately, I don't know what wire size I used. I think I chose something that could handle 500mA DC or slightly larger.  Not sure. It was a long time ago.

[kb3ouk]

The self-resonant frequency of the choke has to be higher than the maximum frequency that the amp will be used on for the choke to do its job.

[W4AMV]

The self-resonant frequency of the choke has to be higher than the maximum frequency that the amp will be used on for the choke to do its job.

You might want to check an earlier post on this subject. While you are correct, the "Series" self resonant frequency should be out of band, the so-called self resonant frequency (anti-resonant or parallel resonant frequency IS NOT). All these plate chokes are typically self resonant well below 30 MHz, at least the ones I have measured. However, that is not an issue. In the end, the choke simply looks like a small shunt C and is easily absorbed into the matching network. Typical large Plate Chokes are anywhere between 2 pF- to -10 pF and usually with a respectable Q. The choke is basicly a high reactance shunt C with nearly a zero ohm continuity! Of-course, if the shunt C gets out of hand, if the self resonant frequency were absurdly low or the electrical L value TOO LARGE, you are correct and troubles will occur.

Alan

[wa3dsp]

Getting back to the original question about the RF choke for the SB200 -

Carl did not have any.

Lou offered a used one for $35 shipped.

RF Parts has a brand new equivalent - RFC55 for $24.95 + shipping.

I have a piece of Teflon to wind a new one.

At this point I think I will opt for either the RF parts new choke or winding my own.

73 Doug

[kb3ouk]

The self-resonant frequency of the choke has to be higher than the maximum frequency that the amp will be used on for the choke to do its job.

You might want to check an earlier post on this subject. While you are correct, the "Series" self resonant frequency should be out of band, the so-called self resonant frequency (anti-resonant or parallel resonant frequency IS NOT). All these plate chokes are typically self resonant well below 30 MHz, at least the ones I have measured. However, that is not an issue. In the end, the choke simply looks like a small shunt C and is easily absorbed into the matching network. Typical large Plate Chokes are anywhere between 2 pF- to -10 pF and usually with a respectable Q. The choke is basicly a high reactance shunt C with nearly a zero ohm continuity! Of-course, if the shunt C gets out of hand, if the self resonant frequency were absurdly low or the electrical L value TOO LARGE, you are correct and troubles will occur.

Alan

I've saw one homebrew schematic that didn't even use a choke, it used a pi-net between the modulation transformer and the amplifier. The pi-net was designed so that it blocked the RF going towards the power supply but passed the modulated dc coming from the power supply

[W4AMV]

There is certainly motive to not use a choke. In any case, not to stray from the original intent of the post, now that you are going to build one, you will know what to expect. There are many misconcpetions on choke operation and characterisitcs. I have some SB220 like chokes in the junk box. When I have a chance, I will measure and post its characteristics.

[WQ9E]

A bunch of SB-200 parts were just listed buy it now on ebay, contact the seller and maybe he has a choke (if it wasn't already listed and sold).

 

[W8ACR]

I have a big stash of RF chokes. If someone could post a picture of the needed part, I may have one or something similar.

Ron

Addendum: Just now posted on Ebay - original plate choke for SB-200. Buy it now for $24.95. (I have no interest in this ebay listing.)

[Opcom]

I made one myself on 1.25" Teflon rod and it turns out I have to redo it with smaller wire to get the inductance up, but it was very satisfying to have made my own. And will be even more satisfying to make another one properly with what I learned from the first one. The rod came from an internet supplier of indulating shapes and was not too costly as the site specializes in short pieces and small orders. Teflon is easy to drill and tap also. eBay may have rods as well.