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Simple mod for brightening the crunch channel on my Marshall JVM410H

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  • Simple mod for brightening the crunch channel on my Marshall JVM410H

    I much prefer the voicing of the OD1 channel to that of the crunch channel. OD1 is much brighter and sharper, whereas Crunch is 'richer', for want of a better word. Looking at the schematic the difference in the voicing seems to be based around the gain control. The crunch channel has an extra cap bypassing a resistor. This cap is not present in the OD1 channel. Another difference is that also in the crunch channel the next cap is of a different value 10n in the crunch channel vs 3n3 in the OD1 channel). I've attached a little annotated image, showing clearly what I mean. I've also attached the full schematic just in case it is deemed necessary. My question is, to you amp voicing experts, that will removing the cap that bypasses the resistor, and changing the value of the other cap bring the voicing of the crunch channel in line with that of the OD1 channel. Many thanks!
    Attached Files

  • #2
    Probably in large part, but note that the 1nf brightness cap is wired different in the two channels.

    Is that what you want, two channels that are the same?
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Yes, ideally I would have 2 channels that were identical, but there are less gain stages available in the crunch channel as compared to the OD1 channel, so it will never be quite the same. As they are now they are voiced so differently that I find it irritating when switching between them. As an aside then, is there a way to duplicate OD1 into the crunch channel?
      Last edited by greengriff; 03-01-2021, 10:49 AM. Reason: fix a typo

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      • #4
        I usually leave the original architecture of the crunch channel (typical of the 2203) since those of channels 3 and 4 have their own problem: the transition in the gain settings is not better. Makes the bass content very dependent on the final gain setting.
        I just readjust a few components on it.
        This image can make it much easier for you to locate the components. I don't want to imagine what it might be like to remove the entire pcb...

        * If you prefer the emphasis that the 1n bypass capacitor produces on moderate gain settings, you can simply leave it.

        Click image for larger version  Name:	jvm2.jpg Views:	0 Size:	49.3 KB ID:	925835 Click image for larger version  Name:	jvm.jpg Views:	0 Size:	111.8 KB ID:	925836
        Last edited by Pedro Vecino; 03-06-2021, 12:17 PM.

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        • #5
          Hi Pedro. That's very interesting actually: when you look at the schematic for the 2203 and the JVM 410 side by side the component values around the gain control are the same. However on the 2203 the output from the gain control goes straight into the grid of the following triode, whereas on the JVM410 there are some extra components: C229 before the gain control, and R93 an R117 after the gain control affecting the signal before it enters the next triode. Do these changes account for the much darker tone of the JVM's crunch channel as compared to a 2203?

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          • #6
            The 220K resistor in series, slightly. But I have never touched it since it is common to channels 3 and 4 and they need it.
            On the crunch channel, limiting the bass slightly (10n to 4n7) and emphasizing the filter effect (470K/470pF to 470K/820pf) is enough to center it globally more evenly.

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            • #7
              Thank you!

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