voila un 1er liens pour des mods du jcm 800 2203 :
http://marshall.redpt.com/clay(...).html
et un autre :
http://www.tone-lizard.com/Ult(...)0.htm
WARMUP VOICING MOD
JCM800's are notorious for being very bright. This is due to an input series resistance which is very different from the Classic Marshall circuit. (470k Ohms vs. 68k Ohm). The following mod details a method of warming up the tone on these amps. This gives your amp a more classic, warmer Marshall tone.
1. Replace the 470k Ohm resistor that is tied to the input tube grid with either a 68k Ohm, 34k Ohm or 10k Ohm resistor. The lower the resistance, the darker the tone, but 68k is the "classic" value. (Don't be tempted to remove this resistor altogether). It has to be there to attenuate the grid circuit and prevent excessive overload conditions.
2. Remove the 470pF cap which bridges the 470k Ohm resistor you just removed.
GAIN MODS
These are gain mods aimed at increasing the distortion available.
3. Locate the 10K cathode resistor (R6 found on V1B) and replace it with a 4.7K resistor (or just solder another 10k in parallel on PCB amps). This will beef up the gain. This seems to be the "sweet-spot" on these amps. You can also try other values like 6.8k, 3.3k, 2.7k, and 1.5k.
If you want insane gain use a 1.5K (but I think it's too much and doesn't sound as good). The 4.7k sounds just right and the gain doesn't get mushy. Since there is no cathode bypass cap here, all the frequencies in this stage are amplified normally (flat response).
It was even suggested to put these 5 values (10K, 4.7K, 3.3K, 2.7K, and 1.5K) on a 5-way rotary switch so you can switch between the different values.
4. The phase-inverter can also be modded for more gain. Rather than the 82K/100K plate-load pairing, try 100K/120K or 120K/150K. Too much gain here will result in a tone that is rather unpleasant.
5. Change R11 in the voltage divider to 120k, then change R10 in value from 470k thru 120k, wherever you like it best. The 120k for R10 gives a richer distorted tone.
6. PPIV (Rich Mod).
VOICING MODS
These voicing mods are intended to tailor the frequency response of the amp. Some of the mods also increase the gain in a certain frequency range as a side effect.
7. Locate the 68K input resistor (R3) and place a 330pf 1kv ceramic cap across it. This will beef up the mids and lows and make the entire amp feel more "meaty" and "thick". I think this mod came from Mike Soldano.
8. Locate the 820ohm cathode resistor (R9 found on V2A) and place a .68uf cap across it. This will increase the gain a little. But mostly it gives you the nice "Plexi" midrange boost. This is called a "gain focusing", because it adds gain, but only in part of the bandwidth (mids and highs here).
Some mention to use a .47uf cap. It was even suggested to use a 5-way rotary switch of (no cap, .22uf, .47uf, .68uf, and 330uf) so you can switch between the different values.
9. Locate the 33K "slope resistor" which is in the EQ section (R15) and replace it with a 47K resistor. This bumps the amp's entire midrange frequencies up a notch. The result is a very "fat", "thick" or "beefy" tone that is really awesome. I think Soldano's use 47k. The Plexis and SuperBasses used 56k with 250pF tone cap.
Some mention to run a toggle switch to toggle between 33K, 47K and 56K resistors.
10. Locate the 22K Midrange knob (VR4) and replace it with a 50K Liner or Log pot. This is for those who are seeking more flexibility out of the midrange knob.
11. Locate the .68uf cap (C1) which is found on V1A and replace it with a .47uf cap. This alters the midrange response and adds a "heavier" tonal quality to the amp. This is also a "gain focused" mod. Altering the value of the cathode bypass cap to a smaller value changes the corner frequency of the low pass filter and thereby adds gain in the mids between the new corner freq. and the old corner freq.
12. Need more bass, look at the purple wire going from the impedance selector to the 100k feedback resistor, Disconnect it from the impedance selector, now get a .0033 cap connect 1 lead of the cap to where the purple wire went and take the other lead and connect the purple to this, BOOM more tight bass (adding bass in the preamp makes the amp turn to mush and you lose gain on higher notes). This is kinda like a fixed resonance control.
RESONANCE CONTROL MOD
13. This mod was first posted by Mark Cameron. It works on Fenders and Marshalls with great results. It adds an extra knob (output jack holes work well for this). Basically, it is the opposite of a presence control, allowing you to tighten up the bottom. I will try to talk you thru the wiring. Looking at the back of a 1meg pot, left to right, numbering pins one two and three. Disconnect your purple feedback wire from the impedance selector and solder to pin three. Solder a new wire from the feedback resistor (where the purple feedback wire used to be connected) to pins one and two. Solder a .0047 cap from pin three to pins one and two.
MISC. MOD
14. Locate R24 and R25. Usually these are 220K for EL34 output tubes, and 150K for 6550 output tubes. Lower both resistors, regardless of tube type, to 100K. This keeps the tubes on a 'shorter leash', and does promote long life. The theory behind this move is that the chance for the output tubes to draw grid current at high volumes is reduced.
