Hey UG!

I've got a Marshall JCM800 Bass Series top that's seemingly died out, but I have little to no knowledge/experience with amps, so I'm hoping some of you could give some advice about it!

Basically, the output is low and distorted. It's just as if somebody's reduced the output by 75% and plugged a distortion/overdrive pedal into it. Needless to say it's impossible to play properly with this type of sound. We've tested whether it could be the bass itself or the bottom stack, but it's none of these - it's the bass amp itself!

My dad bought this amp quite some years ago now and has never really kept it clean etc. or used it at all actually, so I'm suspecting perhaps one or more of the tubes is malfunctioning. This would make sense regarding the massively reduced volumeoutput, and I gueeeess it could cause the distorted tone as well.

I know it's a rather vague basis for a diagnosis - but does any of you know what might be wrong?

Thank you!
Needs new tubes.
Quote by Dave_Mc
I've had tube amps for a while now, but never actually had any go down on me
Quote by jj1565
maybe you're not saying the right things? an amp likes to know you care.

Tubes are good, and good for you.
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
If you are in North America contact Doug at www.dougstubes.com

He'll set you up if you tell him what you have.

I'd recommend JJ EL34s for power amp and pretty much JJ 12AX7s for the preamp. If you want to get a little more elaborate than that just ask. I've bought JAN Phillips 5751s, Tung Sol, and Penta Labs 9th Gen from him and I can personally recommend those as well.

if you are in Europe try www.hotroxuk.com
Cool - so it's pretty positive it's one or more of the tubes that's died out? That's what was making the most sense to me as well! I really don't know anything about how they work, but I'll read up on it a bit, buy some new tubes and get it fixed myself then!

Thanks a lot guys - you just saved me the good couple of bucks having it repaired at the guitar shop!
When you replace power tubes you almost always replace all of them. They work in pairs. Buy matched sets. I have a Blog on biasing an amp that is similar to yours. You can use that as a guide. If any of that seems scary or foreign then pay someone to bias your amp. It is important for the tube life, performance and most importantly - the tone of an amp (fixed adjustable push/pull amp) like yours
Last edited by 311ZOSOVHJH at Oct 28, 2011,
If you are short of a buck, Shuguang EL34's work surprisingly well in those. In decreasing order of goodness: SED, JJ, Shuguang. JJ is the sweet spot. SED are better but twice the price. You get close to SED quality and close to Shuguang price. But if you are really pressed for cash the Shuguangs are fine.
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