Changes in power filtering on JCM800 in late 80s

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by JamesHealey, Jan 23, 2008.


  1. JamesHealey

    JamesHealey Member

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    Hey guys, i've been looking through the schematics of my amp which is a JCM800 1987 built in 1987, i've found some schematics dated for 1988.

    Which seem to fit my amp,

    http://www.drtube.com/schematics/marshall/jcm800pw.gif
    ^^ This is the earlier JCM800 50watt power section

    http://www.drtube.com/schematics/marshall/1987pwrm.gif
    ^^ This is the later JCM800 50watt power section

    I can see differences in the power filtering they've added another 50uf+50uf cap in the later version, and if im correct this matches up to the late 70s metal panel design and the one with less filtering matches up to the earlier 60s power filtering design?

    now correct me if im wrong, wasn't marshall trying to cut costs back then? and this to me seems the wrong way around. I know they cut filtering out of the JCM800 2203 model, so why is this anyone goy any theories? and am i barking up the wrong tree?
     
  2. SatelliteAmps

    SatelliteAmps Member

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    On the earlier schematic there is a cap that is a 100uf that is replaced in the the later schematic as a 50+50uf. It is essentially the same thing.

    Later JCM800s had less filtering (Post 1986). This is not well documented in most Marshall schematics found online. And also, the dates on the schematics don't necessarily mean that is when the schematic was done, it can mean when the schematic was drawn. So dates don't always line up precisely.
     
  3. JamesHealey

    JamesHealey Member

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    it seems that the 1987 model was no effected then after 1986 as mine is 1987 and has all the full power filtering.. althought mine was made in january of 1987 it might have escaped being de-filtered per-se?
     
  4. JamesHealey

    JamesHealey Member

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    19th of jan 1987 to be precise.
     
  5. AdmiralB

    AdmiralB Silver Supporting Member

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    The 50W amps didn't change the number of caps - they always had three cans, they are just configured differently. The 'old style' had one can parallelled for 100uF total at the plate supply, one section of a dual 50 can behind the choke for the screens, and the other section of that can behind the 10K resistor for the PI.

    The 'new' style parallels the second dual 50 can at the screen supply.

    The net change is the screens get more filtering in the new layout, while the PI loses its independent node on the DC rail. The preamp filtering didn't change, but in the 'new style' there is one less dropping resistor in the rail, so (all else equal, which is rarely the case) the preamp in the new layout runs at somewhat higher voltage.
     
  6. JamesHealey

    JamesHealey Member

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    and how does this effect the tone?
     

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