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Unread 09-21-2008, 09:11 AM   #1 (permalink)
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JCM 800 Mod

I am the proud owner of a 1984 JCM 800, have been for about 8 years. Only thing I will ever use. I am wondering, however, what kid of mod I would do to get a little distortion. Amp has only gain, which sounded righteous with my les paul, but I recently picked up a 1981 Ibanez Artist, and would like a little more bite. Any suggestions, or am I being blashphemous even asking?
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Unread 09-21-2008, 09:40 AM   #2 (permalink)
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Re: JCM 800 Mod

Hi,

I would recommend you a Marshall Guvnor2. I recently went thru the mill of trying out several well known pedals as well as some no-names and the Guvnor2 came out on top. However I also admit this was with a non-master volume amp. Maybe the normal Guvnor or Blues Breaker would be enough to push it into 'cream dream' - I would not recommend a distortion pedal as they dislike master volumes amps at higher volumes and produce mud - at least my old JCM900 did with a few....

Cheers Brett
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Unread 09-21-2008, 09:49 AM   #3 (permalink)
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Re: JCM 800 Mod

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Originally Posted by mikeg View Post
I am the proud owner of a 1984 JCM 800, have been for about 8 years. Only thing I will ever use. I am wondering, however, what kid of mod I would do to get a little distortion. Amp has only gain, which sounded righteous with my les paul, but I recently picked up a 1981 Ibanez Artist, and would like a little more bite. Any suggestions, or am I being blashphemous even asking?
Firstly; If you don't understand basic electrical safety procedures such capacitor discharging then you would be better of bringing your amp to a qualified amp tech to do these mods. The JCM 800 is very popualr amp for modding so any tech worth his salt will know how to get more gain out of it.

Anyway if you're happy to have a go yourself then here you go:

Check out The Ultimate JCM 800 on google and it will bring to a great page full of info and actual pictures on some mods you can do for more gain. Your amp will look slightly different as, AFAIK, your pots (being and '84) will mounted directly on the PCB but the same theory will apply.

Could I suggest putting a resistor in parallel with the 10K on V1a and a capacitor in parallel with the 820 resistor (Rk) on V2a.

When putting a resistor in parallel with another you reduce the overall resistance, for example a 10k in parallel with another 10k = 5k effective resistance. Reducing Rk will increase the gain of the stage by reducing the negative feedback and shifting the bias point. The formula is R1*R2/R1 + R2 for resistors in parallel. I suggest putting a resistor in parallel with the existing one as you can easily remove it to bring the amp back to stock at a latter stage. My experience backs up the guy in Voodoo's statement that it's not a good idea to go much below 5k in this poisition as the gain becomes buzzy and you start introducing excessive noise beyond this point. Try out a few different values to see what you like.

Putting the capacitor across the Rk on V2a putting the ac at ground potential thus increasing the gain of the stage as Rk is removed from the ac gain calculation of the stage. .68uF is the classic Plexi value giving that classic mid range gain punch (it's not all just due to the EL34s!!), using a lower value will increase gain at higher frequencies, using a higher value will increase gain at lower freqencies giving a "thicker" gain response.

I'd also suggest using shielded cable for the input wiring as when you increase gain you also increase noise. Shielded cable for signal carrying cable greatly reduces the noise introduced in high gain circuits.

If you want a sound sample of what kind of tones you might expect with these mods the click on my uTube page (link below).
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Unread 09-21-2008, 02:37 PM   #4 (permalink)
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Re: JCM 800 Mod

Is this the site you're referring to Paolo?

Tales From The Tone Lounge; The Ultimate JCM800?
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Unread 09-21-2008, 02:38 PM   #5 (permalink)
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Re: JCM 800 Mod

Seems like the mods on this site are doable but I wonder how they compare to mods by say FJA or David Bray etc.?
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Unread 09-22-2008, 03:00 AM   #6 (permalink)
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Re: JCM 800 Mod

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Originally Posted by steelhorse View Post
Is this the site you're referring to Paolo?

