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Thread: Clip: DIY JCM800 Atomica/Jose

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    Clip: DIY JCM800 Atomica/Jose

    Finished this build recently. Ceriatone chassis/turret board, but changed most of the circuit into an "Atomica" style Jose mod. But maybe a little more aggressive/modern.
    In place of the trimmer on the back, I have a 6-position rotary for different clipping options. Though I find myself using 2N5401s most of the time.










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    Last edited by FourT6and2; 12-11-2018 at 04:55 AM.

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    Do you have a schematic for this?

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    Old Timer olddawg's Avatar
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    A very clean build...

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    Quote Originally Posted by Chasin' Note View Post
    Do you have a schematic for this?
    Typical Jose. Start with a JCM800 type of circuit with dual gains and a pre-tone stack master volume.

    Cathodes are 2K7/.68, 2K7/.68, 820R/.68
    First stage plate is 320K total. In this case 100K and 220K in series with a 330pF bypass.
    First coupler is 0.022uF, second stage is 0.0022uF
    4700pF bright cap on Gain 2.
    68K/68K voltage divider on Gain 1 (68K from wiper to ground and 68K input grid on V1b)
    Typical Jose mod with whatever zeners or transistors you like
    Depth mod with whatever value cap you like
    Everything else stock JCM800

    And just go from there if you want to tweak something.

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    Old Timer J M Fahey's Avatar
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    Typical Jose means nothing to us.
    ... whatever zeners or transistors you like
    ... whatever value cap you like
    Would you care to share an actual schematic?
    Thanks.

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    Juan Manuel Fahey

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    Thanks bro

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    Old Timer J M Fahey's Avatar
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    Thanks
    NOW we know what are you talking about, .

    So basically cold cathode stage removal and added clipping diodes , so end result is way less second
    harmonic and a lot more third ... a woodier sound comopared to original reedier one,which is the core of JCM800 sound.

    Total gain not much changed because what´s increased by bypassing (and rebiasing) cold cathode and increasing V1 plate load is lost by heavily loading first stage with a 68k resistor.

    Rest of circuit looks roughly standard.

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    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    Thanks
    NOW we know what are you talking about, .

    So basically cold cathode stage removal and added clipping diodes , so end result is way less second
    harmonic and a lot more third ... a woodier sound comopared to original reedier one,which is the core of JCM800 sound.

    Total gain not much changed because what´s increased by bypassing (and rebiasing) cold cathode and increasing V1 plate load is lost by heavily loading first stage with a 68k resistor.

    Rest of circuit looks roughly standard.
    The entire circuit is pretty standard. That's why I said "typical Jose." This thing is 40 years old by now.

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    I don't understand how this clipper works. I mean the BE-junctions of the 2N5401s.
    Anyone cares to clarify ?

    Thanks.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by strato56 View Post
    I don't understand how this clipper works. I mean the BE-junctions of the 2N5401s.
    Anyone cares to clarify ?

    Thanks.
    They behave like back to back zener diodes. It's a guess at to what the zener voltage might be as it's not specified in the datasheet other than >5V.

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    Experience is something you get, just after you really needed it.

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    Thanks ! I thought 5V was the breakdown voltage.

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    Yeah, just using transistors for their internal zener-like behavior. They do sound different than using zeners of the same/similar voltage. The MPSA06 sound like 12v zeners and the 2N5401 clip a little more. Maybe like 9v-10v. But the character/feel is a little different than a zener. I have a rotary switch in this amp with a bunch of clipping options on it from 20v zeners, 18v, 16v, 12v, 9v, and a few transistor options. Not in the schematic, but you can see it in the photo.

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    Thanks for clearing that up !

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