Jcm 800 4210 presence resistor?

Discussion in 'The Workbench' started by Tyon sceen, Nov 25, 2020.

  1. Tyon sceen

    Tyon sceen New Member

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    I’ve done a ton of reading, but can find any good pictures online of the resistor to remove to set the presence at 0. From what I’ve read, there’s a fixed resistor instead of a presence pot on these amps. This amp is really bright! I would like to remove the resistor to try and take out the brightness. Can anyone point out the resistor to snip to get rid of the fixed presence. I think this is a January 1983 amp. Thanks.
     
  2. Tyon sceen

    Tyon sceen New Member

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    Here’s some pictures
     

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  3. Gene Ballzz

    Gene Ballzz Well-Known Member

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    @Tyon sceen

    First, :welcome: to this great forum!

    Next, blue arrows below indicate the connections of the "Presence/Negative Feedback Loop" to the Phase Inverter and 4Ω tap of the Output Transformer. Though I do not believe that simply removing the resistor to either eliminate the loop or putting a jumper in its place it is the wisest solution here! Changing the value of that resistor or replacing it with a variable one may be more appropriate. Folks with more expert experience will likely chime in here soon to help!

    And you may also find that modding an amp by a "paint by numbers/point to which component" approach (without full understanding of what is actually going on) may be a sad and/or dangerous path to follow.

    Just My :2c:,
    Gene

    4210.gif
     
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  4. thetragichero

    thetragichero Well-Known Member

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    if it's too bright try removing the 220pf bright cap from the volume pot (reducing the value would only allow higher highs through and might be nice too)
    also keep in mind that unlike a standard tmb tone stack this James tone stack is boost/cut so it is x neutral" with both knobs at noon. i wouldn't see it being bright with treble control at zero
     
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  5. Tyon sceen

    Tyon sceen New Member

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  6. parachute

    parachute Member

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    Is it the bright cap from the volume pot, or the gain pot? Because I had a bright cap removed from the gain pot on mine by a tech (thats what he told me, the gain pot) , and it did help to lower the brightness.
     
  7. parachute

    parachute Member

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    And i did have a presence control pot placed at the back of the amp earlier, but that doesn't really help as much, at least at bedroom levels.
     
  8. thetragichero

    thetragichero Well-Known Member

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    schematic up there shows a stock presence control. i wouldn't have a 50w amp for bedroom levels anyway but yes since it is negative feedback from the output transformer its effectiveness is based on the output level
    normal channel or boost channel too bright? they each have a bright cap on the volume controls
     
  9. Gene Ballzz

    Gene Ballzz Well-Known Member

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    That's a bit odd, considering its the exact amp as initially described, with the dates being in the correct range.

    Hmmm, Very Interesting!
    Gene
     
  10. Tyon sceen

    Tyon sceen New Member

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    Stock control is on the 2205, 4210 has fixed resistor instead of pot.
     
  11. South Park

    South Park Well-Known Member

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    You should look at the tone cap that presence resister is hooked up to . That is what is rolling off frequency’s . Just a idea
     
  12. Tyon sceen

    Tyon sceen New Member

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    I decided to put a pillow in front of the speaker. Read a lot today about people putting blankets over their speaker cabs. The pillow definitely tamed the brightness! Can turn the volume up higher. Great solution for now. This amp sounds great now. What about a speaker change? Thoughts?
     

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