Marshall JCM 900 MkIII


One of my all-time favorite amplifiers is the Marshall JCM 900 MkIII Hi Gain Master Volume. It was made for just 2 years and I wish Marshall would reissue it. I'm glad I still have mine. It doesn't get a lot of play time these days but when it does, it's glorious. Here are some PDF files with info about the amp.  One of them is the page from my old site minus the sound clips. Also included is the handbook, preamp and power amp schematics.








Sound Clips:

These clips are of my personal MkIII 2100. There is no post EQ at all. What you hear is the raw recording.

EQ settings on the amp: Presence = 2-3, Bass = 8-10, Middle = 3-4, Treble = 6-7, Preamp Volume = 10.

For the track named, "Sound Clip March 2009", this one is post-repair of the power transformer, new filter caps, new bias supply components, as well as new tubes; EH power and preamp.

Equipment:
  • Guitar (Frankenstrat) straight in to the amp.
  • Ibanez delay/echo in the loop on the echo setting.
  • Effects loop volume box after the delay/echo in the loop.
  • Cabinet is an Avatar Vintage 2×12 with Celestion Classic Lead 80 speakers at 16 ohms.

Amp Settings:
  • Presence at zero
  • Bass at 4
  • Middle at 5
  • Treble at 5
  • Master Volume at 4
  • Gain Sensitivity at 18
  • Preamp Volume at 10

The Master Volume is a little touchy between 2 and 3 so the effects loop volume box lets me get past that and then choke it back down to reasonable levels.

CAUTION: Do NOT put any volume box between the amp and speaker cabinet. They are NOT attenuators!

Marshall® is a registered trademark of Marshall Amplification plc.

19 comments:

  1. Thanks for the info on this awesome amp. I bought a 2501 model after a side by side comparison with a DSL 60 watt combo. So glad I got the MKIII!

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    1. Greatness. The 2501 is a killer little amp. That's one I'd like to snag myself.

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    2. I'm currently running mine with a early 80's Celestion G12-65 in the combo and the G12T-75 in a closed back cab...fantastic speaker speaker setup...bass from the closed back and scream from the open back combo. Is there an effective way to date these amps? Only thing I could find was a transformer sticker: Drake Transformer 06-90. June '90? Not sure, just guessing.

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    3. There should be a serial number stamped in the back of the chassis beginning with a letter. Yours should start with either a Y (1990) or a Z (1991-92).

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  2. Hey, I saw a 50watt version of the mkIII on sale for $599? Should I pounce?

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    1. If it's in really good condition and everything is in good working order, that's not a bad price. If it's beat up and has torn tolex and such, it might be a little high. Check it out first if you can.

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  3. Hey Erik,
    I Bought a used JCM900mk3 and got an old peavey 4x12 cab with it (Celestion G12t-85).Is there a nice recommended setup to run samll club's gigs? is a 2*12 or a smaller cabinet can fit? 10x for the help, Roy Levy.

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    Replies
    1. Running a 2x12 is a great option. My main cab is an Avatar 2x12 with Celestion Classic Lead 80's. Check those cabinets out or even a used Marshall 1936 2x12.

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  4. Hello, great site! Found a jcm 900 2502 mkiii for $550. Would this be one I should try to add to my collection?

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    1. I would if it is in good working order. Worth checking out.

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  5. Hi Erik, your website makes me enjoy a lot. An old mkiii appeared in my way, and your knowledge help me to know how repair and adjust it. But I have a doubt in the schematic, it's about the bias. Where do I need 38-40 v? Thanks

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    1. It would be best to take it to a tech if you're not sure. I don't bias mine myself. I have a tech do that for me.

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    2. Ok, thanks but i have to search a good one and i am not sure there is one near my city. I would like to learn, if you know where I could find more information please let me know. Thanks

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    3. Best way to find a good one is to ask fellow guitar players who they use to service their tube amps. A good word of mouth beats a Google Search any day. Hopefully you can find a good one near you.

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    4. Thanks, best wishes from Spain

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  6. Love this site!! Thinking of ditching my JCM 2000 DSL 50 for a 900 mkiii 50 watter (EL-34). Playing punk rock, think Bad Religion, etc. Worth it or will I miss my DSL. PS I never use the cleans on the DSL either. Just overdrive.

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    1. That's totally up to you if you think it is worth it. The DSL is a fine amp in its own right. Try out the MkIII and compare the tone to the DSL and go from there.

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  7. Hi Erik. I have a question about the jcm900 MKIII 2502.

    Do you know what is the exact BIAS point en how to measure it the right way?

    Regards
    Marco
    marcoluten@gmail.com

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    1. The exact bias point is noted in the 50 watt power amp schematic. You'll need a voltage meter to read the millivolts at that point. However, if you are not certain on how to do this, take the amp to a qualified tech to have it done. The voltage can kill you if you are not careful.

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