JCM 900 4100 mod

Ken

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quiksilver98, I think the easiest and best 'mod' you could do would be to buy an attenuator. Like all tube amps, these change their character and become monsters (possibly even more so than other amps) as they reach the higher volume settings, so attenuation will get you up there at volumes you need it to be.
.

This is a way better solution than any mod. 4100's share one trait with Plexi: both need to be loud to sound good, which also somewhat limits their appeal. So getting the power tubes in play with an attenuator is the way to go.

Ken
 

900jcm

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my jcm900 4100 into a 1936 cab ( celestion 75s) sounds great. a true rock and roll amp...i gig both ( either the 6l6 vor EL34) versions of the 4100...a good amp by any means. folks question my pedalboard and the dirt pedals i use. it's all amp. out of a 3 set gig , i use the 'B' channel for distortion for about 95% of the time. a 100 watt jcm900 into 300 watts of speaker compared to 150watts... big difference being amp and cab are being pushed. both 'A' and 'B' channel volumes are on 7-8. snare drum volume. any lower with the volume and you wouldn't think twice about this amp. get it above 7 , she rocks mate. a ts9 infront for extra flames...yeeeeowza! aloha
 

ampmadscientist

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Does anyone knows of any mods to do to the JCM900 to bypass the clipping diodes and create a gain cascade within the preamp tubes instead. Basically my 900 sounds good dirty, but it feels very dull on the strings, without punch. I found that if I throw the clean channel on ten and bring the preamp gain to about 2:00, it gets a very nice power amp distortion and it starts to feel more alive. But the problem with this is that the distortion is now after the preamp stage thus destroying my time based effects.

No,
but there are other mods that make it more useable.
It can have a pretty decent sound into the effects loop. (the loop has to be modified too)

Further, you probably need an isolated effects power supply...just saying...because there will be a ground loop between the input jack and the effects loop.

The overdrive channel design is basically built around diode clipping.
Although there are certainly more possibilities to make a preamp tube overdrive rather than diode clipped.
But all of these still involve the solid state preamp circuits. It's a SS tube hybrid...no matter what you do.
But despite that SS, it can still sound pretty surprising.

You can remove LED 1-4 from the clean channel.
This takes the clipping out of the clean side only.

After modifying this amp, I like it quite a bit better. It sounds pretty darn good I think.
Stock "as is" I don't like it very much.
 

tschrama

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So many ignorent posts here on modding the JCM900 DR's. I just had to at my 2 cts: removing diodes clipping, enabeling genuine tube clipping IS easy.

Here it goes:

Old thread, I know.... but I happen to be busy on a JCM900 4100 DR and I would like to share some tips.

The 4100 sounds great as-is with some power amp distortion. I can do that with my attenuator, and it just sounds marvelous. But you can help the preamp to sound a bit better to:

1] remove the boost channel pre-EQ, by removing C27 (4.7nF). This removes the ice-picky fizz and gives some extra sub-bass as a deep mod
2] Make sure the cap aound the second opamp stage of the boost channel is 100pF. Some came with 220pF and that is just too much. Change it to 100pF you might even try 47pF or 22pF as in the JMP-1
3] The input resistor of the 2nd opamp of the boost channel is 10K. You might try to picky-back a 10K over it so it is effectively 5K. Keep 2] at 100pF if doing this

4]Yes, you can easly remove the diode clipping for real tube clipping. I don't believe that sounds much better but it is easy to accomplish:
Remove diode that is next to the diode brigde. This effectively takes out the whole diode bridge. Remove the 220K resistor around the tube of the preamp. Now the tube is running open-loop, with full gain, no more "handcuffs". Now the signal is much larger, so the post-EQ attenuator (470K-470K) must be altered to 470K-47K.

There you have it, no more diode-clipping, genuine tube stage with cathodefollower connected traditional Marshall Tonestack.
 

Ken

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4]Yes, you can easly remove the diode clipping for real tube clipping. I don't believe that sounds much better but it is easy to accomplish:
Remove diode that is next to the diode brigde. This effectively takes out the whole diode bridge. Remove the 220K resistor around the tube of the preamp. Now the tube is running open-loop, with full gain, no more "handcuffs". Now the signal is much larger, so the post-EQ attenuator (470K-470K) must be altered to 470K-47K.

There you have it, no more diode-clipping, genuine tube stage with cathodefollower connected traditional Marshall Tonestack.

This is an interesting mod! Knowing the preamp tubes in this amp do not do the usual tasks, I'm questioning if your mod uses only V1 for preamp gain instead of adding V2 and even using the same tube twice like other Marshalls do.

If the mod only uses V1 you're not going to get a lot of gain unless you get an unusual tube; there's a reason Marshall added the clipping diodes after all. I will say a stock DR with a pedal actually sounds good enough to forget spending significant money on mods. Does the mod retain the cleans because a stock DR has good clean sounds. It's the moderate volume distortion tone that's I've always had an issue with.

Ken
 

tschrama

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Hi Ken,

ref 4]
The amount of overdrive will stay about the same, or a little less. It now is generated from the v1 tube, instead of the diode-bridge arrangement. Clean are unaffected, prolly slightly less overdrive.

I agree, the 4100 with an boost pedal, SD-1 etc, can sound decent. In fact I upped the gain significantly in a prevous mod, but this weekend I but it back to stock.. well almost, because I liked it as is, with the 100pF and a boost pedal.
 
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Ghostman

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...So that's the advice I post: if you don't like your amp's voicing, forget about mods etc. Get an amp whose voicing you like!...
Ken

Ps. Ignore Ken, he is a notorious Dual Reverb hater.

