Early 1980s Marshall Lead 12 mods ideas, would love feedback

Jul 25, 2020
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Hi, I have a Marshall Lead 12 5005 combo MK1 version with the fuzz clipping circuit. I wish to bring this thing as close as I can to a mini-JCM800 as i possibly can. Ive already swapped the stock op-amp for an LM833N based on another thread here and it's been a massive improvement, especially with the mid-range.

For now it is powering an early 2000’s 4x12 1960A cab with two 1982 G12-65s as well as two 1975 G12M-25s with RIC cones (all four are 75hz).

Things im looking at.

Lack of Brightness and Bark
I have not had the opportunity to crank this thing due to living in a dense city with neighbors in all directions, and the current environment keeping every single one of them at home. But at low volumes, even with treble and mids maxed out, the amp is only moderately bright when running through famously bright G12M-25s. As such it's missing alot of the "marshall bark". The EQ is incredibly sensitive to the point where putting all knobs at 0 brings the entire volume down to maybe 5% vs if they were at noon. So i dont think its the EQ. Probably needs a bright cap somewhere i cannot identify.

MKI vs MKII/Reverb/Headphone Out etc etc

This video shows a very clear difference between the MKI and MKII versions of the Lead 12 with the latter version sounding in my ears much punchier. So my current strategy is to try to get my amp as close to MKII specs as possible

Lack of Schematics

Drtuber's website only has latter years Lead 12. The closest i could find to a schematic for the MK1 with the fuzz circuit was a single user in the MyLesPaulForum who seems to have reverse engineered it by hand.


While i cannot verify if it is correct or not, so far looking at the preamp section it looks near accurate and so i am using it as my main schematic for further mods. I have already used it to identify the clipping diodes and the 100ohm resistor they link to after the second op amp stage. Removing these three items seems to have significantly reduced the fuzz. I am wondering if there is anything else to the "fuzz circuit" or if that alone took care of it.

Bass Clipping at high gain
I do not know if this is a side effect from something i might have missed in the fuzz circuit or if it’s the same issue given in the second post of this thread that appears to be solvable by changing a capacitor value. Essentially at high gains, especially with palm muted stuff, the bass audibly distorts instead of remaining tight, giving a nasty clipping sound similar to a cheap overdrive pedal or a maxed out 60’s fuzz circuit. I would love some insight on this.


Amp Gain is not linearly usable
The Gain knob feels exponential when i turn it up. Anything past 8 and i have to do hairline increments to tweak my gain because it becomes incredibly sensitive. Is there some way to mitigate this so that gain is more linearly scaled as the knob progresses? Is this how a normal JCM800 also behaves? Would replacing with a log pot work?

10” vs 12” speaker
Would simply screwing in a 12” internal speaker to the combo while not modifying the hole cause any issues? I’d like to insert a scumback in the combo (there is enough clearance for it) without having to permanently cut out a bigger hole from wood.

I would love input from the seasoned vets here to be able to help out in voicing this little beast to be as close to a JCM800 as it can. Because it definitely is already in the ballpark when run through a full cab. Im not trashing this amp, it actually feels like its 75% there towards the JCM800 tone, im just trying to bump that to maybe 90% Hopefully we can also produce a verified schematic for the MK1 once and for all for anyone wishing to work on these in the future.

Image references as well as some ideas on what to do with this thing.


Any help would be appreciated and thank you.