JCM900 dual reverb problem

[dvoosten]

Hi all,

I have had a jcm900 for some years now, and I am very happy with the sound. There is however one problem, and that occur when I switch from channel b to channel a. Channel a will be as clean as it gets, with plenty of reverb and channel b will be over the top distorted without reverb. If I now switch, the reverb tank will still be filled with noisy distorted sounds from channel b, which will then be output to the speakers in the clean channel. Apparently only the output of the reverb tank is controlled by the channel switch and not the input. Is this how it is supposed to be?
I have made a hack by making a little timing circuit in my footswitch that switches the delay of for a while upon switching channel and I was wondering if anyone had other solutions for this problem.

best,
Dries

[Robert M. Martinelli]

model number?

Hi Dries,
could you check what the model number exactly is ?
Regards
Bob

[dvoosten]

re: model number

Hi Bob,

it's a Marshall JCM900 4100 100-Watt top.

Dries

[Robert M. Martinelli]

Hi Dries,
you won't believe it, but the model number alone could be not enough, as the 4100 has been manufactured in several variants from 1990 until 1999....

Anyway, I think you should check your preamp section, in the reverb recovery circuitry there is a small switching IC named M5201, and I seriously think you should take a look in that area and maybe try to replace it. I don't remember if it is socket-mounted or not; if this is not the case, adding a socket would be a good idea to make future replacements easier.

Hope this helps

Best regards

Bob

[Enzo]

SInce your channel is way out of whack and distorted, fix it first, and the reverb situation may correct itself.

[dvoosten]

reverb recovery question

Hi Bob,

I''m at work, so I can currently not check the exact model number. But I have checked out the marshall preamp schematics and have found the part you are talking about. But the schematic seems to confirm my suspicion that there is indeed no switch before the reverb tank and that the amount of reverb is only set by the potmeter after the tank. That would be completely consistent with the problem I am having, which would mean there is nothing broken, but it's actually a design flaw. Which would mean I should not be the first person to complain about this! Could you explain to me what is wrong with my conclusion?

thanks,
Dries

p.s. Enzo, with out of whack and distorted, I mean the typical marshall crunch sound. So that's ok!

[Robert M. Martinelli]

Hi,
I' ve checked the several variants of that preamp and all of them have the same switching IC. This said, I am thinking about a component's failure rather than a design flaw.

That M5201 is designed to be a sort of "solid state switch", controlled by voltage on pin 1, all I can think about is that your M5201 has some problems and doesn't switch the signals the way it is supposed to be, this could be the reason for the "crosstalk" at the reverb recovery you' re experiencing.

Hope this helps

Best regards

Bob

[Enzo]

Not sure about the problem here. The reverb is after the preamp. After the effects loop even. Whatever comes out of the preamp goes to the reverb. When you switch from dirt to clean, whatever the last thing out of that dirt channel was will still have to reverb itself away. How could the reverb springs quell themselves after a channel change even if the input was switched off? You could switch the inputs, but then you trade off the tailing reverb you don;'t want for a lack of continuity when you do want it.

If you holler in an empty warehouse, the sound will continue to reverberate even if you start talking quietly.

[dvoosten]

So it is a bug or a feature

Ok, so we seem to have a little disagreement here on whether my problem is actually a bug. Enzo points out that the reverb is expected to spew out the loud noises for a while, and therefore suggests that therefore there is nothing broken. Voxrules thinks that there is something wrong and the M5201 switched gain opamp might be fried.

Looking at the schematics I think that Enzo is right and this is standard behavior, which is what I was afraid of.

I do consider this a bug. Normally speaking when you have an effect, just changing the wet/dry balance and the output of the effect is enough. In the reverb in this configuration, the wet/dry balance is changed by the switching between channels. I'm thinking that in the case of a reverb, it might have been a better choice to control the wet/dry balance by changing the input signal of the reverb tank instead of the output.

Anyway, it seems I have to live with it.
Thanks for the discussion guys!

Dries

[Robert M. Martinelli]

Ok, so we seem to have a little disagreement here on whether my problem is actually a bug. Enzo points out that the reverb is expected to spew out the loud noises for a while, and therefore suggests that therefore there is nothing broken. Voxrules thinks that there is something wrong and the M5201 switched gain opamp might be fried.

Looking at the schematics I think that Enzo is right and this is standard behavior, which is what I was afraid of.

Hi,
maybe i got it wrong from the beginning, but nothing in your "thread starting" post said the noise was there "for a while" - it only stated that the noise was there - period - so I understood the noise was there at all times after switching the channel. This made me think the M5201 had problems.

If the noise does disappear "after a while" as you're now saying then I agree with Enzo.

At this point you might start another thread about how to get rid of this bug

Best regards

Bob

[Enzo]

If I sing into a mic with a lot of reverb added, and then mid-note I pull out the mic cord, the reverb tails off afterwards until it dies. If somehow the reverb output were to detect the loss of mic connection and kill the reverb output instantly, you would get that ran into a brick wall thiing with the sound ending. You may not like it, but I don;t consider it a bug.

But considering it as something you would change, then please define exactly how the amp should operate under your guidelines? I am assuming you'd want the unused channel not to be feeding the reverb drive? So adding another set of switching ICs to the drive input could be accomplished. Is that all you need?

On the other hand, we might want the tail off switching the other direction, and it would be gone.

[Steve Conner]

FWIW, I recently built a channel switching amp with spring reverb that does this, and I don't consider it a bug. Like Enzo says you can't magically stop the springs in mid-twang.

[Enzo]

When there is just one reverb and it is after the preamp. I am not sure what we can do.

If we took the reverb tapping point much earlier so there could be separate ones for each channel, you could control things.

[dvoosten]

fixes

When there is just one reverb and it is after the preamp. I am not sure what we can do.

If we took the reverb tapping point much earlier so there could be separate ones for each channel, you could control things.

Something that might be important for the discussion is that when both channels have the same amount of reverb, the behaviour is not a problem.
In that case, you want the reverb of the crunch channel to last for a little while after you have switched to the clean channel. The behaviour is annoying only when the amount of reverb in the clean channel is set much higher than in the crunch channel. I think it is not uncommon to have full reverb on the clean channel, but this would completely screw up the definition of the crunch channel. As an example, when you would be playing Radiohead's creep with just one guitar, you would need you crunchy sound to be very dry and you clean sound to be very reverby

My guess is that changning the reverb pots to stereo pots that control
both the wet/dry balance in the reverb recovery and the gain in the reverb drive, both indepedently for both channel, the behaviour would be appropriate even in that situation.

[jaz10]

Related Problem with JCM900 - 4100

I've read a bit on this thread and appreciate any guidance out there. I've got what I think is a 'related' problem. My symptom is -- No channel switching at ALL - I suspected one of the MS5201, then found that the preamp voltage was off. I've got +/- 29vdc coming off the small rectifier on the preamp, but where it needs to be +/- 15 vdc after the first 2 resistors -- I'm getting +/- 2.9 vdc. I've checked all the caps, diodes and resistors in that area -- and they seem fine -- I can't seem to get +/- 15vdc out of that area.

Any ideas are appreciated. thanks