JCM 900 4100 mod

Discussion in 'The Workbench' started by quiksilver98, Mar 21, 2012.

  1. quiksilver98

    quiksilver98 Active Member

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    Does anyone knows of any mods to do to the JCM900 to bypass the clipping diodes and create a gain cascade within the preamp tubes instead. Basically my 900 sounds good dirty, but it feels very dull on the strings, without punch. I found that if I throw the clean channel on ten and bring the preamp gain to about 2:00, it gets a very nice power amp distortion and it starts to feel more alive. But the problem with this is that the distortion is now after the preamp stage thus destroying my time based effects.
     
  2. Marshallmaniac

    Marshallmaniac Well-Known Member

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    Man, I'd seriously just dump it for a decent price and buy the amp you're looking for.

    I had a 4500 that sounded just terrible! As you describe and more. It had little dynamics, felt thin, and the clean even sucked! You'll be much happier with a new (or new old..) amp. The 900's are one of those amps I put in the 'meh' category. They're just an ordinary, run of the mill guitar amp with nothing special going for them. But others actually like them. Go figure!
     
  3. Roadburn

    Roadburn New Member

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    Just popping in (been a while).

    First of all, I'm no electromagician. Everything I'm going to say is what I found on the web...

    What you try to achieve is not possible, Quick.
    The gainstages in a DR are all "OP amp", no tubes involved.
    And I think it is allready "cascading" (one pre-amp into a second pre-amp)

    Tube 1 = EQ section
    Tube 2 is split into 2 parts, one drives the fx-loop, the other the reverb.
    Tube 3 is part of the power section (no idea what function it has)

    JCM 900 4500 DR schematics:
    http://www.drtube.com/schematics/marshall/cd0189-iss7.gif

    If you look at the upper left hand side of the schematic, you can see 6 triangular shaped items, those are OP amps.
    Somewhere in there (one is at the top) you can find "VR1" and "VR2" indicated with a small dot.
    Those are the knobs/dials for both gain stages (channel A, channel B).

    You can find the diodes, that give you your distortion, between those gainstages (top/bottom).
    The other diodes are Bridge Rectifiers (indicated by "BR1" and "BR2")
    They act like compressors, which could be the cause of the dullness you experience.



    P.S. I asked the same question when I visited DrTube about my MkIII (which has a different pre-amp section b.t.w.). He told me it is possible but not as straightforward as changing diodes or removing them.
    Those diodes change voltages etc. Any modification would ultimately require the same output as the diodes.
    He also advised me not to temper with it because of the vallue of the amp.



    P.P.S. not sure about the age of your amp or if the caps need replacing (most importantly not sure if this would do anything about the dullness you experience)

    P.P.P.S.
    It's not a hopeless amp if you can do this:
    [ame=http://www.youtube.com/watch?v=x0d1HilfLxA]Robin Trower Bridge Of Sighs - YouTube[/ame]
    or this:
    http://www.marshallforum.com/tone-zone/18288-best-tone-competition-results-awards.html <---900DR 4100 winning an ultimate tone competition. Well done Jeffe!
     
  4. Ken

    Ken Well-Known Member

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    The best way to transform a 4100 into a good amp is try and get $500 for it and buy a used DSL for that amount. Modding it to eliminate the diodes is just insane outside of a hobbiest project. You'll spend a ton of money and end up with some abortion more often than not.

    The 4100's are kind of an outlier for Marshall. They have that 90's hair band SS sound with an icepick attack and not a lot of depth, because that's what they were designed to do. These days people seem to prefer the classic rock sound of the 800 series amps or the modern overdrive of the 2000 series or JVM. And the latter two also have great clean tones as a bonus.

    If you have to ask how to eliminate the 4100's diodes and put a real tube preamp stage in, you probably aren't up for the task to begin with. It's not something for most people to undertake and it will never be cost effective; you'll have to build a preamp circuit basically.

    Ken
     
  5. V-man

    V-man Well-Known Member

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    Here we go again...

    After a nice period of "even-keeled" suggestion about his preferences, it seems he's back to talking out of his ass again.


    No, Ken. The best way to transform the 4100 into a good amp is to actually play it at volumes that it was intended to cover and tweak it accordingly. Not fiddle around with it for about a month at bedroom volumes before dumping it off on your bassist and bobbing your head between the thighs of the DSL engineer ever after.

    The amp certainly may not be the best fit for the OP, but your agenda is old and goes way beyond anything useful here.
     
  6. Rahlstin

    Rahlstin Well-Known Member

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    I once saw a schem with all the mods nessesary in red to do it and it basically required a rewire of the whole amp. It would be a mess.
     
  7. American Viking

    American Viking Well-Known Member

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    Australian that was pretty funny.

    I have to admit, when I read someone going against the herd on the DSL, I get a little smile going every time...
     
