Marshall JCM 900 Not the Worst Amp Ever.

I just finished repairing a JCM 900 4100 Dual Reverb amp for a friend. He bought it and the cabinet really right I think about $600 or so and it's very clean and it looks like brand new.

He brought it to me at a gig a week or so ago and said that it sounded bad and the sound was dropping out. It turned out that it had some nasty pots that needed cleaning and that cured the drop out problem. The output tubes were a new looking set of Groove Tubes EL34S tubes. I checked the bias on the amp and they were running at about 15ma idle! According to the book this should be about 38ma at this B+ voltage(475 volts). I noticed then that one of the tubes was gassy. I checked it out and found a crack down close to the base!

What I think happened was that someone stuck the EL34's into this amp which being a 1993 production must have come with 5881's from the factory. The tube retainers were EXTREMELY tight with king kong springs. I don't know if this could have cracked the tube or maybe somebody fumbling around in the amp putting the tubes into the sockets might have done that, who knows.

I happened to have 4 brand new EL34EH tubes on hand so I put those into the amp at 32ma and it sounds MUCH better.

I wouldn't say that this amp has much of a warm tone in either channel but it's not horrible. I know Myles(guitar player in my band) could get by on a gig with this amp. It's very much a hot rodded tone and it comes from that philosophy. Is it worse sounding than say a Peavey 5150? Probably not.

We'll see how he likes the amp now that it's at least working as intended. My question is, how do you tell what output tube a Marshall from this area used originally? Were there any circuit changes? I'm tempted to try it with a 6L6GC and see how the voicing sounds. Right now with the EL34 it's pretty darn bright but maybe the customer will like that in the amp. Do you think Marshall voiced this amp with the Russian 5881 Sovtek in mind? If so it might be the superior choice in a JCM 900 head like this one...