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500-B multi section cap stuffing capacitance question

Discussion in 'Fisher' started by rufleruf, Nov 8, 2018 at 6:54 PM.

  1. rufleruf

    rufleruf AK Subscriber Subscriber

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    I'm working on restuffing some cans and I am wondering how critical the capacitance values are?

    For example, what would happen if I put a 47uf 450 in the place of an original 20uf 300V? I know the voltage is okay, and common sense says the extra capacitance will be fine, since it serves as a reservoir for the voltage at that section level I think, but I am still learning and hate to make assumptions because I know increasing the capacitance is supposed to up the voltage out of the section too

    thx
     
    Last edited: Nov 8, 2018 at 7:33 PM

     

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  2. MikeRam

    MikeRam AK Subscriber Subscriber

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    I asked a similar question and Dave let me know that you don't get much benefit beyond doubling the capacitance, and if you go too much over the voltage rating, the cap will have a harder time keeping its form. That being said, I don't think your values are that much over what is recommended so I say go for it!

    Mike
     
  3. rufleruf

    rufleruf AK Subscriber Subscriber

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    Thanks Mike - so at 47uf, maybe too much? Is there a calculation I can do to see if the voltage increase would be too high?
     
  4. MikeRam

    MikeRam AK Subscriber Subscriber

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    Location:
    Portland Oregon
    Dave Gillespie would be the one to give you a calculation if there is one. I think 47uF is just fine. It is a little over double but not that much. Also, adding 150 volts over 300 is not too bad as far as I can see but maybe Dave will chime in and give his opinion.

    Mike
     
  5. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Matt: You could also see if there is a 39uf 400v-450v cap out there.
     
  6. bhamham

    bhamham AK Subscriber Subscriber

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    I'd use the 47uF/450v and wouldn't worry about it. Ever compare the caps list of a McShane kit to the original values?
     

     

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  7. rufleruf

    rufleruf AK Subscriber Subscriber

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    Thanks guys - I did a lot of reading about electrolytics used in filtering and couldn't find a hard and fast rule - though I found a "not more than about double" reference, which isn't much help!

    I decided to be patient and get the "slightly overs" for the Fisher, but I am going to use what's on hand for a General Radio 740 capacitor checker I hope to get working today. Can cap in it was probably bad when I was born 46 years ago!
     
  8. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    The "not more than Double value" arose out of concern for transformer starting shock when OVERLY LARGE MAIN caps charge up 1st time. An overly large Main filter cap setup can cause a voltage excursion thru the transformer that can damage or ruin it over time. In a Tube unit this can also slam tubes with too high a voltage if not kept under some kind of restraint.....ie: Surge suppression varistor, and or variac @ or below 117VAC. If this is C-91A on the 500C I don't see any reason where it's gonna hurt. It should actually help stiffen the power to the Phono tubes. It shouldn't increase the voltage to the tubes.
     
    Last edited: Nov 10, 2018 at 1:00 PM

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