JCM 900

Discussion in 'The Workbench' started by DieterGuitarJoe, Nov 25, 2019.

  1. DieterGuitarJoe

    DieterGuitarJoe New Member

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    Hi All, I've got a 1995 JCM 900 Dual Hi-gain, modified with EL34 tubes, rewired the presence,treble,mid and Bass adjustment so it works with a Pre-Amp connected. Also put in a split to work with two end-tubes when required smaller volumes.
    My problem is still the highs by higher volumes, the amp and 1960A cab sound great with my Rocktron Piranha on lower volumes, but when I crank it, it produces to much high and the bass seems to disapear. Would it be possible and make a difference te replace two original speakers to two bass-speakers? Or change to a complete bass-cap?
     
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  2. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member VIP Member

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    Welcome to the forum, unfortunately I don’t have your answers, but be patient other more knowledgeable members will be along to help you with your questions
    Cheers Mitch
     
  3. MonstersOfTheMidway

    MonstersOfTheMidway Well-Known Member

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    Hello.

    You can certainly change out some or all of the speakers/drivers. Read the specs to any speakers you're researching and choose one that cuts off the higher frequencies. I'm not sure about bass speakers, but maybe it's worth a look.

    There are a few other things you can do that are usually quick, easy, and cheap. First, you can try an eq pedal. Most EQ pedals are designed as a graphic EQ (these usually have sliders that adjust to boost or cut certain frequencies), or as parametric EQ (usually have knobs which control a certain aspect of tone cluster similar to what you find on amps= low, mid, and high frequencies). Price range can vary, but you can try borrowing one from a friend or buying one from a dealer/store that has a good return/exchange policy (some dealers/stores let you try an item for a certain amount of time during which you can return the item for a full refund or exchange for another item.)

    The second option is to buy a preamp tube with built-in tone characteristics. When used in the gain stages of some amplifiers, certain preamp tubes have a tone characteristic(s) such as reduced high end frequencies and/or a "darker" tone. In my experience, amplifiers that are inherently "bright" sometimes benefit from using a preamp tube that is "darker" than other of the same type. My first attempt at reducing the high end frequencies of an amp that seems too "bright" is to replace a preamp tube in the gain stage(s) with another that I know has the high end rolled off and/or is somewhat "dark." For example, to my ears the JJ ECC83-S is a preamp tube that is generally "darker"/less high end frequency/less "brightness" than most ECC83/12AX7 types. JJ ECC83-S are usually pretty cheap, easy to find, and easy to fit in most amps equipped for 12AX7 or ECC83. First try a tube in the gain stage positions because the tone will be greatly impacted by what preamp tube you place in that position in the circuit. After, you can try other tubes in whatever mix and/or match scheme you like (just make sure it's a tube that is compatible with your amp).

    Good luck with your amp. Let us know what you decide to do.
     
  4. DieterGuitarJoe

    DieterGuitarJoe New Member

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    Hey, thanks for the response, I'll try the EQ first and maybe try a bass-Cab second.
     
  5. ricksdisconnected

    ricksdisconnected Well-Known Member

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    i would try this first in the loop. EVERYBODY needs a EQ pedal.
     
  6. Lance Chambers

    Lance Chambers Well-Known Member

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    I have a JCM 900 and it does exactly what Marshall wanted..........ROARS! :D
     
  7. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    I'm not following 100%, but, you are saying that you moved the tonestack to be post effects loop?

    Sounds like you might have something hooked up wrong. How do you set your presence?

    (edit) I'm going to move this to workbench, because it will probably take some diagnostics...
     
  8. LRT#1

    LRT#1 Well-Known Member

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    I realy like how my 900 4100 did with an EQ in loop. Also seem to remember some one saying you could replace a couple of capacitors for different values to bring the low end out.
     
  9. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    If I understand the OP right, he's saying he's running a pre-amp into the loop, but, somehow has the tonestack wired in... :shrug:

    So, he has plenty of tone controls. Here's the pre-amp:

    upload_2019-11-26_13-8-46.png

    It seems that this is his problem:

    To me, that is 100% opposite of how the presence (negative feedback) should work. Usually, the presence levels off when you crank it. This sounds like the louder he's cranking it, it's becoming more presence, but losing everything else...

    I'm not a tech, so, IDK why it would be doing that...

    He could have speakers hooked up out of phase, but...
     
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  10. LRT#1

    LRT#1 Well-Known Member

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    .
     
    Last edited: Nov 26, 2019
  11. DieterGuitarJoe

    DieterGuitarJoe New Member

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    Hey, it was indeed hooked up after the Loop, but I had this done by a professional Technician, the Presence always at 0.
     
  12. RobS

    RobS Well-Known Member

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    might pay to check your output tube bias is correct and not too cold
    mine, although a 6l6 model when turned up if anything I back off the bass cause its a bit too much
    I run all the onboard eq pretty much at 5 (pres included) but when cranked back the bass off

    also, try less preamp and more volume to get your sound from the output section, sounds much better when turned up

    EQ in the loop can fatten things up easily but can also put too much bass as well if set wrong

    the 4100/4500 is an amp that really shines when cranked as opposed to lower volumes
     
  13. DieterGuitarJoe

    DieterGuitarJoe New Member

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    Thanks man, I'll give it a shot, I do give a lot of volume with the pre-amp. The JCM was original with 6L6 tubes, but changed it to get a warmer sound.
    I have a small Roland Cube as a practice amp and I love the sound of that little fellow and I seem to get disappointed with the big monster. kinda weird.
     
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