2 Marshall JCM800 2410 amps, nearly identical.

[bigdrums925]

Hi all: I have 2 Marshall JCM800 2210 amps (not 2410 as in the title) and one of them is noticeably louder than the other. As far as I know both are working properly.
Everything about these seem identical except the circuit board and an additional 500VDC cap the other does not have.
I need to make the weaker one sound and respond like the other.
Any suggestions how to systematically go about doing this? The first pic is the one that is quieter

[Justin Thomas]

Dumb question, but since you didn't mention it... same speakers? As in, exact same one between two amps? Same exact tubes in exact same positions?

Justin

[bigdrums925]

not a dumb question but yes, exact same speaker cab, the impedance switch is set to 8 ohms on both amps, and settings the same.
The power tubes are Grove tubes in the louder and JJ's in the other. I'll try switching the tubes,
thanks

[Justin Thomas]

Don't forget the preamp tubes, too..

Justin

[m1989jmp]

At which volume setting are you comparing them? The potentiometers can be quite different, try comparing the amps when the pots are set the same resistance-wise.

[fireman]

They are different versions of the amp, one has the IC for Chanel and reverb switching, the other has transistors, have you traced the circuit to compare components and values? Lots of info on the net about how different the earlier verses later amps sound, maybe. Volume levels too.

[xtian]

First question to answer is, are both amps healthy and putting out the spec'd wattage? Measure the before-clipping output wattage of each and report back!

[bigdrums925]

I swapped all the tubes and no change.
One amp is louder than the other at all volume levels, even full on

[Enzo]

OK, so it is consistent. Apply the same signal to both, and trace through th amp to see where it gets bigger/smaller. There may be a problem, or even a wrong part. Or even a production change.

[Pedro Vecino]

As I see it, it has all the appearance of being different models. The top, oldest, with pre-overdrive equalization. The lower one is the modern (more usual, the good one), with a diode in parallel with 10K cathode resistor in one triode (as quick identification method). If this is so, itīs impossible for them to sound the same.
Older: http://drtube.com/schematics/marshall/4210.gif
Modern: http://drtube.com/schematics/marshall/2210prem.gif

[Chuck H]

I swapped all the tubes and no change.
One amp is louder than the other at all volume levels, even full on

You should test the actual output wattage properly with a dummy load, signal generator and a scope. Master volumes wide open of course. Test for actual max clean(ish) watts at any control settings for either amp. That is, you may need to adjust either amp differently for maximum power. It could still be that at identical settings for both amps, one is clipping the preamp before driving the output to full power. Without accurate schematics of both incarnations it's hard to know why this should be the case, but it may well be.

It looks as though either amp uses a different master volume circuit. This in itself would almost certainly alter where either control is set for the same performance. That's why you need to test both amps with the master volumes at full and because there may be other circuit differences (not the least of which would be pot tapers) you cannot expect to set any other controls on either amp identically.

Knowing when either amp is clipping the power tubes or clipping the preamp will require signal tracing as Enzo suggested.

[bigdrums925]

Seems to me the only polite thing to do when a problem is resolved is a post stating that. So here goes: First and foremost this is one awesome website and particularly because of all the collective experience of those willing to spend their time helping others, Incredible!
The result of all the feedback was finding a website doing mods on these Marshalls saying:

Important Modifications do not apply to models manufactured in 1982 & 1983 - Marshall Date Codes
◾The 82' & 83' models were not designed the same & have little/no gain by comparison
◾Modding these models requires removal of stock circuitry & installation of a custom hand-wired circuit (1 or 2 channels) - Call for details & pricing

So apparently there's little chance these two amps will ever respond similarly i.e. little/no gain by comparison
Tells me all I need to know to tell the client

I again want to thank each of you for your comments, suggestions, and valuable experience shared so generously. Years ago I had a tutor that taught me the basics. He is declining and not able to provide the details of his vast experience any longer. Having access to this site and the wealth of experience is invaluable.
Thank you.

[Chuck H]

One incarnation of that model having "little/no gain by comparison" to others in that line might be a misnomer. It's probable that the distortion available from either amp is different, but distortion isn't gain, per se. Distortion is an artifact of too much gain. But I wouldn't want this to turn into a semantic muddle. My point is just that I'll bet either amp makes the almost the same clean watts when set up properly. And those settings would likely be different for either amp. If the user wants two identical amps then this doesn't work, but one could probably serve as a backup for the other if the player is willing to learn both amps control setting peculiarities.