#1
I recently bought a JCM 900 off of a friend of mine and I was hoping to get a little advice on the sound. I'd like to get as much gain out of the amp as possible; is there a best course of action for me to take? I realize it will never sound like an 800, but if anyone has an idea how to get a little more out of the amp that would be very much appreciated.
#2
Which JCM900? There are 3 different types.
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#3
Quote by Cathbard
Which JCM900? There are 3 different types.

Seconding this.

An SLX should have PLENTY of gain on tap, hell it's actually better suited to modern metal the best of all the 900s.

Also, 800s are pretty low gain amps compared to pretty much anything newer. Every 900 has more gain on tap inherently than the 2203/2204 (the good JCM800s).
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Quote by Anonden
You CAN play anything with anything....but some guitars sound right for some things, and not for others. Single coils sound retarded for metal, though those who are apeshit about harpsichord probably beg to differ.
#5
There are some pretty simple mods for the Mark 3 and the Dual Reverb that will improve the gain sounds. The SL/X is a gain monster in stock trim, way more than a JCM800. It doesn't need modding, it just needs good tubes.
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
#6
*praying for not dual reverb*
WTLT 2014 GG&A

Quote by andersondb7
alright "king of the guitar forum"


Quote by trashedlostfdup
nope i am "GOD of the guitar forum" i think that fits me better.


Quote by andersondb7
youre just being a jerk man.



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#7
Quote by trashedlostfdup
*praying for not dual reverb*

You know it's gonna be a DR, don't kid yourself.
Current Gear:
LTD MH-400 with Gotoh GE1996T (EMG 85/60)
PRS SE Custom 24 (Suhr SSH+/SSV)
Ibanez RG3120 Prestige (Dimarzio Titans)
Squier Vintage Modified 70s Jazz V
Audient iD22 interface
Peavey Revalver 4, UAD Friedman BE100/DS40
Adam S3A monitors
Quote by Anonden
You CAN play anything with anything....but some guitars sound right for some things, and not for others. Single coils sound retarded for metal, though those who are apeshit about harpsichord probably beg to differ.
#8
Quote by oneblackened
You know it's gonna be a DR, don't kid yourself.


you are probably right.

DR=

WTLT 2014 GG&A

Quote by andersondb7
alright "king of the guitar forum"


Quote by trashedlostfdup
nope i am "GOD of the guitar forum" i think that fits me better.


Quote by andersondb7
youre just being a jerk man.



****** NEW NEW NEW!
2017-07-07 2017-07-07 Update and a Chat On Noise Constraints *** NEW FRIDAY 7/7
2017-04-13 RUN AWAY from COMPUTERS!!! TCE? RANT ALERT!!!
2017-03-02 - Guitar Philosophy 1001- Be Prepared For the Situation (Thursday 2017-03-02)
2017-02-21 How to Hot-Rod the Hell of your Stratocaster for $50! (Tuesday 2017-2-21)
Resentments and Rambling from a Guitar Junkie
---> http://trashedengineering.blogspot.com/
#10
They're a hybrid. There's as much solid state amplification stages as tube in the preamp of a 4100. And they have really dumb shit circuitry - like really ****ing dumb. They're pretty fixable though - you just remove the dumb shit. They still have a metric shitload of op-amps but that's not the really dumb part. Their hybridness isn't really the problem. It's the stoopidity - and that's monumental.
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
#11
Quote by trashedlostfdup
you are probably right.

DR=



haha yeah you guys are right, it is dual reverb. It's my first amp head and I got a deal on it that was too good to pass up ($700 for the head and cab).
#12
Quote by Frothybike
haha yeah you guys are right, it is dual reverb. It's my first amp head and I got a deal on it that was too good to pass up ($700 for the head and cab).

Cathbard has some mods that are easy to do and it will make the amp better
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#13
Quote by Robbgnarly
Cathbard has some mods that are easy to do and it will make the amp better


Awesome, thanks!
#14
Here's the preamp schematic:
http://drtube.com/schematics/marshall/cd0189-iss7.pdf

BR2 and D10 are the problem on channel 2. They form a diode clipping circuit that actually reduces how much signal is available to be passed onto V1. What they effectively do is replace tube clipping with SS diode clipping. It's where the fizziness comes from on that channel.
To remove it you have to remove the bridge (BR2) and replace it with a resistor (or trimpot). The resistor goes vertically on the schematic. I use a 100K resistor but that may be a tad radical for you. It means that the volume of that channel is never run anywhere near as close as channel 1 until you get the volumes up very high. This is because the IC6 is driving what is following so much higher. So balancing out the two channels at low volume can be pretty tricky. If you use a 100K trimpot you will be able to tweak the gain to where you prefer.
With 100K you will actually get more distortion than before and it is tube distortion. BR2/D10 form a type of fuzz box that limits the signal. They are perhaps the worst piece of circuit design I've ever seen. They are a fuzz box that reduces distortion - wtf is that all about?

Moving onto channel 1. Channel 1 isn't as bad, I don't normally touch that. It does have some SS clippers (LED 1-4) but as you can see on the schematic, they are run in the NFB loop of IC4, tubescreamer fashion. If you want, you can simply remove them but they don't really come into play until you run the gain pretty hard and even then, they don't sound bad.


Hope that helps.
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 Nov 28, 2014,
#15
13 replies and no pictures of bees?

You all fail.

Edit:



All is well now.
Everyone is entitled to an opinion.

Feel free to express yours so I can make an informed judgement about how stupid you are.
Last edited by bubb_tubbs at Nov 29, 2014,