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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).


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21 comments:

  1. Willy10 February 2018 at 23:51

    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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  2. glasspak7312 February 2018 at 17:56

    This comment has been removed by the author.

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  3. Hozy3114 February 2018 at 18:20

    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. Willy15 February 2018 at 08:05

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

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    2. Hozy3115 February 2018 at 16:42

      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. Unknown23 February 2018 at 02:01

    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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    Replies
    1. Alex23 February 2018 at 07:06

      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. Unknown23 February 2018 at 13:57

      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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    3. BlondinLux29 November 2018 at 08:21

      Since then did you solve the oscillation problem ?
      I built the first layout too and have the same oscillation problem

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  5. Unknown23 February 2018 at 03:35

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

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  6. Willy11 March 2018 at 10:02

    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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  7. Willy12 March 2018 at 02:37

    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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  8. VVP30 April 2018 at 17:05

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

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  9. Willy20 May 2018 at 03:58

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

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  10. Unknown9 October 2018 at 05:34

    This comment has been removed by the author.

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    1. Unknown9 October 2018 at 05:36

      Sorry i mean offboard wiring schematic layout

      Delete
  11. BlondinLux27 November 2018 at 22:56

    Hi Alex!
    First, Thank you so much for all your great work for the community !!

    I built the first layout couple of days after you published it.
    Having some trouble, no sound.
    I tried to debugged it; no luck so I let it aside until today.

    With plenty of courage ^_^
    I tried to debugged it again and I found my mistakes
    I forgot two cuts! Shame on me :o

    Now it's sounds great!

    But, I still have a problem with the 3PDT switch 1:
    When 1+2 OD1 engaged, I've got a huge squeal
    Do you know how I could solve this ?

    I used BC547 and 2SK117

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  12. BlondinLux14 March 2019 at 07:24

    Ok I tried to figured out with this squeal so I disconnected one by one the leads 1,4 and 7 to switch 1 and it resulted the squeal is coming when I connect sw1-7.

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  13. BlondinLux14 March 2019 at 07:33

    I connected sw1-7 directly to the BC547's base, just above the 2k7 resistor, and it's working great know but I'm not sure if this is correct compare to Bajaman's schematic

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  14. Salin19 November 2020 at 09:00

    This comment has been removed by the author.

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  15. pakkee6616 December 2021 at 16:58

    build with the 23x23 layout, and switchable marshall tonestack, every pots n switch works fine, only a 'Weird tails'appear that follow by the sustain note, bypass the tonestack it still appear, no idea what is it.

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