JCM900 100 watt 2100 - Charge the paddles to 300...

Discussion in 'Marshall Amps' started by coolidge56, Mar 28, 2020.

  1. 6550

    6550 Active Member

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    Awesome! I have the exact same amplifier but I'm not good enough to get in there and start fiddling around. Kudos to you for having the stones.
     
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  2. EADGBE

    EADGBE Well-Known Member

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    Please post some sound clips after you're done. [​IMG]
     
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  3. KingsXJJ

    KingsXJJ Active Member

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    Awesome. Fingers can do so much.
     
  4. What?

    What? Well-Known Member

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    Looks to be well done. Why replace the cement resistors with wire wound?
     
  5. coolidge56

    coolidge56 Well-Known Member Double Platinum Supporting Member

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    All done! :jam:

    jcmr16.jpg

    jcmr17.jpg

    jcmr18.jpg
     
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  6. coolidge56

    coolidge56 Well-Known Member Double Platinum Supporting Member

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    A guy cut one of those CHINA Cermet type cement blocks open and found a carbon film resistor inside. lol That's messed up.
     
  7. scozz

    scozz Well-Known Member Gold Supporting Member

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    I agree,....well done! :yesway:
     
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  8. What?

    What? Well-Known Member

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    Nice, and wtf?
     
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  9. cozmacozmy

    cozmacozmy Active Member

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    How does it sound?
     
  10. patrice

    patrice Member

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    JCM900_BE100_2 (Large).jpg Nice job !
    I bought a 4100 for 400€ and used it has a "host" for a BE100 clone. What was done:
    - drilled the tubes holes to move them on the side of the chassis, changed the sockets
    - added a preamp tube and socket
    - added a choke
    - added and drilled for new pots
    - replaced the original pcb with a main clone board and a custom loop
     
  11. coolidge56

    coolidge56 Well-Known Member Double Platinum Supporting Member

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    LOUD lol But seriously by the time I finished last night I was too pooped for more than a quick sound check to confirm it was working properly. It's very quiet noise wise, a major goal of the rewiring.

    Biasing - It's more difficult to get at the power tube sockets with my meter probe, 5x more so since I ran the heater (filament) wiring along the top of the sockets away from the signal wiring. Also discovered some new info on the JCM900 which further makes it a PITA to bias. Pins 1&8 don't go directly to ground, they flow through a pair of fuses and some other junk. The bias procedure was looking kind of dodgy. For now I set the negative bias voltage to Marshall specs -46.5vdc for U.S. but I'm blind as to actual current.

    So I have a TAD bias test gadget in a shopping cart as I type this. I'll bias it from the top when it arrives and before I unleash this beast at full throttle.

    In other ANNOYING news, damn you Marshall, damn you all to hell (channeling Charlton Heston). The R30 bias resistor :pissed::realmad::wallbash: apparently Marshall used the wrong 56k value 1990-1992 and corrected this in 1993 with a 47k. With the 56k you can't bias the tube hot enough you run out of adjustment. Found ample reports of this in old forum threads. This explains why I found a 270k resistor half ass soldered on top of the original 56k.

    So there I am yesterday, amp finished and R30 needs to be changed out. Yep I had to pull the rear board back out to replace it. Didn't have to disconnect any of the wiring but was kind of a pain to R&R that resistor.
     
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  12. coolidge56

    coolidge56 Well-Known Member Double Platinum Supporting Member

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    Years ago a guy cut open a bunch of 'high end' capacitors and found cheap ass 10 cent capacitors wrapped inside. lol There's nothing they won't counterfeit these days. I got burned last year on a counterfeit Fender neck plate. Genuine Fender neck plate is steel, this pot metal counterfeit, I bent it and broke it in half with my bare hands. (face palm)
     
  13. coolidge56

    coolidge56 Well-Known Member Double Platinum Supporting Member

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    Nice, points for repurposing! You know I was drooling over the new BAD SLO 100 white board yesterday. Now I'm not so sure, the brown solder flux residue kind of ruins the whole white board thing for me.

    badsloboard.jpg
     
  14. patrice

    patrice Member

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    Yes I prefer a black, red or brown board over white.
     

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