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maxkotzen 07-10-2009 09:36 AM

jcm 900 5881 to EL34 conversion by
Maybe this could help too i own a jcm 900 dual reverb with 5881 tubes.
i sent a mail to and here's the answer i got from them:

Biassing Marshall JCM900's

To be able to measure the bias current I'd put an 1 ohm/1W
resistor in series with the brown wire going to the output
transformer (see attached picture). That way you can measure
the bias current flowing through all EL34's. I'd bias at
around 35mA per EL34 (= 70mV for 50W amps or 140mV for 100W
amps over the 1ohm resistor)

When replacing power tubes ALWAYS rebias!!!!

Changing from 5881's to EL34's in JCM900's.

If you compare the schematics fo the 5881 and EL34 versions
you'll see that the differences are minor.

The power tube sockets are allready ready for EL34's, and
don't need to be rewired.

The EL34's need a "less negative" bias voltage than 5881's,
so the bias circuit has to be changed slightly. I'd suggest
adding a resistor of 47k parallel to R28 (should be 47k).
That way the bias current range shifts up (less negative
control grid voltage).

Furthermore I'd replace the screen grid resistors (R31 & R34
470R/5W) to 1k/5W ones and not to 2k2/5W ones as suggested
on the EL34 schematic.

just wanna know what do you all think about this....since i don't understand nothing about electrical stuff and im afraid to destroy my jcm 900

RobS 07-10-2009 09:41 AM

Re: jcm 900 5881 to EL34 conversion by
can you include the pictures that were sent to you?

maxkotzen 07-10-2009 10:03 AM

Re: jcm 900 5881 to EL34 conversion by
i would love to but he forgot to add the pic.

gemather 07-10-2009 10:34 AM

Re: jcm 900 5881 to EL34 conversion by
my amp was build for germany,
allways thinking build for people understanding nothing.
It was configured to fullfill both,EL34 and 5881 tube´s.
With the less power to EL34.

From the difference of volume there was no manner between them.

There was build in 2k2/5w from R31toR34,and i never change what is not broken,because you have no offer.
You may destroy the bord by soldering.

maxkotzen 07-10-2009 10:51 AM

Re: jcm 900 5881 to EL34 conversion by
can you explian better??
did you modify your amp to fit both 5881 and El 34????

and if i can ask...doy ou like it more with 5881/6L6 or EL34???

i read on the EUROTUBES site that mostly 99% of jcm 900 players who goes there to get their amps retubed locally, they leave with 6L6 because it just sounds better.

instead wrote me in his mail that jcm 900 were designed to run EL 34
and they just sounds better with El 34s but,
due to the world crisis of early 90s, EL 34 were not avaible, so Marshall converted the amp with the 5881.
As the El 34 were avaible again the went back on using them on their latest jcm 900 series.

HEADSMASHER 07-23-2009 11:16 AM

Re: jcm 900 5881 to EL34 conversion by
just done liams suggested conversion to el34's for a dual reverb and the mods found here and am very impressed with the tone. awesome is the word. Get an amp tech to do this to your jcm900 dual reverb and you wont want another marshall. all the usual ac/dc to zep tones are now present and correct. Of course also make sure you got good tubes and good filter caps or it aint gonna be worth doin anything!

American Viking 07-23-2009 03:52 PM

Re: jcm 900 5881 to EL34 conversion by
the e-mail above has good information in it but he's left critical components out of the equation.
You can't just willy nilly bias tubes up to 140mV and 70mV or whatever he is talking about, there's a formula involved and you should figure that out because diving into a tube amp, these are primitive damn near stone age electronics and they are sensitive and if you don't know what's going on you can kill yourself or kill your amp.
You don't need to change anything within the amp until you know that you cannot get the bias in the proper range. Changing something too soon may mess you up.
All you have to do is simple, put whatever tubes you want in those sockets, I don't care if they are 6550's, KT77's EL34's, 5881's or whatever, they'll all work... Check your plate voltage, which you take a multimeter set to 600-1000 DCV and touch pin #3, the third pin clockwise from the guide pin of the tube and read the number that comes up on your meter.
As far as soldering some 1 ohm resistor in series with brown wi...- whatever, forget that. Spend $25 and get a bias probe from Eurotubes or anywhere you can find one. This will make your life so much easier and if you're confused about getting inside these amps these "tech guys" need to help you find the easiest way not telling you to go crazy with a soldering gun for the first time. You put the tube in the bias probe, put the bias probe in the tube socket, set your multimeter on the 100 mA scale and you can leave your multimeter on while you check and change your bias. Here's that formula...

