Marshall JCM 800 2204 (1985) gain

Discussion in 'The Workbench' started by CROWEMAG, Jan 23, 2020.

  1. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    You can use that if you want...
    There is no DC voltage at that point in the circuit. It's just audio signal.
    If you were using this capacitor for a DC circuit, you would need to find the voltage rating of the capacitor.
    If you look you will see that there is no ratings or specs for the caps you are buying.
    Normally if I were buying parts, I would buy rated parts.
     
  2. CROWEMAG

    CROWEMAG Member

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    Would this one be better? https://www.amazon.com/001uf-ORANGE...words=.001uf+capacitor&qid=1581528428&sr=8-21
     
  3. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    The 600V rating works. I would not buy anything that does not give basic specifications or at least brand/model so that I can look it up. The other capacitor's voltage ratings may be too low for the AC signal. A lot of people go for a nice ceramic or polypropylene in that position.
     
  4. neikeel

    neikeel Well-Known Member

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    1kV is 1000pF ceramic disc is basic spec.
     
  5. CROWEMAG

    CROWEMAG Member

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    Is 50V too low to use?
     
  6. CROWEMAG

    CROWEMAG Member

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  7. South Park

    South Park Well-Known Member

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    400 volt is the lowest you want for coupler caps . For a bright cap try it
     
  8. neikeel

    neikeel Well-Known Member

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    This is low voltage in this site (potentiometer) bypass cap. Only down side of the ones you have bought is that they are very small. That is why I would use 1kV size.
     
  9. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    That would be the lowest rating to use but with no, I repeat, no boosting between the guitar and input.
    Enough boost up front could compromise the AC component of that capacitor.

    Honestly, there is no reason not to just go with 100V or greater.
     
  10. CROWEMAG

    CROWEMAG Member

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    I put the 50v guy in there and immediately the urgency and grit of an 800 I am used to reappeared but now I am worried. I have people telling me voltage rating doesn’t matter for a bright cap and and people saying it does! Hahahaha. I do boost my 800 with a Boss SD1. Are you saying that could be problematic and possibly ruin the component? I am ok with that as I am in a testing phase any way. I just don’t want it to ruin anything else.
     
    Last edited: Feb 15, 2020 at 9:16 AM
  11. neikeel

    neikeel Well-Known Member

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    Test it and get a better rated one on its way.
     
  12. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    The deal is with capacitors, you really need to look up actual specifications, is that the AC rating is anywhere from 1/3 to 2/3 of the DC rating.
    So a 50VDC capacitor is probably good for an AC value of 16VAC to 33VAC.

    Do this. Take a multimeter, place it on AC voltage and measure the AC voltage across the capacitor at all preamp volume positions with whatever you have to cram or maximize the signal to the amplifier's input.
    Let us know what you find.
     
    Last edited: Feb 15, 2020 at 10:51 PM
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  13. CROWEMAG

    CROWEMAG Member

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    I removed it and have a 1k v unit incoming. Thanks for the advice.
     
  14. South Park

    South Park Well-Known Member

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    The bright cap is not in the power rail. So what is the power rating of bright cap that Marshall used in the first place.
     
  15. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    We do not know. Do tell us.
    Marshall is the great know it all and always had it right.
     
  16. South Park

    South Park Well-Known Member

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    The bright cap is bleeding off guitar signal . No power is in involved just signal . So non rated caps will work .
     
  17. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    What you just said makes little sense, electronically speaking.
    Can you please speak proper electronic language in proper English?
     
    Last edited: Feb 16, 2020 at 5:06 PM
  18. South Park

    South Park Well-Known Member

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    So how much power is going Thur the bright cap.
     
  19. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    You should evaluate that and let us know.
    No one has given that information yet. You should be the first.
     
  20. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    The factory has used 500 volt bright caps but there is no DC voltage in that control circuit.
    In circuits where the DC is already de-coupled (like this one) 50 volt caps have been used.
    So there is no real problem with using a 50V cap technically for audio bypass.
    Personally I use 500 volt caps but it's not critical.

    If you notice there is series resistors hooked to the bright cap.
    470K from the plate side and up to 1 Meg on the volume control side.
    You will also notice that there are actually 3 capacitors in series, this will increase the voltage capability.
    True the audio voltage can be 225 volts AC...
    it's not really the entire audio voltage that is across the bright cap; because it's in series with other caps.
    and the series caps /resistor severely limit the possible current if there was any.
    marshall2204cap.png
     
    Last edited: Feb 16, 2020 at 6:04 AM
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