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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. Hipocrates

    Hipocrates Anti-Muppet Subscriber

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    I know
     

     

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  2. triode17

    triode17 Super Member

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    Alan, The value drift with temperature is a good reason to use polysty. as they are very stable with temperature.
    I don't use them for coupling caps because I can't get high enough values in polysty. They generally stop at 10,000pF. Plus the small, clear ones are rated 50v. Russia does sell 450v versions though , I believe. I use polypropylene for couplers. Or Russian paper in oil caps.
     
  3. BinaryMike

    BinaryMike Pelagic EE Subscriber

    Unless I'm badly mistaken, C0G/NP0 caps are used in RF circuits specifically because they don't drift with temperature. Other class-1 ceramic formulations are available with controlled drift characteristics, to compensate thermal drift in precision tuned circuits.
     
  4. Alan0354

    Alan0354 Super Member

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    Ha!!! Tell me more, I love to be wrong on this.
     
  5. I LIKE MUSIC

    I LIKE MUSIC Super Member

    NP0 means negative-positive zero in terms of capacitance change.

    upload_2018-1-12_21-9-16.png
     
    Last edited: Jan 12, 2018
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  6. BinaryMike

    BinaryMike Pelagic EE Subscriber

    There's a table of the various available temperature coefficients for class-1 caps in this article on Wikipedia.
     
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  7. Alan0354

    Alan0354 Super Member

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    Guys, I am happy to be wrong. I assume all ceramics drift!!!

    So the next question is why are we still using film caps for low values values like below 0.0022uF?
     
  8. PakProtector

    PakProtector AK member

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    Have seen many caps in the FB path and they are usually Silver/Mica. The question is 'why'? Do film caps have more ESR or ESL that can do things that are less than useful? Consider just exactly what these have to do, and what makes the Mica caps useful. Then plug it in and have some measurements and a good listen. There are more SS amps out there with vanishingly low distortion figures that are quite un-listenable than I care to comtemplate...so don't count on a single measurement to tell the tale, yes?
    cheers,
    Douglas
     
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  9. drtool

    drtool It might get loud In Houston Subscriber

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    Yes Douglas. Having La Scallas new in 82. Having a number of SS amps, none sounded anywhere as good with them as my DG SE Maggie.
     
  10. ConradH

    ConradH Addicted Member

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    C0G/NP0 caps are second to none for most parameters. Oddly, though they can make them have close to zero TC, very few are dead on zero. They actually seem to shoot for slightly off zero, but it's still very low. Now you can get them in MLCC form, so higher values are available than used to be. Silver-mica is a strange beast. They have very low losses (dissipation factor), yet have higher than expected dielectric absorption. I don't detect any sonic problem with them, but one has to be a bit suspicious. They're getting quite expensive considering they only compete with C0G/NP0 if you need higher voltage or high values- though those tend to be huge and crazy expensive. I think the largest I've got is 65000 pF, 0.5%.

    There's a trick used for precision measurement bridges like some old General Radio. The TC of silver-mica goes one way. The TC of polystyrene goes the other. If you proportion a parallel combination of the two correctly, the resulting TC will be near zero. That's how they got their reference caps, usually about 0.15 uF, to be temperature stable.

    It's always horses for courses. Not every circuit needs a low loss capacitor, and some will work very poorly if you use one. Not every circuit needs great TC, though it's rarely a bad thing.
     
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  11. PakProtector

    PakProtector AK member

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