(plate dissipation / plate voltage) x 0.7 = correct bias

here's the plate dissipation of tubes...
EL34, E34L, KT77, 6CA7 = 25%
5881, 6L6, 6L6GC = 30%
6550, KT88 = 35%

It's really easy if you have two multimeters, one to just have the bias probe in the whole time. If you have the bais probe hooked up to the meter and turn it on after the amplifier it will blow the fuse in the multimeter. When you check the plate voltage be sure to have the black (neutral) lead on your multimeter clipped to the chassis of the amp for ground. All readings and adjustments must be done with the amp hooked up to a cab and in the on (play mode) position with no instruments hooked up to it.

The only reason you would have to change the final bias resistor is IF the bias won't reach the correct range. This is a big IF because it is not typically necessary but it is sometimes. I would always try to bias normally before going nuts and tearing my amp apart.


HEADSMASHER 07-24-2009 02:57 AM

Re: jcm 900 5881 to EL34 conversion by
"As for that "Liam" mod above, that's not a conversion mod that's a complete tone overhaul. You don't need to do any of that crap just to change tubes over that's not even what that mod if for and he's not even the first one to do that mod anyway."

Put yer glasses on mr grumpy viking and read my post again, i dont say "liam mod" you just made out i said that and i aint making any claim i was the first! Jeez, wot some people do to big themselves up eh? Nice reply to my first post mate, all misunderstood and jump down my throat why dont ya?
I posted it so people would know the liam suggestions work and give an awesome sound and that the other mods also work.
If you contact marshall themselves they will tell you around ten items need changing inside the amp to convert to el34, seeing as they designed and made the amp i think i'll go with their advice. They have been making the odd amp for a few years now! It's also worth noting that the amp was made originally for el34's but due to a shortage of those tubes when the amp was being sold it was converted by marshall to use other tubes! Either way you always get some oldtimer saying you shouldn,t do this or leave that alone but how boring grandad, life better with options and adventure!

I dont think i'll bother hanging around in here if its gonna be smothered by bullies like viking grump.
I got my own succesful amp manufacturing and repair business and i'll hang out elswhere, there's plenty other forums.

gtrman 07-24-2009 05:17 AM

Re: jcm 900 5881 to EL34 conversion by
Sorry to hear that .. this forum has many excellent contributors and American Viking certainly is one of them. He has had a lot of informative posts.
Just ease up little and take a deep breath... have a beer .. and try again. This forum is actually pretty "mild" as far as snipe remarks. I have seen plenty worse ...
Enjoy your stay I'm sure you will find the forum very helpful ! (absolutely no sarcasm whatsoever)


American Viking 07-24-2009 11:22 AM

Re: jcm 900 5881 to EL34 conversion by
Not trying to bust your balls but this isn't the first thread for the 5881 to EL34 in a 900 where people are looking for straight answers about the conversion so that kind of post throws em off. I'm sure your intentions were fine but you threw an off topic mod in a thread where some, like myself, may construe it as misinformation on how to achieve conversion from the two tube types, seeing as how that's what this thread is about. I wasn't trying to jump down your throat, I just stated that that mod had nothing to do with the conversion and when I said "he wasn't the first" I wasn't referring to you. Sorry for any confusion.

I'm seeing a lot of dudes tear their amps apart because of bad/misunderstood information and I'm just trying to protect my beloved Marshalls around the world and save some players the heartache of losing their pride and joy.

I wouldn't let little old me keep you from the rest of this forum. There are a lot of cool and helpful people on here and a couple of asses too, me included.

Shawn Fate 07-24-2009 05:17 PM

Re: jcm 900 5881 to EL34 conversion by
keep in mind not all the JCM 900's need 10 items changed,Even Marshall will tell you,Yours might not even need the screen resistors changed.It depends on what marshall had at the time it was built.
best thing to do is get the schematic and see what is already in the amp and what you will need.

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