  8. Oculophilia

    Oculophilia Active Member

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    Yeah, if you just don't like your 900 trade it in before you have it modded. Modding kills the value even if it makes for a better sounding amp. That said, I would have all the normal stuff checked,caps and tubes before I did that. I like my 4001 even at lower levels, though it really rocks at 4 or more.
     
  9. Ken

    Ken Well-Known Member

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    "Bedroom volume"? That's the WORST way to play a 900, which we both know. You have to get the power tubes in play. That was part of my issue: a 100 watt amp in a smallish practice space can't be played full out. It's disrespectful to the band and wouldn't do my ears any good either.

    Amps that derive tone from the preamp stages are much better suited for reasonable (as well as bedroom) volumes, which is why the 2000, JVM, etc. series are popular.

    I like a NMV plexi too, but unfortunately those would be as impractical for me as the 4100 was.

    The OP asked about undertaking a fairly complex mod to improve his amp by getting more tone from the preamp. My advice was that this is not cost effective and he may not even like the results. I stand behind that advice.

    Ken
     
  10. V-man

    V-man Well-Known Member

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    The advice is perfectly valid (IF the stock config can't be tweaked to user's needs, it IS cheaper to replace than mod). The point at issue was that "the way to get a 4100 to sound good is sell it for a DSL" business. While that may be a viable option for the OP, it's couched in the spirit of bashing an amp that you don't like, and serves no constructive purpose.
     
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  11. Ken

    Ken Well-Known Member

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    That's a valid point. It comes down to personal taste. My theory has always been to get an amp that you love right out of the box, and if you want to then tweak it, go for it. There are lots of threads on how to improve a 4100.

    But an amp you can't get your head around with guitar straight in...well you can get your pedals, your EQ, jumper the FX loop and if that all fails, spending money to mod it. At some point I have to ask, "WTF?". Why not start with an amp you LIKE?

    So that's the advice I post: if you don't like your amp's voicing, forget about mods etc. Get an amp whose voicing you like! Especially if the mod is fabricating an entire preamp section!

    Ken
     
  12. SmokestackElRopo

    SmokestackElRopo Active Member

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    As a forum, on this particular amp, we can bash or we can love. But when it gets down to the brass tacks, Robin Trower just proved that it is a viable amp. If he is indeed playing a 900. I own a 5881 4102 combo, but aint no way I can make it sound like that.
     
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  13. Roadburn

    Roadburn New Member

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    He played 2x JCM900 DR's.
    There is a video floating around the intarwebs where he talks about the gear he used during that tour (sorry I can't find it).
    Robin used a few custom pedals to create that sound (he plays VM's nowadays).
     
  14. Söulcaster

    Söulcaster Well-Known Member VIP Member

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    I had a 4500, killer amp if you know what you’re doing.
     
    Last edited: Jan 30, 2018
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  15. Lowlife

    Lowlife Well-Known Member

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    Do you have the FX-loop patched, and the loop volume turned up to the sweet spot. That actually helps giving it some "Oomph"?

    Ps. Ignore Ken, he is a notorious Dual Reverb hater. The DR's are great amps, how old is your tubes? do you have a good one in V1 (V2 and 3 are soundwise of no importance).

    Pps. But don't ignore the fact, that the modding required to do what you want to do is so significant, that you'd be better off getting another amp.
     
  16. Ken

    Ken Well-Known Member

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    Oh, so he should ignore me but you suggest the same thing I suggested!! You say he should ignore me so he won't know you stole my advice???? I suppose he should ignore both of us and just pretend he likes his DR. After all, it works for Robin Trower.

    Ken
     
  17. LesPaulopolis

    LesPaulopolis New Member

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    There's a VooDoo Amps / Labs mod that rebuilds the preamp and you lose your reverb (since that tube is now used for gain) but it costs around $400 I believe??? Not worth it IMO.

    Maybe you can get a tube preamp pedal and run that into the FX return.

    Ch. A gain cranked and an EQ in the loop was the best tone I got outta that head. Don't judge all Marshalls by the 4100/4500.
     
  18. Lowlife

    Lowlife Well-Known Member

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    Hehe, maybe he should, however plenty of people out there doesnt even have to pretend, <gasp> :naughty:
     
  19. Ken

    Ken Well-Known Member

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    And may the Goddess have mercy on their souls.... :D

    Ken
     
  20. iron broadsword

    iron broadsword Well-Known Member

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    quiksilver98, I think the easiest and best 'mod' you could do would be to buy an attenuator. Like all tube amps, these change their character and become monsters (possibly even more so than other amps) as they reach the higher volume settings, so attenuation will get you up there at volumes you need it to be.

    Try diming the amp before you do anything and see if you still don't like the diodes. I love em'! The whole 'eww clipping diodes' thing bugs me, cause the truth is they sound great if you know how to use the amp. Which goes for any amp.
     

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