1. 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”.
2. Locate the 10K cathode resistor (R6 found on V1B) and replace it with a 4.7K resistor. This will beef up the gain. This seems to be the “sweet-spot” on these amps. If you want insane gain use a 1.5K.
3. 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.
4. 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.
5. 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.
6. 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.
- Note: All resistors are 1/2watt unless otherwise specified and caps/resistors are marked according to the Marshall schematics
thanks to schematic heaven for the above mod