Disk cap ? Looking for a 850Pf in feed back circuit. No 850. Will a 800 + 50 work

Discussion in 'Tube Audio' started by drtool, Jan 2, 2018.

  1. drtool

    drtool It might get loud In Houston Subscriber

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    I know I would need to wire in parallel. 1% silver micas I found at Just radios. 500/1000 V. Thanks, stay warm, Wayne
     

     

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  2. Alan0354

    Alan0354 Super Member

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    It should work.

    But if it is feedback from OPT output, you don't need that high voltage requirement. I would look for film cap.
     
  3. Alan0354

    Alan0354 Super Member

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  4. BinaryMike

    BinaryMike Pelagic EE Subscriber

    Some standard values that yield your exact total: 750pF + 100pF, 820pF + 30pF

    Digi-Key & Mouser stock many of these. It's best to avoid anything other than C0G ceramic, polystyrene film, and polypropylene film.
     
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  5. Alan0354

    Alan0354 Super Member

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    Is NPO/COG ceramic caps good enough? That would be really easy.

    Also, if OP willing to go SMD, you can have a much wider selection.
     
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  6. GordonW

    GordonW Speakerfixer Subscriber

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    What amp is it? Is the feedback from the secondary of the output transformer, or is it from somewhere else in the amp? Most feedback caps can be low voltage (the OPT secondary has no more voltage than the peak output voltage to the speaker- usually less than 50V except for huge amps, and less than 150v even for monstrously big amps)- but if it's getting feedback from a tube plate or screen pin (think Dynaco ST70 or Mk III, for an example), then those definitely DO need to be VERY high voltage (even up to 1KV).

    Regards,
    Gordon.
     
  7. Alan0354

    Alan0354 Super Member

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    This is actually my question:

    Is NPO/COG ceramic caps have as low distortion as the film caps? I want to put a low pass filter to roll off some highs. I don't mind the value drift off......so if the poll frequency is 900Hz instead of 1KHz, it does not matter to me. But distortion matters.
     
  8. Hipocrates

    Hipocrates Anti-Muppet

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  9. drtool

    drtool It might get loud In Houston Subscriber

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    GordonW It is one of my Eico HF22. I believe it is secondary. Remember I am still learning this. I have rebuilt one. All stock, sans 3 Rs and all the can, wax caps. All disc caps were reused. One disc cap did have a small crack in it at the bottom of the leg. But for now, it is playing fine. Now this time I am trying to go a little farther on amp #2. So I can learn more. I really love theory and why something will work better, or not, and why. The new 500V disc cap, came in the 1% flavor. So that is why I picked the 500V. I do know one cap is specked at 1K. I know, (or at least believe) on the feedback cap, value is important. I know that if Eico did not speck the voltage, it is going to be low Voltage. I would think there is no downside with a disc cap rated for 500V . I know on some caps you do not want to go crazy high voltage ratings, but on disc caps I believe no problem?

    BinaryMike first time I have heard of COG ceramic. Going to poke around on Mouser. I was wondering if getting values 425 pF + 425 pF would have an advantage over 800 pF + 50 pF. I am leaning toward one polypropylene film cap.

    Hipocrates, thanks for the links.

    Wayne.
     

     

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  10. BinaryMike

    BinaryMike Pelagic EE Subscriber

    I can't think of any reason to prefer a pair of equal value caps except for the possibility of statistical accuracy improvement. There is evidence that higher voltage ratings improve performance of film caps, so that polyester might be acceptable if its voltage rating is far in excess of what's required.

    Also see Jung and Marsh:

    http://www.waltjung.org/PDFs/Picking_Capacitors_1.pdf
    http://www.waltjung.org/PDFs/Picking_Capacitors_2.pdf
     
    Last edited: Jan 9, 2018
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  11. drew_t

    drew_t Well-Known Member

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  12. ConradH

    ConradH Addicted Member

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    NP0/C0G are close to perfect in every way. Very few things would care about the difference between 820 and 850 pF, except for maybe a phono eq section.
     
  13. Alan0354

    Alan0354 Super Member

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    Very interesting result. This is particularly good as Bateman actually design capacitors. I used to think about voltage coef. only, but he added the loss tangent. Looks like the COG/NPO are almost as good as the best of film caps. ( about 3db different when measuring at -126dB!!!) That's more than an order lower than the lowest distortion amp can do.
     
  14. Hipocrates

    Hipocrates Anti-Muppet

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    Pretty cool isn't? Bateman have quite interesting readings there. Even when is hard for me to understand sometimes.
     
  15. triode17

    triode17 Well-Known Member

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    Don't use ceramics in audio circuits. I tried it once as experimental output coupling caps and they sounded terrible. Harsh and spitty treble. Use polystyrene.
     
  16. Alan0354

    Alan0354 Super Member

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    You use it for output? What value you use? Are you using COG/NPO? Output coupling cap usually needs higher value above 1uF. I can't find any COG/NPO this value.
     
  17. Hipocrates

    Hipocrates Anti-Muppet

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    They go until 0.47 or 1uf i think ... polystyrene are similar in values but if you look at them with a solder on your hand they drift in value, they are very heat sensitive.
    So all in all, as you well know it, depends on the application
     

     

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  18. triode17

    triode17 Well-Known Member

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    Please read the orig. thread again. He's referring to a feedback cap. Low voltage.
     
  19. triode17

    triode17 Well-Known Member

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    Polystyrene are excellent audio caps. Yes, they are heat sensitive during installation, but I have been using them for years and haven't damaged any with the iron yet. Just be careful.
     
  20. Alan0354

    Alan0354 Super Member

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    No, I was responding to your post on your experience of ceramic cap as output coupling cap. Nothing to do with the feedback cap. I know using COG/NPO cap for feedback WORKS. I use them in my SS amp and I am getting 0.0035% THD at 60W 20KHz. Way better than any tube amp can do. That's not my question at all.



    My question for you is what value you use? Are you using COG/NPO ceramic. Because regular ceramic caps are really really bad, very high distortion. COG/NPO is a completely different animal all together. Only problem is you cannot use COG/NPO for precision frequency control as the value drift with temperature. That is, if you try to design a band pass or notch filter, don't use COG/NPO as the the peak/notch will drift with temperature.
     
    Last edited: Jan 12, 2018

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