#1
Guy is selling a Marshall JCM900 2x12 combo for $450 locally. He says the only issue with it is that some of the knobs have broken off, but the pots are still intact.

I'm trying to find out what I can about this amp online, but I'm not finding a ton of solid info. My main questions are

1. Is it a fair price?

2. Does this amp have an effects loop?

3. Would my money be better spent on something else?

4. Is it some form of a hybrid amp? Some stuff I've found online has mentioned it not getting it's drive from tubes?

Any general info about this amp would be great too, thanks!
Telecasters-->Dunlop Volume Pedal-T1M Pearl-AMT Japanese Girl Wah-Line 6 M9-Ibanez DE7-EHX Cathedral-->Mesa Boogie Nomad 4x10 combo
#2
This is a 4102, yes? I have one and I mod them. So is it a Dual Reverb? We need to establish that, there are three distinctly different types of JCM900.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#3
Quote by Cathbard
This is a 4102, yes? I have one and I mod them. So is it a Dual Reverb? We need to establish that, there are three distinctly different types of JCM900.

Whoops, I missed that part in his posting, it says it's a 4102. I'll just copy and paste the text of the posting (He doesn't have any pictures with it)

"For sale I have one Marshall model 4102 JCM900 100 watt all tube combo amp. Loaded with two celestion G1275T's. Louder than a freight train, really. Operationally perfect. Only issue is a couple of the pots have the shafts broken off but they turn easily and function flawlessly. Inherent design flaw, the knobs were really close to the top edge without much protection. Otherwise, it looks good and sounds like your typical JCM900 Marshall. No trades, firm on price."
Telecasters-->Dunlop Volume Pedal-T1M Pearl-AMT Japanese Girl Wah-Line 6 M9-Ibanez DE7-EHX Cathedral-->Mesa Boogie Nomad 4x10 combo
#4
Ok, now we can talk turkey.
Here's the manual
https://docs.google.com/viewer?a=v&q=cache:eYNBsOBYSJMJ:www.drtube.com/schematics/marshall/jcm900.pdf+jcm900+dual+reverb+manual&hl=en&gl=au&pid=bl&srcid=ADGEESjS5HcqeJ_Q9Gg8kCAkT22peBtqc8eQVo6WEHiiOk__hw2A_YJZDAfTctw0QGtsmulsgK-ukwb0ypa8uQcuZ0iqMK8JHkRB3OmSXhdkMvYgO6wk-cgeXPh8pzvz-Ax2Ayk5i9M9&sig=AHIEtbQuaxozf-Zq9J0KThuP60u79XEDaQ

To answer your questions first.

1. Sounds like a fair price but in Australia that would be a really cheap price. I got mine for $250.

2. Yes, it has a series effects loop. Op-amp driven.

3. They are a great punk amp. With some modding they are a pretty good thing. Very nice cleans and if you drive the power tubes hard the power amp is standard old Marshall fare. It depends on what sound you are after. One thing, damn they are loud, but playable down low too.

4. This is where it gets interesting. Yeah, I'd definitely call them a hybrid. The power amp is pure Marshall valve but the preamp is certainly hybrid. Here's the preamp schematic:
http://www.drtube.com/schematics/marshall/cd0189-iss7.gif
All those triangles are op-amps. Lots, huh? That's how they can get such high gain with only two preamp tube. (the third one is the phase inverter in the power amp)
Now here's the stupid, stupid thing about the 4100 - it has clipping diodes even though you could get enough actual gain out of all those op-amps to drive those two 12AX7's into flatline.

So here's what you do. See BR2 and D10 up the top? Those are the same clipping diode arrangement as a 2210 JCM800, you know? The shitty sounding 800? They are the clipping diodes for the lead channel. You replace those with a 100K resistor. Easiest to just cut the trace on the board where BR2 joins to earth and solder a resistor across (so from C36 down).
You'll still have bucketloads of gain, all those diodes do is add fizz.

Now look at LED 1-4. Those are clipping diodes for the first channel. You can just remove those. Personally, I have found those to be less offensive and left them in.

It's BR2/D10 that are the cause of the dreaded JCM900 fizz. That has to be one of the stupidest design flaws every perpetrated on an unsuspecting public.
It's like they were set on using the clipping setup from the 2210 and designed everything around that and never tried it without them. With all those op-amps why would you want clipping diodes ffs? If you want more gain, change a resistor on the opamps and pump more signal into the tubes. Why try to distort it first - that's just dumb. If I want a fuzz box I'll plug in my own, you dumbass bastards. What were you thinking?
Without those clipping diodes the opamps aren't such a big deal really.

Any other questions? This is one amp I know really well.


Edit. Oh, pots. The pots on JCM900's are crap. You will almost certainly have to change them yourself anyway at some point. They are PCB mounted. Standard sized PCB pots but it does mean pulling out the board - or at least unscrewing it and pulling it up to where you can solder underneath. It's a bit of a pita job tbh but something every JCM900 owner has to deal with at some stage if he keeps the amp for long enough.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
Last edited by Cathbard at Feb 7, 2012,
#5
Quote by Cathbard
Ok, now we can talk turkey.
Here's the manual
https://docs.google.com/viewer?a=v&q=cache:eYNBsOBYSJMJ:www.drtube.com/schematics/marshall/jcm900.pdf+jcm900+dual+reverb+manual&hl=en&gl=au&pid=bl&srcid=ADGEESjS5HcqeJ_Q9Gg8kCAkT22peBtqc8eQVo6WEHiiOk__hw2A_YJZDAfTctw0QGtsmulsgK-ukwb0ypa8uQcuZ0iqMK8JHkRB3OmSXhdkMvYgO6wk-cgeXPh8pzvz-Ax2Ayk5i9M9&sig=AHIEtbQuaxozf-Zq9J0KThuP60u79XEDaQ

To answer your questions first.

