Marshall JCM 800 Tube Question

Discussion in 'Amps and Cabs' started by gpecoulas, Sep 25, 2006.

  1. gpecoulas

    gpecoulas Gold Supporting Member

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    Does anyone know if a JCM 800 with 6550 power tubes can run EL 34s without any modifications to the amp?
     
  2. 6789

    6789 Member

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    bump. I want to know too. Plus what sound difference would there be?
     
  3. Blue Strat

    Blue Strat Member

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    No. You have to have the tube sockets rewired and possibly a resistor change to the bias circuit.

    Not a difficult change, but not a plug-n-play situation.

    Tonal difference would be lower volume, softer, breakup.
     
  4. BadAssBill

    BadAssBill southofnash.com Silver Supporting Member

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    If you change them...let me know what you think please. I've thought about doing this as well.
     
  5. mmorse

    mmorse Member

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    Tube sockets rewired? I have an '82 2204 originally setup for 6550s. I simply piggybacked a 47k resistor on the bias resistor and was then able to bias 34s properly in the amp. I've done a couple other simple tonal mods to tame the harsh highs but the single resistor was all I had to do to get the bias pot in range to run the 34s.

    I also changed the NFB wire from the 4 ohm tap to the 8 ohm tap on the impedance selector but I didn't like the loss of gain so I put it back. I like my crunch from the amp instead of a pedal.
     
  6. StratTone

    StratTone Member

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    +1 You don't have to do anything to the tube socket.
     
  7. DEMENTED

    DEMENTED Supporting Member

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    My '85 JCM 800 is 2204 model from Canada and it came stock with EL34's. I personally prefer them to 6550's. Right now it has the Mullard reissues but I expect some Mullard XF2's from the Tube Tramp very shortly and they are going right in!
     
  8. cap'n'crunch

    cap'n'crunch Member

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  9. Blue Strat

    Blue Strat Member

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  10. Beam Tetrode

    Beam Tetrode Member

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    I disagree. Two bias resistors are generally safer than one.

    The socket distinction is recognized. Most Marshalls from the JMP/JCM800 era do have pins 1 and 8 wired together on the socket. The original schematics verify this. No socket rewiring necessary, both pins are grounded.

    The real techs are busy disposing of JCM2000 units.
     
  11. mmorse

    mmorse Member

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  12. mmorse

    mmorse Member

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    If the Mullard reissues you have are the New Sensors, changing to real XF2s will be a real eye opener for you. :dude
     
  13. mmorse

    mmorse Member

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  14. myles111

    myles111 Member

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    Mike,

    I have found that if you have enough bias swing to get the EL34's into the proper range you are fine.
     
  15. Gretsch6120

    Gretsch6120 Member

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    Just wondering what you did to tame some of the harsh highs, I find that I have the same problem with my 800 and I would like to try and get rid of them
    Thanks
     
  16. Beam Tetrode

    Beam Tetrode Member

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    The 2203 and 2204 amps are built for maximum tonal aggression. A tube swap won't tame the high end at all. However, you might try removing the bright cap on the gain pot.
     
  17. Gretsch6120

    Gretsch6120 Member

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    I have changed tubes many times and haven't noticed a difference, so removing that bright cap will it change it drastically? what can I expect when I do that? Have you done it and what was your thoughts on it? I have about a million more questions after these ones are answered
     
  18. Beam Tetrode

    Beam Tetrode Member

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    Actually, I've always enjoyed my 2204 heads stock.
     
  19. mmorse

    mmorse Member

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    Removing the brite cap will not make a dramatic difference. But it may be enough for your taste. You just have to try it. It wasn't enough for me so I also removed the brite circuit and replaced it with a 68k carbon comp resistor. You can also tame some of the harshness by using a 5751 in V3. Or you can try an AT7. I tried the AT7 first but it robbed too much gain for my taste. The 5751 doesn't sacrifice any gain to speak of. Just makes the top end a tad smoother.

    How loud you run the amp also affects the overall brightness. As you turn the gain up, the less bright the amp will be. Maxing the gain will cut the bright circuit out altogether. But I found my 2204 gets over saturated with the gain dimed.

    I did the mods so I could have a nice, smooth, full sounding yet aggressive amp at lower volumes. At home, I never get the master past 2 yet it still has plenty of lower mids and bass and is nice and crunchy with the gain up around 7-8. Prior to the mods, the amp sounded thin and bright at that volume. Of course, running a NOS Mullard in V1 and NOS Mullard 34s for power had a lot to do with it. :AOK
     
  20. Rex Nomad

    Rex Nomad Member

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    There are lots of mods you can do to a 2203/2204 to tame the highs. I have two 2203s and have messed with them a lot and here's what I've found helps (I'm not a tech by the way - just a tinkerer):

    Buy an RFT 12ax7 from Blue Strat (KCA NOS tubes) and put it in V1

    Clip the bright cap: might be all you need to do.

    Change the feedback resistor from 100k to something lower like 50k or 68k. Brings up the lows, softens the highs and adds a little compression.

    Insert a 100K (Brown-Black-Yellow), 1/2 watt resistor between the middle wiper of the Master Volume and the wire going out to the phase inverter circuit.

    Replace the 470k Ohm resistor that is tied to the input tube grid with a 68k Ohm. (personally I don't like this one because the low end gets too flabby for me but some love it)

    Check these out for more details:
    http://marshall.redpt.com/clay/mv_marshall_circuits.html
    http://www.tone-lizard.com/Ultimate_JCM800.htm
    http://www.obsoleteelectronics.com/Marshall_Tech/marshall_tech.htm
     

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