JCM 900 Dual reverb Mod question

[Kev 335]

Hi Guy's

I recently bought a used JCM 900 Dual reverb amp with a view to add some mod's
I completed a few as recommended by Soundguruman ..so far so good

However, I noticed a mod on the board I didn't recognize!
I searched the planet for clues but came up with zero reference to this change
as in the pic below





R6 (1 M) is piggy backed with an RC circuit ...I have no clue why this has been done .......Anyone?

[Chuck H]

The mod you're questioning is a high frequency bleed to ground. A low pass filter circuit. Directly on the input like that I don't consider this a good practice. It might have been put there to achieve stability in the amp which may have been modified in other ways you haven't discovered.

I may be speaking out of turn but most here (myself included) would not implement mods suggested by soundguruman without researching them for technical accuracy. He never made friends here and sometimes his input was detrimental to progress. He no longer participates for good reasons. If you research his interactions here you'll no doubt find threads where he was contrary to best practice and negated by other members of the forum. That said...

He is someone that has put some time into the craft of modifications and wasn't always wrong. But the fact that he actually was wrong sometimes, sometimes in a dangerous way and then acted like an indignant dick about it did inspire forum members to tar and feather him and drag him to the outskirts. We haven't witnessed his participation in many years.

If his suggestions have worked for you so far then happy day. It looks like that amp you have has been modded already. So no way to know what you're actually working with before any changes have been discovered.

I would undo the input mod you found for starters. And I would undo ANY mods otherwise. Return the amp to stock and functioning and THEN ask here about changes to make it to your liking. Otherwise the actual circuit in question is nebulous and impossible to quantify for advice.

JM2C

[Kev 335]

Thanks for your in depth reply Chuck, I guess I'll snip that RC circuit out for sure and will be reviewing the cranky guru's mod's too.
My main mission with this amp is too cut shrillness and harshness from the High's, also the amp's bass response needs a hefty boost!
It really lacks low end and body...if you have any mod suggestions? I would certainly be thankful for that!

The board is a JMP-52-C

The only mod's I did was removed R7 R43 C33 C34 and jumper-ed C27....the R13 mod didn't have any effect to my ears.

[The Dude]

It might be helpful to list the mods you've already done to the amp.

[Kev 335]

These mod's I removed R7 R43 C33 C34 and jumper-ed C27 I'm happy with the results tone wise for both channels.

The most recent mod's were

4] D10, (diode) snipe one leg off
5] R60, (470k) Pickybag 47K over it
6] R13, (220k) snipe one leg off

This mod is all over every forum....addresses channel B exclusively..and I have to say it transforms
the amp..it comes alive!

The only problem now is channel B is very loud compared to ch A ..apparently R60, (470k) Piggyback 47K over it
is supposed to lower the ch B volume so that mirrors ch A's volume pot position....Anyone have a solution for this?

My first thought is increasing the value of VR10 ch b master vol pot...it's blasting at no 2 on the dial!..???

[Chuck H]

For future discussion...

[Chuck H]

Here is the same schematic with the stated modifications in red. Snipped or removed components have a red circle around their designators.

[Kev 335]

The only problem now is channel B is very loud compared to ch A as mentioned in my last post
I hear it's the nature of most marshall amps to be louder sooner than later with the 900 being the exception.
So a solution maybe to increase the sensitivity of ch A or lower it for ch B

I don't wish to alarm you further with soundguruman's solution for this, but it may be an option
I actually only used 1 - 3 of his mods.

14. Now, because we made the clean channel cleaner, we are going to increase the sensitivity of the Clean Master Volume...
14a. Pin 2 of IC8 goes to ground. Cut that track, right in the middle...
14b. Install a 1 meg resistor between pin 2 of IC8 and ground.
Now, the settings of clean master volume and overdrive master volume will be "closer" together

Weird that he would say "because we made the clean channel cleaner" ...doesn't gel well for the reason for the mod!

Anyway the reason Ch b is now so loud is because of this:

4] D10, (diode) snipe one leg off
5] R60, (470k) Pickybag 47K over it
6] R13, (220k) snipe one leg off
popular mod that's almost everywhere I looked...might be part of the Liam mod!

Actually the amp now sounds a very bluesy and tasty beast!

If you recommend staying away from the crankyguruman mod...I'll trust you on that ....but if you think it will work then all good!

however if you have an alternative solution I would be glad to hear it.....

I was thinking of just changing the values of VR8 or VR10 but i'm not sure if that would work...Guess that's why I'm on this Forum...lol

[Kev 335]

Just an update for my first post, I embarrassingly named the components on the Wrong PCB
"R6 (1 M) is piggy backed with an RC circuit" this is actually on the power board.
and R6 on the proper board is 47k
Apparently it is a manufacturing mod exclusive to the jcm900 head and not for the
combo version. not sure what it does?

Also tried Soundguruman's mod to increase the sensitivity of the Clean Master Volume

14b. Install a 1 meg resistor between pin 2 of IC8 and ground.

Which didn't really increase that much in line with Ch B volume

So I'm open for any suggestions?

[strato56]

IC8a is an inverting operational amplifier : so piggy back another 220k across R46 : that will give you a raise of 6dB.

Restore the original connection from pin2 !

[g1]

"R6 (1 M) is piggy backed with an RC circuit" this is actually on the power board.
and R6 on the proper board is 47k
Apparently it is a manufacturing mod exclusive to the jcm900 head and not for the
combo version. not sure what it does?

To me, it looks like a fixed presence circuit. It's in the place of the more traditional spot for a Marshall presence control, where in this model the actual presence control is in the preamp area.
Off the top of my head, maybe it has something to do with open back cab (combo) vs closed back cab (head).
edit: hold on, what do you mean about "R6 on the proper board is 47k" ?

[Kev 335]

edit: hold on, what do you mean about "R6 on the proper board is 47k" ?

R6 47k on the power PCB as opposed to R6 1 Meg on the Preamp PCB..sorry for the confusion g1

[Kev 335]

IC8a is an inverting operational amplifier : so piggy back another 220k across R46 : that will give you a raise of 6dB.

Restore the original connection from pin2 !

Thankyou Strato56 I'll give it a try

[g1]

R6 47k on the power PCB as opposed to R6 1 Meg on the Preamp PCB..sorry for the confusion g1

What I meant was, R6 on power board is 4.7K, not 47K.