Master Lead Marshall Circuits

Master Lead Marshall Circuits - MV Models 2203,2204,...

This describes the circuitry of master volume Marshalls. It shows most of the major components and describes the circuits used in the 2203 (100w) and 2204 (50w) models. Starting in 1975, Marshall decided to create new master volume models to add more gain to their product lines in response for more demand for hard rock and metal type tones.

Similarities to non-MV Models 1987,1959

The 2203/2204 MV models are very similar to the 1987/1959 non-MV models. They share the same type and number of tubes. The main difference is where the non-MV models have 4-inputs and two separate channels. The MV models only have 1 channel with high and low sensitivity inputs. Basically, they took the 2nd channel from the non-MV model and added it as an extra gain stage. There is a mod to the older non-MV amps, called the One-wire Mod, that cascades the two input channels, similar to the Master Lead amps.

Circuit Board Layout

Block Diagram with Clickable Details

Click on the blocks for a detailed explanation.

2204 Pre-Amp Schematic

2204 Power Amp Schematic

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.

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.

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).
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.
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.

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).
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.
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.
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.
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.

Resonance Control

This mod was first posted by Mark Cameron. It works on 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.

Special thanks to:
Wizard of Ozz at Marshall Amp Forum
Plexi Palace Marshall Forum
Les Paul Forum Amplifiers
Trace Allen from Voodoo Amps
Bruce Clement's Chassis Pics
Mark Cameron