JCM resurrected from the dead

Discussion in 'The Workbench' started by DennyMarshall, Apr 6, 2020.

  1. DennyMarshall

    DennyMarshall New Member

    Apr 6, 2020
    Likes Received:

    New to the forums here. I recently picked up a 1981 JCM 800. The board was in shambles, years of guys and wannabe amp techs poking around and soldering (poorly) resisters on top of resisters.

    Long story short, I took the amp to a local repair tech who put the amp as close to original as possible and he spent some quality time cleaning up the board and replacing caps, and resistors that were either shot or the wrong values.

    I love the sound of 800’s and I’ve owned at least 50 over the years.

    The only issue I have right now is it’s lacking just a little distortion and I mean a little.

    What values and where could I change a resistor that might open that up just a little. Maybe a more aggressive 12ax7?

    Here is a fat old guy clip of myself farting around with the amp and the distortion on 10.

    Thanks in advance for any info.

    Here are some of the issues as posted by Curran the amp guy

    Last edited: Apr 6, 2020
  2. South Park

    South Park Well-Known Member

    Sep 14, 2019
    Likes Received:
    V1a is where the gain stage is drop the value on the ground resister and v1b . The lower you go the more noice you get . Or boost pedal
  3. neikeel

    neikeel Well-Known Member

    Feb 12, 2014
    Likes Received:
    A higher output 12AX7 in v1 is easy to do, it might be all you want.
    These things are so much down to taste it’s hard to recommend any particular one.
    Certainly lowering v1 cold clipper resistor from 10k down to 8k2 or at a push 6k8 will work (any lower and you lose the 2203 Kerrang, IMO). You can test this by temporarily croc clipping a 33k or 22k resistor across the 10k and seeing if you like it. other subtle things are 120k plate resistors on V1 or adding a bypass cap to v1a cathode (eg o.1uF ) but i find it a bit too much. Another common one I do like is subbing the 470pF treble peaker cap (the flying one between jack and pot) with 1800pF or 2200pF. Again you can try that by just croc clipping a 1800pF cap across the current 470/470 and see if you like it.
    Also worth checking if the bright cap (1000pF) was omitted by your tech during the rebuild - although that gives more brightness up to about 7 on the pot. After that it’s effect is over.

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