Pps. But don't ignore the fact, that the modding required to do what you want to do is so significant, that you'd be better off getting another amp.

LMAO!!! :lol::lol::lol:

Classic thread!
 

iron broadsword

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So, that was a fun read.

And now I am really interested in trying out the mod you posted, tschrama. Where did you come by this, or did you figure it out on your own? And also, do you have any clips of the diodeless DR?

Also, could you post the names of the resistors and caps referenced in 2], 3], and 4]? I'd love to do this and record samples before/after.
 

tschrama

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NP, these are solely my idea's:


1] C27, (4n7)snipe one leg off
2] C35, (220pF) to 100pF
3] R42, (10k) piggy back another 10K over it. But i Like it as is.
4] D10, (diode) snipe one leg off
R60, (470k) Pickybag 47K over it
R13, (220k) snipe one leg off
 

tschrama

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While I'm at it: thiese are my mod idea's I currently have been enjoying in my TSL60:

jumper R90 1nF
jumper R92 470K
clip out C84 (470p)
jumper R83 1Meg
 

ampmadscientist

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Does anyone knows of any mods to do to the JCM900 to bypass the clipping diodes and create a gain cascade within the preamp tubes instead. Basically my 900 sounds good dirty, but it feels very dull on the strings, without punch. I found that if I throw the clean channel on ten and bring the preamp gain to about 2:00, it gets a very nice power amp distortion and it starts to feel more alive. But the problem with this is that the distortion is now after the preamp stage thus destroying my time based effects.

Bypass WHICH clipping diodes?
the clipping for the clean channel or the overdrive channel?
BOTH channels have clipping diodes!

It's not "power amp" distortion, it's DIODE clipping.
You are off base for sure.

1. The preamp gain for the overdrive channel is ALL solid state.
2. This signal is clipped by diodes, not tubes.
3. Then, this signal enters a single tube gain stage and a cathode follower.
4. Which then, drives the tone stack.

The distortion, in the stock amp, is pretty much all solid state.
It's Op Amps, and clipping diodes.

You want the effects send to be CLEAN?
Then you want the distortion AFTER the effects return?
You can't do that.

HOWEVER
we modify the amp, so that the distortion is coming from the preamp tubes, and the solid state is cleaned up...
BUT this mod should be done by a qualified technician.

I DO NOT recommend that you try to modify the amp yourself.

http://www.marshallforum.com/workbench/77383-un-official-4100-mods-thread.html




 

tschrama

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1st link
No, not me, wish I could play like that. Not bad sounding btw, but I think you now hear the opamp clipping to the -15/+15 rails, instead of the diode bridge clipping -2/+2 volt. So you loose quite some overdrive.

2nd link
Small error in the first post, but he corrected it in post#7. Seems that that pre-EQ fixed tonestack is something most players dont like. If you just cut-off one leg of C27, you accomplish the same thing.

Jumping the diode lowers the clipping threshold from about +/-2V to +/- 1.4V, giving more overdrive. Seems perfectly fine to me.

The 4100/4500 design is a complete mistery to me. All the components are in there to made a perfectly fine all tube high gain moster, but instead marshall used 2 tubes to buffer a solidstate preamp, a solidstate fx-loop and solistate reverb. What were they drinking?????
 
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iron broadsword

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I'm gonna go with rampant drug problem in England during that time. :D FWIW I like the amp a lot I just know it can be better.

Ok, so gotcha. I am hearing something I don't like in the 1st one (youtube clip).. is the mod you posted similar or does it bypass the opamps as well? It seems that your 4] accomplishes this but I wasn't sure.
 

tschrama

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Well, the opamps are still there, and since the diode bridge is disconnected, indeed the clipping of the opamp is could be heard, BUT...

Since 4] drasticly increases the sensitivity of the V1, V1 will start to clip, before the opamp does. So the V1 is now the clipping part, definening the overdrive amount.

V1 , after step 4], has a clipping input threshold of about -/+5V, so you have to snipe the diode (threshold +/- 2 volt) to make sure the V1 is clipping, and not the diode. this also means you loose about 10dB of overdrive.

Offcourse that can be fixed.....

BTW
JCM900 DR are cheap mod platforms. Buy them 300-350 Euro, take out the preamp PCB, and put your favorite tag board in. I turned one of mine JCM900 into a Roccaforte HG50. :cool:
 

iron broadsword

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Very cool, thanks for the explanation.

Yeah, I have been fiddling with the idea of turning the preamp into an 800.. I just haven't taken on the task of learning to make it all work together. I could build the preamp no problem.. tying it into the 900's power amp.. not without a lot of reading or a paint-by-number's approach from a google score. I could flip it for $850 locally and just buy something else too, which I've been thinking about recently. I guess I am just at the point where as much as I love the sound of it, the 800 tones are enticing me.
 

tresmarshallz

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I understand the desire to mod this amp, but from my observation it is not necessary.
I put a 6 band EQ in the loop and was able to solve ALL the issues I had with my 4500 not being punchy enough Now it wounds ridiculously good and makes all my friends jealous LOL
 

tschrama

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Well, I still think the 4100/4500 series has too little bass attentuation and gain before clipping. It can crunsh OK, but wont do chugga chagga. You cann't fix that with a post Clippping EQ... oh well... hence the SD-1.

How do you set your EQ pedal? I would like to try :yesway:
 

tresmarshallz

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I also run a keeley modded sd1in front
On MXR 6 band, I run them from left to right on a scale of 1-10....
1st on 6
2nd on 5
3rd on 5
4th on 3
5th on 5
6th on 6

So, slight bass and treb boost and mid cut. I make up for the mid cut by boosting mids on the amp
 


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