JVM410H Crunch Channel - what to do with it.

Discussion in 'The Workbench' started by greengriff, Mar 5, 2021.

  1. greengriff

    greengriff Active Member

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    As a long time (13 plus years) JVM410H owner I never felt that the crunch channel met its stated goals of offering plexi, 2203, and modded 2203 tones. The sounds on offer were all perfectly pleasant, but voiced way too rich/dark, which also led to what was a rather jarring (to me at least) change in character when switching between crunch and OD channels.

    The tonal differences were confirmed when I recently got lucky enough to have my JVM and a bunch of other amps, including 3 varieties of plexi (Marshall 1959, Bugera 1960 Infinium and a Blackstar Artisan 100) in the same room with a couple of hours to mess about with them. The good (great!) news is that the JVM can really nail a plexi tone (OD1 orange, gain just under 1, resonance up to around 8, presence, EQ and volume to taste). I mean it’s really, really good, and it’s definitely not in my imagination as it was tested in the same room, with 3 great sounding other amps, run through 6 or 7 different cabs. All of the above also confirm some things that I kind of knew anyway: the JVM can be made to work beautifully with any speaker, and is never left wanting, even by some very expensive competition.

    Anyway, the not so good news is that the crunch channel is pretty much redundant to me. So, I looked at the schematic and it’s clear that you can brighten up the crunch channel by altering a couple of things around the gain control to make its behaviour the same as the gain control on the OD channels (remove one cap, and change the value of another).

    However the question is, do I really want to do that? Ideally I’d make the crunch channel into a clone of OD1, as there are so many ridiculously good sounds in that channel that it would be nice to have another one, but that’s probably not feasible without major surgery of the kind that I’m not prepared (or skilled enough!) to carry out.

    So, what I’m after are some ideas of what I can do with my crunch channel. I’m not bothered about it necessarily mimicking this amp or that amp, but I’d like to give it a brighter, more aggressive and attacking character. Does anyone have any suggestions?
     
  2. tallcoolone

    tallcoolone Well-Known Member

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    Did you try a Klone or a TS in front?
     
  3. greengriff

    greengriff Active Member

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    Yeah, and it works ok, but OD1 orange produces all the plexi - through 2203 - through hot rod - through insane gain tones that I want, so I was hoping to do something else with crunch.
     
  4. spacerocker

    spacerocker Well-Known Member

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    First of all - what valves have you got in the pre-amp? JJs sound very gainy, but dark....Shugang are slightly less gainy, but very bright and alive sounding!.....

    If you are able to solder to a reasonable standard - you could do the "AFD Mod" (100F Capacitor in series with 1K0 both in parallel with R96) and the plexi mod - 1uF or 680nF capacitor in parallel with R97. Very simple mods which are easy to do (if you have good soldering skills) Another simple mod is the negative feedback mod - disconnect one end of R58 and add a resistor in series with it something like 250-500K.

    These mods transform the crunch channel, and the amp in general and make it a much bigger, punchier dynamic beast!..

    A tech could do all these mods (in their simplest form) in less than an hour. Longer if you want the mods to be adjustable/switchable

    Last thing to try (or maybe the first?) would be an EQ in the loop, with a mid boost (3KHz to 6Khz). I agree with you that the JVM can be relatively dark sounding, and these mods improve things no end!
     
  5. greengriff

    greengriff Active Member

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    Hi. Valves are stock Marshall (so rebranded Sovtek maybe?). The amp itself isn't too dark for me, only the crunch channel, so any solution really needs to target just that rather than the entire amp. The AFD mod sounds intriguing. I shall check it out. My soldering is good enough but I have a friend with much more experience of soldering PCBs than I have, so I'd probably get him to do it. NFB is intriguing but I'd do it a different way and splice a 250k pot into the tap wire from the OT so that I could make it variable rather than having to choose a fixed value.
     
  6. greengriff

    greengriff Active Member

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    Looks like the AFD mod is targetted at OD1. I don't want to mess with OD1. I did see that @Casey_Butt had revoiced his crunch channel, so maybe he'll chime in.
     
  7. em07189

    em07189 Active Member

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    For me the best crunch channel is od2 in green mode before i moded the amp.
     
  8. spacerocker

    spacerocker Well-Known Member

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    Yes - I used this method for the negative feedback mod as well - and have it on a 1 Meg Ohm log pot on the front panel. I used a 1 Meg Log pot as it gives a lot of adjustment in the first half of its movement adding 0 to about 300K to the stock value, then the second half of its movement gives 300K to 1 Meg with quite big increases in resistance. Having said that, now I have found a setting I like (about 500K) I never touch it! I suggested a fixed value to you as it greatly simplifies the mod - but yes you can put it on a pot if you want to be able to adjust it.

    The AFB mod affects ALL channels and modes (not just OD1) - EXCEPT clean green ad crunch green. it adds a lot of gain and brightness which is especially noticeable in the Crunch Orange and Red mods! It gives the amp a lot more clarity and "bite" and also increases the gain on the crunch Orange and Red mods. I actually prefer the modded crunch channel to OD1 most of the time....the mod does add gain to OD1/2 Orange and Red - but this can be offset by either 1) turning the OD gain down slightly, 2) doing the "Anti-compression mod" (affects OD1/2 Orange and red modes and clean red only) or 3) doing the "gain reduction mod" which also only affects OD1/2 Orange and red modes and clean red only. I used a combination of 2) and 3)....

    Overall these mods give the JVM much more bark and bite, and give it a more "classic" hot-rodded Marshall feel!
     
  9. greengriff

    greengriff Active Member

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    OK well it sounds like the AFD mod is the way to go then, as long as you're sure that I can retain the original character of OD1/2 with just a bit of knob twisting, or a couple of minor tweaks?
     
  10. spacerocker

    spacerocker Well-Known Member

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    That has been my experience - yes! Try it - Together with the negative feedback mod, I think you will love it! If not, you can easily put it back to stock?

    Keep us posted!
     
  11. greengriff

    greengriff Active Member

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    Yep, will do.
     

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