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JCA22H too bright

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  • JCA22H too bright

    Really digging my new JCA22H. About the only thing I'd like to change is the treble response. Its very bright.Ive tried a zillion different tubes-new and old stock and different speaker pairings. Right now I leave the presence off and the treble on 1 . Would be nice to dial down the treble output and be able to use the treble knob for varying cabs and such. Any idea's? Thinking some simple "treble cut" with a pot and cap. TIA Bob

    Schematic here. Couldn't figure out how to link it directly.

    Last edited by rockon1; 12-30-2013, 12:33 AM.
    "Reality is an illusion albeit a very persistant one " Albert Einstein

  • #2
    It appears to be a Marshall tone Stack.

    The treble cap (C14) is 470pf.
    Both mid & bass caps (C15 & C16) are .022uf.

    I would download Duncanamps Tone Stack Calculator & play around with it.TSC
    I do not think there is much to be done in the tonestack, short of a redesign.

    Edit: I ran the calculator.

    Turning down both the Mid & the Treble seems to kill the treble.


    • #3
      Hmmm...I like the mids up but treble almost off. Wonder what a different slope resistor would do? Bob
      "Reality is an illusion albeit a very persistant one " Albert Einstein


      • #4
        Run the calculator.


        • #5
          OK ran the calculator a bit. Maybe a 500k or 1M treble pot? Bob

          Nevermind...that wont work.
          Last edited by rockon1; 12-30-2013, 01:18 AM.
          "Reality is an illusion albeit a very persistant one " Albert Einstein


          • #6
            Here is a nice read on Fender & Marshall stacks, with Duncan TS sweeps.

            Link: GM Arts - Guitar Amplifiers


            • #7
              I know it may sound like a lame idea but have you tried turning all of the controls off?


              • #8
                Thanks for the link! Good read. I haven't turned them all off but have dropped the treble and mids real low. I'll tinker with adjusting the controls with my ears instead of eyes. Some one mentioned on another forum to try bumping up R21(grid stopper) to 470k. Any thoughts on that? Thanks Bob
                "Reality is an illusion albeit a very persistant one " Albert Einstein


                • #9
                  Lots of treble peaking in that circuit. C1, C8 and C13 are 1uF metal film cathode bypass capacitors. You could just remove them, or replace them with 22uF at 16V or more electrolytic caps. The trick is you have to know which terminal is ground so that the minus side of the electrolytic goes to ground and the plus side goes to the tube's cathode. C1 and C13 affect both channels, C8 only affects the lead channel.

                  C5 and C6 affect the lead channel only. C7 affects the rhythm channel only. You could remove them or lower the value perhaps by half or one fourth. C6 probably has the biggest effect, especially when the lead volume control is not turned up very far.
                  WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
                  REMEMBER: Everybody knows that smokin' ain't allowed in school !