JCM900 circuit differences ?

[Valvehead]

busy Marshall day at my house....


So, ive got 2 , 4100 model jcm900 Marshalls here for repair .

Check this one out-


AMP #1 has 5881 tubes with the Marshall sticker on the amp that says --5881 tubes only"

this amp seems ok--just needs the pots cleaned (causing the cutting out)---bias looks fine . everything else looks fine, a nice loud Marshall.



AMP #2 has EL34's in it--i dont see the " 5881" sticker on the chassis. This amp shows low bias (about 22ma per tube) , and low volume. This one also has USA stamped next to the 4100............


Heres the weird part--- The Marshall scem shows R30 should be 22k for the 4100 model.

Amp #1 has the 22k
Amp #2 has a 10k

both have the 150k for R26+R27 .

So, is R30 changed for usa vs Europe versions? Or did someone get in there and change that booger to 10k causing the low bias ?

As far as i remember these JCM900's all were meant to have 5881/6L6 tubes ?


looks like someone wanted some EL34's in amp #2 ?

[Alex R]

Don't know about the US but there are plenty of UK jcm900s with EL34s in. I am looking at one right now...

Not sure about R30 but if you ring up Marshall in the UK you can talk to the very nice people in the tech department about it.

[Valvehead]

this is the scem im going by . From the Marshall book:

http://www.freeinfosociety.com/elect...iew.php?id=488

model 4100

[Alex R]

R30 drops the voltage to the preamp. Probably the change doesn't make a whole lot of difference.

[Valvehead]

Alex-----

the one with the eL34's says usa on it , but there are no numbers on the OT.

Is it the same OT as the other head with the 5881's? Thew 5881 amp ot is a part # TXOP-00001


If they are the same trans, why couldnt i install some EL34s in that one and re-bias ?



By the way , the el34 head sounds WAYYY better than the 5881..

[Alex R]

- not up on those numbers, so not sure. However EL34s expect a different OT primary impedance from 6L6s and the transformers are likely to be different. This question of impedance - there's a broad ball park, and it is possible to have a compromise value in your OT that will do for either type of tube, but uness we know the turns ratio in your OT we can't tell.

It is possible to calculate the turns ratio by disconnecting the OT, putting a small AC voltage (eg the 6.3v filament supply) across the speaker side of the OT and measuring the much higher AC voltage that will be generated on the plate side. Methods are around on the web, easy to google. If you can't find them ask and I'll look something up.

Once you have the turns ration you can work out the primary impedance from the speaker impedance you have connected. Then you will know what tubes will be ok on that particular OT.