Friday, 9 February 2018

Baja JVM Overdrive

This is based on a Marshall JVM410.
You can find Bajaman's original thread and schematic on the FSB forum here.
There are two versions:
First one with just one toggle switch to select between Cruch, OD1 and OD2.
The second one uses separate footswitches, Gains and Levels.
You need to add an extra pole for LEDs.
Can use any small signal N channel JFet for the buffers (mind the pinout).


14 comments:

  1. I've started on this one, but it will be a while before it's finished. Thanks heaps for putting it up!

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  3. Verified the second layout. Used 2SK30As as i had no 117s. Sounds real good. Just a minor niggle with the level 1 pot when both are maxed. It causes a dip in volume when switching OD settings but it comes back if you lower the level 1 down a bit. I ordered a 3PDT on/off/on as i think having one level and one gain seems nicer, so i'll build the first version when it arrives as i'm quite liking this OD w crunch. Thanks Alex/Bajaman!

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    1. Here's the big question - does it sound like a JVM????

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    2. Listening to some YT vids its close but maybe not as much gain. However, that maybe because i'm using 2SK30s rather than 117s. I have some on order so will see.

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  4. Thanks for sharing, nice post! Post really provice useful information!

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  5. I’ve got the first version of this fired up but I think I’ve done something wrong with the Crunch/OD switching. In both the up and middle positions I get the same slightly overdriven sound. In the down position I get weird noise/oscillation.

    What does switch 1 - 2+3 mean? I think this is where I’m messing up.

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    1. It means when switch 1 (Crunch/ODs) lugs 2 & 3 are connected you get the Crunch.
      That's not the wiring but just the way the switch works.
      Switch 1 lugs are numbered like this:
      1 4 7
      2 5 8
      3 6 9
      It's an ON-OFF-ON (3 positions) toggle switch.

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    2. Thanks for the reply Alex! And for all the other stuff you do here of course. :)

      Last night I tried the wiring as described above and got the same result, only this time the noise was in the up position of the switch, with middle and down being the same sound as far as I can tell.

      Sorry to be bothersome. Troubleshooting has never been my strong point.

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  6. I also just noticed that the drive knob doesn’t seem to be functioning...

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  7. Ok, I've built the second one and after some messing around clearing tracks with a knife I've gotten my gain pots working correctly (they were a bit weird in the sweep (nothing, tiny sweetspot, unbiased type sound).

    The volume pots are interactive though. One (OD) is silent-loud-silent through the sweep, and the other (Crunch) is working properly when the OD is selected. Pretty certain this swaps over when Crunch is selected. I must be missing something.

    I'm using BC459 and 2ks30 as well. There's plenty of gain, but it's not a metal machine.

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  8. So what I've finally done is ditch the crunch and OD volume pots, and run those to separate Marshall tonestacks w/LPB1 veros, omitted the volume pots for those,and run them back to the JVM vero, going from the output on that to a single volume knob.

    If that makes sense.

    Anyway, now both the OD and Crunch channels have their own EQ, and that makes a nice difference to the sound. Much closer to the YT demos of the amp, especially in the high gain areas.

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  9. ehi guys, is it verified?
    i didnt understand which was the problem to verified it.

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  10. http://www.madbeanpedals.com/forum/index.php?topic=27946.msg270655#msg270655

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