1. Sounds like a fair price but in Australia that would be a really cheap price. I got mine for $250.

2. Yes, it has a series effects loop. Op-amp driven.

3. They are a great punk amp. With some modding they are a pretty good thing. Very nice cleans and if you drive the power tubes hard the power amp is standard old Marshall fare. It depends on what sound you are after. One thing, damn they are loud, but playable down low too.

4. This is where it gets interesting. Yeah, I'd definitely call them a hybrid. The power amp is pure Marshall valve but the preamp is certainly hybrid. Here's the preamp schematic:
http://www.drtube.com/schematics/marshall/cd0189-iss7.gif
All those triangles are op-amps. Lots, huh? That's how they can get such high gain with only two preamp tube. (the third one is the phase inverter in the power amp)
Now here's the stupid, stupid thing about the 4100 - it has clipping diodes even though you could get enough actual gain out of all those op-amps to drive those two 12AX7's into flatline.

So here's what you do. See BR2 and D10 up the top? Those are the same clipping diode arrangement as a 2210 JCM800, you know? The shitty sounding 800? They are the clipping diodes for the lead channel. You replace those with a 100K resistor. Easiest to just cut the trace on the board where BR2 joins to earth and solder a resistor across (so from C36 down).
You'll still have bucketloads of gain, all those diodes do is add fizz.

Now look at LED 1-4. Those are clipping diodes for the first channel. You can just remove those. Personally, I have found those to be less offensive and left them in.

It's BR2/D10 that are the cause of the dreaded JCM900 fizz. That has to be one of the stupidest design flaws every perpetrated on an unsuspecting public.
It's like they were set on using the clipping setup from the 2210 and designed everything around that and never tried it without them. With all those op-amps why would you want clipping diodes ffs? If you want more gain, change a resistor on the opamps and pump more signal into the tubes. Why try to distort it first - that's just dumb. If I want a fuzz box I'll plug in my own, you dumbass bastards. What were you thinking?
Without those clipping diodes the opamps aren't such a big deal really.

Any other questions? This is one amp I know really well.


Edit. Oh, pots. The pots on JCM900's are crap. You will almost certainly have to change them yourself anyway at some point. They are PCB mounted. Standard sized PCB pots but it does mean pulling out the board - or at least unscrewing it and pulling it up to where you can solder underneath. It's a bit of a pita job tbh but something every JCM900 owner has to deal with at some stage if he keeps the amp for long enough.


Aaaaaawesome, thank you so much man. OK, new set of questions after reading through all that.

1. I'm used to sticking with a slightly dirty sound, and throwing pedals on top for more gain (But I'm starting to like a good clean channel more and more). Would this work well for the drive channel on this amp?

2. You mentioned getting it modded. Would these mods be expensive to have a tech do? (I have a guy locally that I trust to not BS me, and I'm not very familiar with circuity/mods yet. Hoping to get a BYOC pedal or Monte Allums kit soon to get my feet wet)

3. Same issue with the knobs, would this be pricey to have a tech do?

4. Finally, the sound I'm going for. I'll try to make it short and sweet.

More or less, this whole song

The solo that starts at about 1:42 in this


Hopefully that gives you a good ballpark of what I'm looking for. Ballsy, but defined.
Telecasters-->Dunlop Volume Pedal-T1M Pearl-AMT Japanese Girl Wah-Line 6 M9-Ibanez DE7-EHX Cathedral-->Mesa Boogie Nomad 4x10 combo
#6
1. Yes, that's probably the best way to do it.

If you are getting pots replaced then he should do the mod pretty cheap. The hardest part is pulling out the board. All up it's a bout an hours labour. The actual soldering part is the quick part of the job. So all up you should get out of it for say $100 give or take, less if he's a friend.
Give the guy that schematic and say you want BR2 and D10 replaced with a single 100K resistor. If he is a tech he'll work it out in a second and it's a quick thing to do. One cut, two solder joints.

If you are mainly going to be using channel 1 then you'll be fine without the mod. There's no real need to mod the rhythm channel.

For those tones it will be fine with no modding at all. It'll do that stuff standing on its head.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#7
Quote by Cathbard
1. Yes, that's probably the best way to do it.

If you are getting pots replaced then he should do the mod pretty cheap. The hardest part is pulling out the board. All up it's a bout an hours labour. The actual soldering part is the quick part of the job. So all up you should get out of it for say $100 give or take, less if he's a friend.
Give the guy that schematic and say you want BR2 and D10 replaced with a single 100K resistor. If he is a tech he'll work it out in a second and it's a quick thing to do. One cut, two solder joints.

If you are mainly going to be using channel 1 then you'll be fine without the mod. There's no real need to mod the rhythm channel.

For those tones it will be fine with no modding at all. It'll do that stuff standing on its head.

Awesome. You have been a huge help, thank you so much. It sounds like this is pretty much the perfect amp for me, hopefully I'll have a NAD pretty soon.
Telecasters-->Dunlop Volume Pedal-T1M Pearl-AMT Japanese Girl Wah-Line 6 M9-Ibanez DE7-EHX Cathedral-->Mesa Boogie Nomad 4x10 combo