Free Easy Mods For Jcm 900 2500/2100

Discussion in 'The Workbench' started by NewReligion, Jan 9, 2018.

  1. NewReligion

    NewReligion Well-Known Member VIP Member

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    I wanted to post this as I was messing around. When I was younger everyone said there really id not anything to be done about making the JCM 900 2500/2100 sound improved. I disagree. Now the 4500/4100 are another story I have not played with so far.

    Of course more can be done but I will finish at this point.


    1.) R5 3k3 add .68uf bypass cap. (Do not decrease resistor value).

    2.) R8 1k5 add .68uf bypass cap.

    3.) D2 Remove single 1N4007 diode and replace with set of 5.1 volt zener's (1N4733).
    Stripes (Cathodes) Outward.

    4.) R4 Plate = 320k.

    5.) R7 Plate = 320k.


    Straightened out some issues on this old 1990 JCM 900 50 watt 2500 head. It belongs to my son. It has had a lot of issues over the last 18 years.

    At one point it was no kidding on fire and in pieces. He blew a transformer as a kid after redplatting and burning up a lot of components.

    It blew the pico fuse and harmed the Effects Loop to boot.

    He was still complaining about the loop over heating so I had another go at it. It seems even in 1990 Marshall still could not stick to their schematics.

    I made some modifications to the circuit as there is only so much you can do with the ic based 900 series but it sounds a lot more Marshall to my ears now. Better diodes and values, plate and cathode values as well.

    Plugged straight in.

     
    Last edited: Feb 25, 2018
  2. JCMDOUG

    JCMDOUG Well-Known Member

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    I reduced the Neg-Feedback in my 2500 and replaced the filter caps and added a choke.
     
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  3. JCMDOUG

    JCMDOUG Well-Known Member

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    I just cut the wire from the output trans and installed a switch. I added about 66k to reduce the feedback sig. 900 Neg mod.jpeg
     
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  4. twothemax

    twothemax Active Member

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    what did the 66k do?
    could a person just install a 66k resistor inline with the violet wire?
     
  5. NewReligion

    NewReligion Well-Known Member VIP Member

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    Great Idea's...

    The Friedman SAT mod works on this amp but my intent was to get away from the "clipper" tone. The Zener's really made a difference to the "Gain Sensitivity"...♫
     
  6. LPMarshall hack

    LPMarshall hack Well-Known Member

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    Nice job Dave. I’d love to hear some crunchy chords with the mods.
     
  7. NewReligion

    NewReligion Well-Known Member VIP Member

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    I think you can hear a open g chord ring out on this short video. I already returned the amp to my son who plays a Diezel VH4 Blueface. This was his first Marshall as a kid.

     
    Last edited: Feb 25, 2018
  8. NewReligion

    NewReligion Well-Known Member VIP Member

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    R2 I believe it was on the effects loop board just inside of the pot adjustment calls for a 3k3. Marshall had a 2k3 in that location stock. So I changed it too. Not sure if it made much difference but it didn't hurt as the loop according to my son was "acting up" and over heating...Hmmm:hmm:
     
  9. twothemax

    twothemax Active Member

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    new religion the mods sounds fantastic.
    r4 and r7 did you just add values to get 320k or did you remove originals.
    also r5 and r8 are the caps wire // with the original resistors.
    got a 2100 that i would like to try this on.

    what type of .68uf caps did you use? what value also ?
    any other info on these would be great
    thanks
     
    Last edited: Jan 10, 2018
  10. NewReligion

    NewReligion Well-Known Member VIP Member

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    Glad you liked it. Don't forget to change out D2 (Diode 2).

    I just added to the plates to = 320k.

    .68uf caps I used...50 volt Nichicon Aluminum Electrolytic caps. Mouser part number is 647-UPM1HR68MDD.

    LOOKS LIKE MOUSER HAS A 16 WEEK WAIT TIME.

    I just ordered another 100 from https://www.digikey.com/products/en

    Keep me in the loop and good luck...♫
     
    Last edited: Jan 10, 2018
  11. twothemax

    twothemax Active Member

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    thanks
    so the resistors are added to r4 and r7 in series to make 320k
    caps added are in // to r5 and r8.
    i see the caps you used are eletrolytic does it matter which end of the cap goes where on r5 and r8? other words is there a pos and a neg. these are radial correct
    yes will do d2 also. so many d2 any particular one?
    man if you could post some pics of this mod it would be great,
    thanks
     
    Last edited: Jan 11, 2018
  12. JCMDOUG

    JCMDOUG Well-Known Member

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    The 66k is a value I kept. My goal is to get the amp to breakup and open up the power amp more like a JCM 800. Just cut the wire and put a resistor inline. I did that on most of my amps. others I put a switch so I can compare stock to mod.
     
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  13. NewReligion

    NewReligion Well-Known Member VIP Member

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    Correct on resistors and caps. Cap negative side should face ground or potentiometers. You asked what I use. I’m sure you can use whatever you like. In fact experiment with caps and resistor types. By the way I used carbon film resistors rated 1/2 watt as the 900 series is loaded with 1/4 watt resistors.

    Diodes I highly recommend the 1N4733 Zener 5.1 volt...
     
  14. NewReligion

    NewReligion Well-Known Member VIP Member

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    I covered every thing in heat-shrink tubing. And the amp has been returned.
     
  15. twothemax

    twothemax Active Member

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    Thanks again.
    what would be the difference if a guy used a 100uf caps instead .68uf. the reason i ask is i have a few of the 100uf caps.
     
  16. NewReligion

    NewReligion Well-Known Member VIP Member

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    100uf will not filter the lows out. .68uf filters the lows allowing perceived gain by accentuating the higher freq’s and smooths out the harsh flub. It is quite common with all Friedman products and historically known as the Plexi mod as the Plexi’s had no bypass cap on V2a. For that matter neither did the MV 2203/2204. I have used it there as well in addition to much more fine tuning.
     
  17. NewReligion

    NewReligion Well-Known Member VIP Member

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    Keep in mind my goal is to make Marshall’s better stand alone high gain amps. But if you place a Boss SD-1 for instance in front the amp screams! Fine for home fun, but at live volumes there is enough compression and saturation to cut through the mix without the thinning of freq’s that is pedals most often result in.
     
  18. NewReligion

    NewReligion Well-Known Member VIP Member

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    Also you may want to try experimenting with D1 when you have finished all other mods.

    I pulled it out. It seemed to make no difference. Perhaps because of the pair of ultra low voltage zeners installed in D2. I just reinstalled the stock diode in D1.

    Kind of wonder what a pair of 1M4733’s would do there. Too much perhaps as I reached my sonic target. Part of this is knowing when and where to stop.
     
  19. twothemax

    twothemax Active Member

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    thanks. placed the order today. so now the waiting game.
    thanks for the info new religion. been looking for some mods to try on the 2100 for a while.
     
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  20. NewReligion

    NewReligion Well-Known Member VIP Member

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    You are welcome.
     

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