Tone stack change

Discussion in 'Marshall Amps' started by Jimmi59, Aug 4, 2020.

  1. Jimmi59

    Jimmi59 New Member

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    Has anyone changed a JTM 45 tone stack with a JCM 800 tone stack. looking to get less bass out of my JTM 45 clone. Any ideas would be appreciated, was thinking of changing the 250 pf with a 500pf and changing the 56k resistor with a 33k resistor, and the 250ka treble pot with a 250kl will this do anything? good, bad, or am I clearly not thinking this out right as far as tone goes?
    The clone was my first build so I'm looking to gain some knowledge.
    Thanks
     
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  2. Rockat

    Rockat Member Silver Supporting Member

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    Hi cant answer all your questions except if the 56k is the slope resistor, then I recommend you leave it. Changing to 33k will actually add more bass! Hopefully someone else can here can help you more
     
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  3. Jimmi59

    Jimmi59 New Member

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    Thank you, for the feedback. any help is gained knowledge
     
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  4. South Park

    South Park Well-Known Member

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    I believe it is called the Hendrix mod for the tone stack. 33 k slope resister. The tone stack is a two band pass filters so the values can be changed.
     
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  5. Jimmi59

    Jimmi59 New Member

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    Thanks, I appreciate the info
     
  6. marshallmellowed

    marshallmellowed Well-Known Member

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  7. neikeel

    neikeel Well-Known Member

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  8. Jimmi59

    Jimmi59 New Member

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    Last edited: Aug 5, 2020
  9. Jimmi59

    Jimmi59 New Member

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    Changed the tone stack on my JTM 45 clone this weekend, To a JCM 800 tone stack.
    The amp sounds fantastic, the muddy tone is gone it sounds a lot like my SV 20 Head.
    Also changed the master volume from a type 3 in Rob Robinettes pages to a type 4.
    Thanks for all the input from the folks in the fourm.
     
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  10. mixn4him

    mixn4him Active Member

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    Changing the tone stack really wont make it less bassy, it will add mids and highs to balance better. To remove bass you need to change out the V1 bypass cap to something much smaller and output couplers to SL spec. Some people like the scooped mids of the Bass Spec's tone stack.
     
  11. Jimmi59

    Jimmi59 New Member

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    Thanks for the input, like I stated newbe at this any and all help is welcomed.
     
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  12. Fusedbrain

    Fusedbrain Active Member

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    Agreed. I also might have suggested lowering the value of the first coupling cap from .022uf to .0047uf or .0022uf.

    But an even better alternative may be to lower the value of that massive electrolyic cathode bypass cap on V1. If it's something like 220uf or 330uf, try going down to something between 1uf and 4.7uf. This will accomplish the same goal of cutting the amount of bass produced by V1.
    Many times a JTM45 clone will be built with NOS coupling caps, so sourcing a few values of electrolytic bypass cap to try out can be a much easier and cheaper option when experimenting.
    I wouldn't worry about the .1uf output couplers.
    The last JTM45 build I did was a verbatim clone with the 56k / 250pf tone stack, .022uf preamp couplers and .1uf output couplers etc, but the V1 bypass cap was 1uf.
    Still sounded like a nice JTM 45, but just a little tighter and a little less flub in the low end, and not quite as "wooly" when cranked up.

    But anyway, if you like the way your amp sounds now, then mission accomplished and congrats on a successful build!!
     
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  13. South Park

    South Park Well-Known Member

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    Tone shaping starts at the first tube . The tone stack is the last stage of that . The gain stage works the frequencies that are not roll off . The idea is to keep things as leaner as possible . The gain and tone shaping work together . V1 and tone stack is a good place to mod
     
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  14. Jimmi59

    Jimmi59 New Member

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    I do like the sound now, I wanted to do a build so I could experiment with some different mods to learn how to shape tones and and add different things to the amp. I added an effects loop to it and a master volume.
    Could you lower the first coupling cap along with lowering the bypass cap? I mean if I lower the first cap and still want to try to get a different sound then change the bypass?

    Thanks for your input it has really added to my knowledge of this.
     
  15. South Park

    South Park Well-Known Member

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    The coupler caps block DC and let AC pass from one stage to the next tube . You can go smaller and Get A little more gain . The by pass caps you can work with if go different sizes the power curves may not work right for all preamp tubes . You can go with different types of tone caps paper in oil orange drops so many .
     
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  16. Jimmi59

    Jimmi59 New Member

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    Okay, I see what your talking about. Thanks I appreciate it
     
  17. mickeydg5

    mickeydg5 Well-Known Member

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    What exactly is a two band pass filter?

    Some Bassman designs had 100k slope resistor in the tone stack to hinder bass and midrange.
    A 330pF - 470pF or so will allow more highs to flow through the treble control.
     

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