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parasitic choke

Discussion in 'Amplifiers' started by Crusher, Dec 12, 2010.

  1. Crusher

    Crusher Active Member

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    anyone have suggestions on making a parasitic choke for a sb220 amp. was thinking either using 68ohm 3w resistor and wrap 3-4 turns around it. metal oxide of course. Or use a couple of 3w 100ohm resistors in parallel and then parallel a small 3 turn coil about 3/8" diameter across these. either one will probably work. just wonder what everyone has used. i also have 100ohm 5 watt resistors metal oxide.


     
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  2. wavrider

    wavrider W9WDX Amateur Radio Club Member

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  3. Toll_Free

    Toll_Free Active Member

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    Agreed. Best part is, call Rich up, tell him your tubes, and plate voltages, you'll have a set of suppressors delivered.

    BUT, if you are a "ten" meter operator, beware! The Nichrome suppressors at 10 meters will turn a slight orange when you're keyed up.... The nichrome DOES dissipate some power at 10.

    --Toll_Free
     
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  4. Crusher

    Crusher Active Member

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    yup, it would be on 11 meter band, so I am sure it would heat up as well.
     
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  5. Crusher

    Crusher Active Member

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    I was reading a few articles and looks like copper and silver are the best conductors. Nichrome is very lossy and best, or using stainless next for a parasitic choke as the have a much lower Q and more lossy. I think I read about 10 times more lossy. How does brass compare. Didn't find a chart and was wondering. Have lots of brass stock and figured would have more loss and maybe use this instead of braid strap as well. Any input would be welcome. Also the articles above were very helpful. Thank you.
    .
     
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  6. Toll_Free

    Toll_Free Active Member

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    Brass makes a great parasitic choke... Just make sure you look at a 'skin depth gauge' and ensure you're using brass that's thick enough....

    I PREFER to make hairpins for my inductors.... I've had better luck with them, you can assemble the resistors to be IN the airflow, AND that helps keep them fresh, so to speak... Look to the Pride DX300 / KW-1 for this type.

    --Toll_Free
     
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  7. Crusher

    Crusher Active Member

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    Yeah, i know whatyou are talking about. On my 35B project, i used 2 100ohm 5W resistors in parallel and then did a horseshoe. if i remember correctly end to end about 1.5" and 2" deep.
     
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  8. Crusher

    Crusher Active Member

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    will have to check, but have plenty of 1/4", 1/2", 3/4" and 1" .032 and .064 thick brass stock.
     
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  9. Crusher

    Crusher Active Member

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    ended up using .032 x 1/4" for parasitics, then used same thickness but half inch wide to blocking cap. which i replaced with same value, but double voltage.
     
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  10. Beetle

    Beetle Well-Known Member

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    Be sure and read that article, in which Measures gives the reasoning behind using Nichrome wire, even with its losses. Don't disregard this - go for a low-Q on the suppressors.
     
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  11. Crusher

    Crusher Active Member

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    I read it, it is very informative. Definitely a great arcticle for reading. I am going to see how the brass works and if not than I will go with the Nichrome setup.
     
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  12. Crusher

    Crusher Active Member

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    Was on ebay, picked up a 50' roll of 18 gauge NiChrome wire. If the brass doesn't work, I will use this stuff.
     
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  13. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    50'

    That is enough to build parasitic chokes for a few years.

    Let us know how it turns out.

    I salvaged a few plate transformers from some medical equipment today.
    Got plenty of KV's for that all bander pair of 3-500's I am planning to build.
     
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