JCM 800 mods for more gain?

S

ss888

New Metal Member
Aug 10, 2010
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Hi guys

There are so many modifications that can be done to the 2203 and 2204 that it's starting to get extremely overwhelming.

Can any of you recommend any mods that boost the gain significantly without adding an extra preamp tube or make it sound like crap? Somewhere along the line that I wouldn't really need to use a pedal to boost the signal.

I generally play stuff like thrash, old doom and older black and death. Nothing super saturated in which I'd need another amp all together.

Thanks guys
 
very easy...
just fiddle with the anode resistors and/or the kathode resistors/their parallel caps, I've done that a million times.
the sound will change a bit though.
BUT
these are the easiest and most basic mods you can do to an amp, if you don't know about these you shouldn't even open it since the voltages in the amp are lethal even after you unplugged it!

I'm sure Wolfe will chime in to tell you more about this
 
Lasse Lammert said:
very easy...
just fiddle with the anode resistors and/or the kathode resistors/their parallel caps, I've done that a million times.
the sound will change a bit though.
BUT
these are the easiest and most basic mods you can do to an amp, if you don't know about these you shouldn't even open it since the voltages in the amp are lethal even after you unplugged it!

I'm sure Wolfe will chime in to tell you more about this
Click to expand...

That was quick.

Thanks for that, I hope this Wolfe person does too!

And I'll be taking it to an amp tech, just need to pass on the details/schematics to them.
 
If I still had a 2203 and really only wanted more gain, I'd probably just use a pedal. Either a clean boost or some sort of TS, depending on the music. That will leave you more flexible and won't affect the core-sound of the amp (as Lasse said). And another thing: modded 800s not always do well on the used market as their are many around who had quite shitty mods done to them. The mods Lasse described are quite easy to reverse though.
 
If you are taking it to an amp tech, just tell him what you want. There are literally hundreds of things you can do to a JCM800 (many modern amps are modded JCM800's to begin with!)

How much more gain are you looking for? Adding a gain stage would be the way to go if you like 'modern' amp gain(think 5150, SLO, Rectifier). But since you don't want saturated, you may want to keep it as is and change some components.

You can get a bunch more gain out of it by doing a few things that Lasse already mentioned, which would be cheap and easy to do. By changing a few cap and resistor values, you are essentially just changing what frequencies end up getting amplified by the tube, which gives a perceived increase in gain. But as Lasse also said, it changes the character of the amp a bit since you now have different frequencies emphasized or cut.

I would start with some things like this-
Change the 10K resistor on V1B. The more you lower the value, the more of an increase in gain you will get. 4.7K seems to be what the amp likes best, but you can go as low as 1.8K(more SLOish). You could use a switch to flip between a few different values, or have the tech add a small 10k trim pot and you can make it variable to find what you like best! (When prototyping amps, that is the method I use. Rotate the pot til it gives you what you want, measure the value, then just remove the pot and add the fixed resistor. Awesome way to work!)

You could add a bypass cap on that resistor also. Try a 1uf. Also try throwing a bypass cap on V2A's cathode resistor.

Ditch the tone stack slope resistor and go with a Soldano value(47k). This can also be put as a pot on the amp and tweaked for a variable midrange!

Change the value of V1A cathode bypass cap from .68uF to 1uF.

Change the resistor between V1B and V2A that goes to ground (470K) to a 1 meg. Change the bypassed resistor in the same area to a 220K or 100k. This will let a ton more level go to the next stages, instant gain increase.
Also, you can remove that cap bypassing the resistor, and see if you like it(or add a switch).


All of these mods(other than the tone stack mod) should be done one at a time. If you do everything at once you may not like the results. Start slow and work your way up.
 
BTW are you in the US? I'd love to help you out with this, modding is fun. Send her to me, I work for cheap!
 
In my book, the Lee Jackson mod wins.
It will turn your amp in to the more Ampeg VL/Metaltronix kind of range.. still that nice Marshall crunch and mid range, but much more "omph" to it.

Btw, its the mods Zakk used on "No More Tears"(The GOOD Zakk tone :lol:)!
 
Notuern said:
In my book, the Lee Jackson mod wins.
It will turn your amp in to the more Ampeg VL/Metaltronix kind of range.. still that nice Marshall crunch and mid range, but much more "omph" to it.

Btw, its the mods Zakk used on "No More Tears"(The GOOD Zakk tone :lol:)!
Click to expand...

I believe he adds a tube right? That's what I would do, but it's a lot of work for a newb
 
I'm with slashvanyoung on this one. What's wrong with just using a boost pedal? Pedals are cool, and easy to change around if you want to have a different flavor suddenly.
 
krokit said:
Just for the hell of it, here's some pics of my 2303. It's a 1978 model which is the daddy of the JCM 800:

http://dl.dropbox.com/u/1404714/Marshall 100w 2203 1978/SDC15038.JPG

http://dl.dropbox.com/u/1404714/Marshall 100w 2203 1978/SDC15039.JPG

http://dl.dropbox.com/u/1404714/Marshall 100w 2203 1978/SDC15040.JPG

http://dl.dropbox.com/u/1404714/Marshall 100w 2203 1978/SDC15044.JPG

www.ascapestudios.com
Click to expand...

Isn't this the amp Yngwie Malmsteen uses? Hmm... does anyone know which mods I would need to turn a 1989 JCM 800 to old JMP specs?
 
korpse pop said:
Might be youtube quality / micing issue, but I don't like this tone at all. Sounds almost like a cheap solidstate amp to me
Click to expand...

Thats the FJA mods for ya'!
Every single clip ive heard from an FJA modified amp sounds like a solid state amp.
 
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