Tales From The Tone Lounge; The Ultimate JCM800?
Yup, that would be it alright. Dunno why I didn't just go ahead and past the link. It was a lazy Sunday............
Personally I don't agree with everythin he says but it's a good site and his suggestions are spot on. The pics of the actual amp guts also really helped when I was starting out. Have a look through the rest of the site, it's also has some great general info.

I'm not sure what Bray or FJA actually do in terms of actual mods but I'd say they use their own variations of these (they are tried and tested) as well as a few of their own. Things like resonance control, mid shifts, depth controls, increased filtering for tightness etc. These are much more difficult to get right but sound great when you do!!

I'm actually working on a circuit at the moment to make my 2203 dual channel. I'll be using the high input for guitar and the low input for a footswitch and I'll be able to select either the high input for dirt and low input for cleans. Just trying to get the level of attenuation right for the high channel so the two are reasonable close volume wise.
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Unread 09-25-2008, 12:09 PM   #7 (permalink)
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Re: JCM 800 Mod

Will these mods work with JCM 800 reissues? If anyone has tried, what was the outcome?
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Unread 09-25-2008, 12:16 PM   #8 (permalink)
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Re: JCM 800 Mod

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Will these mods work with JCM 800 reissues? If anyone has tried, what was the outcome?
While the comonant lay out may be slightly different, AFAIK, the circuit and values are faithful to the original so there should be no problem. I haven't worked on one myself so I cant say for certain.
FJA Mods seem to do all their work on re-issues also so I'm pretty confident that it should be possible.
Do you wanna take a gut shot and post it, I can compare it to the original and let you know how similar it actually is layoutwise and which bits are what on your model? I'm wouldn't mind having a luck anyways, for me this stuff is like looking at high grade porn!!
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Unread 09-25-2008, 12:45 PM   #9 (permalink)
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Re: JCM 800 Mod

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Originally Posted by PaoloJM View Post
Do you wanna take a gut shot and post it, I can compare it to the original and let you know how similar it actually is layoutwise and which bits are what on your model? I'm wouldn't mind having a luck anyways, for me this stuff is like looking at high grade porn!!
I have never taken an amp apart aside from taking the back panel off for tube replacement. I have never learned how to discharge caps and I am terrified of electrocusion. And I was afraid to mess up $2300 amp if anything went wrong. But, I am pretty handy by nature and I am gonna take a shot.. discharing the caps sounds simple enough based on what I have read. I plan on starting with brite cap mod and try other ones one by one. Do I need to separate the amp gut from the case by taking all the knobs out for the brite cap mod? How long doe it take to do that? I will post pics when I take it apart..
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Unread 09-25-2008, 01:09 PM   #10 (permalink)
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Re: JCM 800 Mod

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Originally Posted by slacker72 View Post
I have never taken an amp apart aside from taking the back panel off for tube replacement. I have never learned how to discharge caps and I am terrified of electrocusion. And I was afraid to mess up $2300 amp if anything went wrong. But, I am pretty handy by nature and I am gonna take a shot.. discharing the caps sounds simple enough based on what I have read. I plan on starting with brite cap mod and try other ones one by one. Do I need to separate the amp gut from the case by taking all the knobs out for the brite cap mod? How long doe it take to do that? I will post pics when I take it apart..
No, just remove the chassis from the casing and snap a few shot's of the circuit board. The chassis is heavy (those damn transformers!!) so be careful. Obvioulsy make sure to plug out the AC cord but you don't need to worry too much about charged caps as you wont have to put your hands into the chassis.
You may not even need to remove the circuit boards to do the basic mods. At that stage you do need to worry about filter caps but we'll cross that bridge when we get to it.
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Unread 10-05-2008, 10:41 AM   #11 (permalink)
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Re: JCM 800 Mod

Install a .68 uf would across the 820 ohm resistor (R9) on V2a. Adding the capacitor thickens up the mids and gives a slight boost. It gives the tube a bit more gain and low end response.
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