Marshall JMP 50 Watt 1979 questions

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jbcrow, Feb 26, 2009.

  1. jbcrow

    jbcrow Member

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    Hi Guy,

    Just wondering what you use for tubes in your vintage Marhalls. I've always just used the usual. Im reading different things. Some say 5881's some say 6550 or EL34's. Havent tried 6550's yet maybe someone can recommend a good brand for me. Ultimately when I use the JMP its more for vintage tones. I was getting alot of hum and extra noise so I just snipped one wire on the cap that connects to the volume and that seemed to help alot. Im not big on mods, ever, but this seemed to help. Just seeing if you might have done small mods that helped the tone as well. Thanks for reading, J
     
  2. DEMENTED

    DEMENTED Supporting Member

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    I always prefer EL34's in my Marshalls...it makes them sound more like a Marshall should IMO. snipping the bright cap usually sounds good too.
     
  3. sugarlou

    sugarlou Supporting Member

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    I had a JMP that came w/6550's ..I converted it to EL34's and swear it sounded better w/ 6550's!!
     
  4. Rod

    Rod Tone is Paramount Supporting Member

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    I played JMP's for years and always converted them to EL34's....If the conversion is done correctly, they sound killer with EL34's...the 6550's back then just sounded to bright and to tight,,,sort of sterile...The el34's have the sould for Marshall's...but, I'm a classic rock/blues LP player..NOT into anything metal...ever
     
  5. jasonh23

    jasonh23 Member

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    Nothing quite like properly biased (and high quality) EL34 tubes. The new production svetlana el34s sound incredible in a Marshall.
     
  6. jasonh23

    jasonh23 Member

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    One more note.. most hum problems can be taken care of with a thorough cleaning. Make sure all of your pots and tube sockets have been cleaned and cared for. As well, don't underestimate the 12ax7 tubes. The last tube before the power tubes (phase inverter) is crucial. Buy high quality pre-amp tubes and buy extras.. they are not all created the same, even straight off the production line.
     
  7. kurtsstuff

    kurtsstuff Member

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    I prefer the 6550's for more headroom basically because most times your running some "Dirt Provider" into the amp..better response eq wise across the board...
     
  8. FrankieSixxxgun

    FrankieSixxxgun Member

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    Which JMP are we talking? 1987 or 2204? For a 1987 EL34's in the power section, for a 2204 I'd go with 6550's. I don't care for 6L6/5881 tubes in any Marshall. Too Fendery sounding.

    What you did was clip the bright cap, which is the most common mod to do to any Marshall. Probably the only one you'll need to do too.

    I'm only talking new here, and my favorite new EL34's are actually the Groove Tube EL34-M ones. Make sure it's got the M in the model number, and you can talk the guy down at GC or Sam Ash to $36.99 for a pair. For 6550's the new Tung Sol reissues are about as vintage as you're gonna get. My bassist runs six of them in his '72 Ampeg and man do they sound nice.

    I like to describe EL34's as classic rock sounding and 6550's as LA/80's rock sounding. Seems to be the best way to get it across to people.

    Edit - I should also add that unless it was originally a USA model Marshall, you might need to change the bias resistor to get 6550's hot enough in there. Also FWIW I run a pair of RFT EL34's in my 1987 and run a pair of GE 6550's in my pop's 2204.
     
  9. Swarty

    Swarty Member

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    Actually it is the other way around. Amps set up for EL34s are going to run 6550s very hot and amps set up for 6550 are going to run EL34s very cold.
     
  10. FrankieSixxxgun

    FrankieSixxxgun Member

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    Really? I thought 6550's took more juice than EL34's, so they're need more mA than the EL34's. School me Swarty, you just blew my mind!
     
  11. sickboy79

    sickboy79 Member

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    EL34s all the way. I've never cared for the 6550 tone myself.
     
  12. jerrydyer

    jerrydyer Member

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    yes el34 for sure BUT, I had this set of old (real) Tung Sol 6550's Those sounded a lot like el34 but with a little extree sizzle on top or actually it was hard to describe but they are phenominla tubes. They are still in the amp I beleive. I sold it to someone a couple years ago. It was a 79 50 watter.
     
  13. danj

    danj Supporting Member

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    I currently have KT88s in my 79 2204. Almost a little too much headroom, but the low end is awesome!
     
  14. gldtp99

    gldtp99 Member

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    Both 6550's and EL34's sound good in these amps---i have both versions--- an '81 JMP 2204 w/Sylvania 6550's, a mostly Metro 2204 w/EL34's--- also a '76 1987 that was orig 6550's but converted to EL34's.
    All are different, but very good, sounding-----it depends what sound you're going for, if and what kind of pedals you use...............gldtp99

    PS--- I'm trying out a pair of the GT EL34M's in a build now--- i'll compare them to JJ, Shuguang, and RFT EL34's in this head this weekend---- it's a Blonde Bassman (started as a 6G6-A circuit---now has several changes) using EL34's and repo Marshall iron---- sounds like a Blonde Fender despite the Marshall-type OT and EL34's.
     
  15. Swarty

    Swarty Member

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    Well it's like this; Bias the amp for EL34s and you're going to want somewhere in the neighborhood of -35V DC on the grid (in a typical Marshall) and that might have the tube drawing 40mA or whatever. Put the 6550 in there without changing anything and the 6550 is typically going to run too hot, say 90mA or whatever and you'll need to increase the grid bias voltage to say -50V DC to put it in its comfortable range say 45-50mA or whatever.

    Bottom line; 6550s will typically run too hot when put in a circuit biased for EL34s.
     
  16. jbcrow

    jbcrow Member

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    very helpful, cheers! JC
     

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