Noisy JCM800 help?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by JamesHealey, Jan 21, 2008.

  1. JamesHealey

    JamesHealey Member

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    Hey guys I've recently just given my JCM800 1987 a bit of an overhaul.
    Put it back to non-master volume spec, installed and biased a full set of JJ EL34 and 12AX7's and it's sounding great.

    But god is this thing noisy, im certain it's the caps that need replacing.
    so which ones do i replace?
     
  2. John Phillips

    John Phillips Member

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    If it's definitely the caps, replace all of them - the three large cans and the two small bias caps on the circuit board near the power switch end. If even one of them has failed it is simply not worth doing anything less.

    Marshalls aren't always noisy because of the caps though.

    The most common reason is because some previous owner has modded them to increase the gain :rolleyes: :).
     
  3. JamesHealey

    JamesHealey Member

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    I don't think it's been modded for a gain increase. It doesn't look to have been tampered with at all, looks to have had a power tube socket replaced at some point but nothing else; and the resistor on the tube socket obviously along with it.

    I'm almost certain it's the caps, if i find afterwards it's still noisy i'll be looking to take it to a tech. But I seem to be doing well so far.

    Any suggestions on what caps to use? JJ any good? or try get some LCRs?
     
  4. John Phillips

    John Phillips Member

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    LCRs have been out of production for some time now, and you should not buy 'NOS' electrolytic caps, they're not worth fitting if they're over about five years old.

    I've been using ARS (which are what Marshall fit in the reissues now) but I've heard the JJs are good too. Any type will be fine for the bias caps.
     
  5. JamesHealey

    JamesHealey Member

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    Found a reasonably priced website here in the uk that stocks Mundorfs, which have been written up as being extremely good.. I'd be still interested in the JJ's or ARS' if someone else wants to chip in? especially if they've tried a few and can tell a difference tonally.

    Im guessing the bias caps are just a value and it doesn't effect the actual tone of the amp.
     
  6. JamesHealey

    JamesHealey Member

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    just tried the amp without the anything plugged into the input jack and it's a lot quieter when turned up.. it's not noise free but it's much better, i think i can hear noise from radio waves? and possibly a background sound of a radio station in the noise.

    anyone got any ideas? im not 100% convinced it's going to be the filtercaps now..
     
  7. PRNDL

    PRNDL Member

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    RF can be difficult to remove from a high-gain amp.

    Some people add/increase grid stopper resistors right next to the tubes. The general idea is that the wire wounds have a tiny bit of inductance. Small inductors could also be used, but they will cut the highs. Ferrite beads were used in some amps.

    Simpler methods involved moving the wires with chopsticks while the amp is on. Often one wire acts like an antenna. Once you find that wire, you could move it to minimize the noise, or replace it with shielded audio wire.

    Sometimes RF noise transmits from the power transformer onto the output transformer. I saw a cool web site describing how someone put a bent piece of metal (in a Z shape) between the two and bent it till the noise went away.

    It's also possible that RF is coming in through the house AC. You check this by changing outlets, rooms and houses. Some people use an AC line conditioner to remove this. These are recommended for stage use as they also protect equipment from damage.

    Preamp tubes can also act as antennas. Another technique involves trying different tubes until the problem goes away. Usually it's the first preamp tube, but not always. Also, these days it's much harder to know what brands are higher quality, both for new and NOS.
    This would be the first place to look since the problem occurred after a tube change. If you saved the old preamp tubes, put them in and see if the noise goes away. If it does, substitute in the new tubes one at a time to find the problematic one (or keep using the old ones).

    BTW - Did you match and bias the new output tubes?
     
  8. JamesHealey

    JamesHealey Member

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    well i'll change the caps for what it's worth they're 21 years old and have probably seen better days.. think it would be the house to be fair the electrics in here are appauling and im in the loft... we'll see.
     

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