astrodoubs a écrit :
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.
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.
Remove the 470pF cap which bridges the 470k Ohm resistor you just removed.
Cette mod va te rendre l'ampli plus bouheux, le son de la guitare sera moins déformé par le filtre assez brutal du 470k/470pf au lieu du 68k simple et traditionnel des plexi. Le son sera moins tranchant. Je pense qu'il faut mieux se concentrer sur les résistances et condensateur de cathode des étages de puissance. D'après ma petite expérience sur ce schéma, une bonne partie du coté bouheux/sec/tranchant se joue ici. C'est ce qu'il fait la différence entre le canal sourd du plexi et son canal bright. IL suffit de jouer sur deux résistances/condo pour changer la texture de la saturation (mais pas la tonalité globale de l'ampli, seulement la saturation)
Gain Mods
These are gain mods aimed at increasing the distortion available.
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, 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).
Idem, je parle de cette résistance. En mettant un condo en parallèle, on choisit quelle fréquence va être boostée. Ici, il n'y en a pas et le 10k va booster TOUTE la plage de fréquence reçue. En baissant on augmente radicalement le gain. Pour augmenter tu peux passer à 4,7k mais le son va devenir de plus en plus bouheux. Pour y remédier il faut filtrer et empêcher le boost des basses qui rende le son trop gras. On peut mettre un 0.68uf comme sur le plexi pour casser le gras.
You can also increase the plate resistors values to add more gain from 100k up to 150k or 220k (insane). I don't like this mod as much, it gets buzzy.
Pareil, je ne toucherai pas perso
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. [/color]
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.
Find the 68K input resistor (R3) and place a 330pf 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 also.
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). The resistor and capacitor values form a 1st order low pass filter (6db/octave drop below the corner frequency, where f = 1/(2∏RC).
C'est ce que j'ai dit plus haut. Sans condo on boost tout. Avec un condo on choisi la plage de fréquence qui va être boostée. Il y a des standard après, pas la peine de chercher pendant trois ans.
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 awsome. I think Soldano's use 47k. The Plexis and SuperBasses used 56k with 250pF tone cap.
pas trop d'accord ! Remplacer 33 par 47 coupe un peu le signal de 1 ou 2db environ. D'après le simulateur, les mids ne sont pas
Locate the 22K Midrange knob (VR4) and replace it with a 50K Linear or Log pot. This is for those who are seeking more flexibilty out of the midrange knob.
A fond ça boost mieux les medium, au minimum on a un énorme creux à 500hz
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.
Encore ce que j'ai dit plus haut. On joue sur la plage de boost.
Need more bass,look at the purple wire going from the impedance selector to the 100k feedback resistor, Disconect it from the impedance selector, now get a .0033 cap connect 1 lead of the cap to whare 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
Bon voila. Après on ne sait pas quel schéma as tu sur ton 800 car il y en a un paquet.
