capacitor confusion

Discussion in 'Marshall Amps' started by hotpockitrockit, Feb 17, 2020.

  1. hotpockitrockit

    hotpockitrockit New Member

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    hello all,
    im new to tweaking Marshalls and have a question.
    I own a 1976 JMP 50 lead MKII (metal panel, rocker switch 4 hole PCB) and I noticed the bright cap on ch1 high input is missing.
    now the confusion im having is that when I look online for a replacement there are a million different people saying a million different things.. I would like to know what is the STOCK value and type of cap for this amp including the voltage of the cap. on my schematic it says ".005" (im assuming this means 5000pf or 500uF.) it doesn't say what voltage (im assuming 500v or higher)
    also, does the type of cap affect the tone? ceramic disk, metal film etc.
    thank you in advance, this can get overwhelming fast haha
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    You may also find that the bright cap was removed because the amp is just too bright....a lot of people say this and take the cap out.

    It would be 5000 pf but the closest standard value is 4700 pf.
    Mostly the factory used ceramic disk caps and they were rated 450-500 volts.

    [​IMG]
    It's become common to use silver mica caps also in guitar amps for these purposes.
    upload_2020-2-17_13-45-50.jpeg A silver mica cap.

    One other type of cap is a ceramic dog bone which you will find in older tube amps mostly from the 70s and earlier.

    upload_2020-2-17_13-43-51.jpeg A ceramic dog bone.

    The type of cap does change the sound in small ways mostly but there is no real way to describe tone unless you stick the cap in and listen to it yourself....
    it's just a matter of personal taste or preference. I use silver mica caps mostly for those high frequency circuits.
     
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  3. hotpockitrockit

    hotpockitrockit New Member

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    thanks so much, id like to try the stock value or others and see what works for me. when I play ch 1 I have the presence and treble all the way up and its not killing me, so id like to put a cap back in then I can blend the channels to my liking
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    The trick is to crank the amp to at least "7."
    That's when it mellows out and becomes smooth.
    People who mostly complain are playing the amp at "2." They are always crying about "ice pick" highs, or they are calling the amp "harsh."
    Marshalls do not like to be played on 2.

    The trick is to crank it until the high frequencies compress. At that point, the lows will be clean the the top will be crunchy.
     
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  5. hotpockitrockit

    hotpockitrockit New Member

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    I play at or near full volume, as well as 4 or 5 with an attenuator. I know at full volume the cap will be out of the circuit, but I do roll the volume back to get less gain. so im curious to see what effect the bright cap will have at those volumes
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    At full volume rolled back a bit, it will work perfect. Like 7-9 is about right.
    It adds a little crispiness in the top end.
    I would use like 50-250 pf instead of 5000pf.
    Even 500 pf might be too much for me.
     
  7. hotpockitrockit

    hotpockitrockit New Member

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    ya 100pf seems to be popular, ill have to play around and see what works for me. thanks for all the help.
     
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  8. MarshallDog

    MarshallDog Well-Known Member Premium Member

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    I Have a 74. The brite cap is gone and I have a 100 pf cap i stalled sounds amazing!
     
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  9. black knight

    black knight Member

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    Ampmad, Is there a reason the voltage rating is so high (450 - 500V)? What would the consequences be of using say a 50V rated cap in this position?
     
  10. junk notes

    junk notes Well-Known Member

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    Q: What is the cab?
     
  11. junk notes

    junk notes Well-Known Member

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    Q: What is the cab?
     
  12. ampmadscientist

    ampmadscientist Well-Known Member

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    You can use a 50V cap if you want. It won't hurt anything.
    Traditionally the caps in tube amps were 500V but for the bright cap it's not critical to use 500V.
    I do use 500V but I also know 50V will work fine.
     
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  13. ampmadscientist

    ampmadscientist Well-Known Member

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    Are you talking about the speaker?
     
  14. mickeydg5

    mickeydg5 Well-Known Member

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    Same topic in another thread.
    http://www.marshallforum.com/threads/marshall-jcm-800-2204-1985-gain.111382/

    All capacitors have both DC and AC characteristics. You cannot blow off the AC component at the capacitor as if it does not matter.
    A pushed INPUT can give the first gain stage a considerable swing.

    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.

    If a capactor is only good for 20VAC and the signal passing is 25VAC then it is being compromised.
    That is plain and simple electronics.

    Push the front of one of these amplifiers and measure the signal at those circumstances on the apprpriate capacitors in order to actually evaluate.
     
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  15. hotpockitrockit

    hotpockitrockit New Member

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    im running it through my mesa boogie 4x12 slant cab with v30s
     
  16. South Park

    South Park Well-Known Member

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    You should solder some leads and clips so you can try different caps
     
  17. ampmadscientist

    ampmadscientist Well-Known Member

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    There are 3 caps in series.
    The entire voltage of the preamp is not across the bright cap. That's why a 50V bright cap will be OK.
     
  18. spacerocker

    spacerocker Well-Known Member

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    As a side issue:


    AMS - Can you explain why this would be? As an electronics Engineer, I have always maintained that it is only the VALUE of the cap that matters (combined with the tolerance). Now I am aware that real-world capacitors have other things going on, like ESR, series resistance, leakage (parallel resistance) and Inductance - but at audio frequencies, I would expect these other effects to be negligible! - I am prepared to accept that there could be a difference - but I have never heard anyone put forward a technical explanation (in terms of the parameters I have described) as to why that should be?
     
  19. ampmadscientist

    ampmadscientist Well-Known Member

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    Because the value of a disk cap changes when the voltage changes....it's not what you would call the "most stable." :lol:
    There is so many different types of caps, with so many different audio response and temperature ranges, they are not all the same.
    Different caps also have different "Q" factors.

    2. When 1 capacitor is near another capacitor, it forms a 3rd capacitor between the 2.
    This is stray capacitance.
    In a guitar amp this stray capacitance makes a wild difference.

    In this circuit: there is 3 capacitors in series. The whole AC audio voltage is not across just one capacitor.
     
    Last edited: Feb 18, 2020
  20. spacerocker

    spacerocker Well-Known Member

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    Great - so you are saying the efficiency of the capacitor (it's ability to store charge, compared to energy dissipation due to ESR) changes with construction type, voltage, frequency, etc - OK I get that. Thanks! That is the first technical explanation I have heard. People usually say "It just sounds better"!
     
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