Excessive noise on JCM2000

Floydoid

Hi everyone,

You have a real nice forum here and this is my first post. ?

My question: I have been looking at getting a JCM2000 DSL100 head recently but the few that I've tried all have much more inherent noise compared to my JCM800.

I've read on another US forum that it has something to do with fitting a "choke" to the amp or changing the output transformer.

Can anyone here shed some light? ???

inflames

Welcome to the forum Floydoid!

I have a JCM 2000 DSL and don't have any funny noises on it!
It could be the output transformer.

When you test these noisy amps do you have your JCM 800 with you?
You could then check if your 800 give the same noise at the place where you test the 2000.

If both amps have a noise then it is dirty power and you will need to get a power conditioner.

Floydoid

When I switch my JCM800 on, with the standby still on, my amp is dead quiet and when I swith the standby off the amp is still quiet - no hiss.

By contrast when I do this to a DSL100 there is hum coming from the amp and hiss from the speakers when I turn the standby off.

Maybe it's just the nature of the beast?

inflames

Ok so I assume you tested the JCM 2000 at your place.
So the JCM 800 don't hum, but the JCM 2000 hums.

So it is definitively not a "dirty power" issue.
So I will go with the output transformer.

All I can say is that mine is as quite as a mouse!

Gearhead

inflames wrote So I will go with the output transformer.

I can't really say that I would understand why an output tranny would cause hum. If there are as many windings to the positive side as there are to the negative (and I'm sure Marshall can get that right) then there is really no reason for hum there. I can think of a dozen other hum sources in a tube power amp (mentioned them a couple of weeks ago in another thread), none of which you can't solve spending some time on it. If the hum comes from the power supply (would not be my first bet) then, yes, a choke might be a good idea. For this size amp it is costly and heavy though...

The other part of the story is: what is so bad about a little hum? If it is excessive hum or hissing , yeah, I can understand but a little 50 cycle tells you you're good to go...

The last thing about this thing that I don't understand (call me stupid) is why you would want a JCM2000 having an 800. Care to explain?

matta

Gearhead wroteI can't really say that I would understand why an output tranny would cause hum.

Hey Gearhead,

I can think of one.... induction... I've worked on some newer/modern amps and the placement is TERRIBLE! 80% of the hum on the Fender Champs I mod comes from the transformer, The power transformer is just too close to the OT and there is induced hum, you can PULL the tubes and still hear the hum... the Champ is not alone in this, seems not as much thought goes into production amps these days... a sign of the times.

Cheers

Matt

Karel-Mars

Floydoid wrote When I switch my JCM800 on, with the standby still on, my amp is dead quiet and when I swith the standby off the amp is still quiet - no hiss.

By contrast when I do this to a DSL100 there is hum coming from the amp and hiss from the speakers when I turn the standby off.

Maybe it's just the nature of the beast?

I've worked on two DSL100's and have noted a few strange things about them. Build quality is good, but there are a lot of pc boards and connectors inside. If one of the earth leads are broken off (happens easily when you fiddle about the innards of the beast) the amp is going to hum. The JCM800 has a much simpler circuit - less components in the signal chain - less white noise.

Another thing I picked up from a web article, that proved to be right (do a search on JCM2000 problems) is that wrong value resistors are inserted in the EL34 grid stopper positions: in stead of 2K2ohm I found 220Kohm. That's going to cause some hum and tube failure. This can be fixed.