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Marshall JCM900 tube help


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a friend of mine has a Marshall JCM900 4100 dual reverb and wants to get it retubed. he asked me what tubes to buy and I said 5881 (6l6), but we opened it up to see what brand it had so he could buy the same ones. when we pulled the tubes, they turn out to be Marshall branded EL34s. were both super confused now...

 

i looked up the owners manual on marshalls website, it says it comes with 5881 tubes...

 

he bought this used a couple years ago, and its always had volume dropping in and out issues but never retubed it cause it barely gets used. could it be the tubes just being bad (they look factory, so theyre deffinatly bad) or could the previous owner have put the wrong tubes in it and caused some permanent damage?

 

its a white tolex covered head, which weve never seen before... could it be like a limited edition head with el34s? probably not...

 

 

 

so bottom line, should we buy 6L6s? or stick with EL34s that are already in there?

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The did EL34 & 5881 versions. I guess England had EL34 & the States got 5881, but I couldn't say for sure. Someone could've re-covered it or they do odd exclusive tolex's for shops now and again. Like, a chain of shops here had the current Marshall range in British Racing Green and old school grillcloth last month. I don't think it would be more than that.

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Hey xbryanx,

 

I own a 1990 Marshall 4501 1x12 combo amp. The previous owner had 6L6GC power tubes in it and I had my local amp tech rebias the amp to the factory original EL-34 power tubes that came with the amp. It made a big difference to the sound of the amp for me. Your friend's 4100 amp head comes stock with EL-34 power tubes. The Marshall factory did change over 5881 power tubes during the mid '90s due to scarcity of EL-34 power tubes at that time. I would recommend keeping the EL-34 power tubes in there and unless you're going for different sort of sound.

 

The possible reason the volume is dropping in and out is the effects loop jacks. They need to be either cleaned with contact cleaner and plug the cord in & out of the effects loop jacks to clean the contacts or the effects loop jack needs to be resoldered to the circuit board. I had a similiar problem to my 4501 and sprayed contact cleaner in the effects loop jack and used a patch cord to clean the contacts of the effects loop jacks.

 

I also did an EQ/gain mod to my 4501 that improved the tone and gave it a little more gain. If you're interested in it, let me know.

 

Guitar George

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The 900 did indeed come from the factory with both tube types. I've always preferred the 6L6's since they added some girth to the overall tone and made the cleans a bit nicer. For more grind you could go with the EL34 though. Really, whichever you prefer.

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Hey xbryanx,


I own a 1990 Marshall 4501 1x12 combo amp. The previous owner had 6L6GC power tubes in it and I had my local amp tech rebias the amp to the factory original EL-34 power tubes that came with the amp. It made a big difference to the sound of the amp for me. Your friend's 4100 amp head comes stock with EL-34 power tubes. The Marshall factory did change over 5881 power tubes during the mid '90s due to scarcity of EL-34 power tubes at that time. I would recommend keeping the EL-34 power tubes in there and unless you're going for different sort of sound.


The possible reason the volume is dropping in and out is the effects loop jacks. They need to be either cleaned with contact cleaner and plug the cord in & out of the effects loop jacks to clean the contacts or the effects loop jack needs to be resoldered to the circuit board. I had a similiar problem to my 4501 and sprayed contact cleaner in the effects loop jack and used a patch cord to clean the contacts of the effects loop jacks.


I also did an EQ/gain mod to my 4501 that improved the tone and gave it a little more gain. If you're interested in it, let me know.


Guitar George

 

Hey dude, I'm not being funny but how do you know what date his amp is? I just had a look and the '93 USA schematic is for 5881's.

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The early 900s shipped with EL34s. My 2500 mkIII has EL34s in it. I've tried both JJ E34Ls and JJ KT77s (which are in there now) and have had great success with both. I've tried a lot of preamp tubes, and I like the JJs the best.

 

You'll be fine with any kind of EL34 tube, biased correctly of course.

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Hey dude, I'm not being funny but how do you know what date his amp is? I just had a look and the '93 USA schematic is for 5881's.

 

Hey Megadeth Man,

 

The early Marshall JCM 900 amp heads/combo amps came stock with EL-34 power tubes. Marshall did make revisions to the JCM 900 amp series over time so the schematics changed over a period of time. There's a book called "The History of Marshall" that has schematics of the Marshall amps including the JCM 900 series. I highly recommend this book if you're a Marshall aficiando like me. The early 1990 schematics in "The History Of Marshall" book show the JCM 900 amps to have EL-34 power tubes. You're reading a later version of the JCM 900 amp schematics. This website has some pretty good information on Marshall JCM 900 amps:

http://www.drtube.com/marshall.htm#JCM900

 

It's easy to change from 6L6GC power tubes to EL-34 power tubes or vice versa with a qualified amp tech. The true Marshall tone is with EL-34 power tubes, IMHO. The smooth midrange is its signature tone. I also own a '92 blue vinyl Marshall 30th Anniversary 6101 1x12 combo amp that totally kicks ass onstage.

 

Guitar George

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Hey Megadeth Man,


The early Marshall JCM 900 amp heads/combo amps came stock with EL-34 power tubes. Marshall did make revisions to the JCM 900 amp series over time so the schematics changed over a period of time. There's a book called "The History of Marshall" that has schematics of the Marshall amps including the JCM 900 series. I highly recommend this book if you're a Marshall aficiando like me. The early 1990 schematics in "The History Of Marshall" book show the JCM 900 amps to have EL-34 power tubes. You're reading a later version of the JCM 900 amp schematics. This website has some pretty good information on Marshall JCM 900 amps:

http://www.drtube.com/marshall.htm#JCM900


It's easy to change from 6L6GC power tubes to EL-34 power tubes or vice versa with a qualified amp tech. The true Marshall tone is with EL-34 power tubes, IMHO. The smooth midrange is its signature tone. I also own a '92 blue vinyl Marshall 30th Anniversary 6101 1x12 combo amp that totally kicks ass onstage.


Guitar George

 

Oh, I get you. You're not saying his actual amp came stock with EL34's just that the original issue of 4100's came with EL34's. I believe I've scanned Doyle's book once or twice before. ;)

 

Totally with you on the true Marshall EL34 tone thing. :thu:

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hey im the guy that owns the JCM900. as far as a dsl goes im just not a fan of the tone. i love the sounds of my 900 way too much. i want to thank everyone for letting us know that the el34 was first in the jcm900 line. i wasnt even aware of the tubes in my amp and why i loved the sound so much. its why i find myself playing it more than my triple rectifier.

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