I like the original BB Marshall. It sounds a little harsh but not too much let’s say bright … one of the flaws : the overdrive starts from 3 hours (before, there is not much, and the overall level of output remains fairly low), the pots spit (that’s marshall) and even treated with KF Contact (or equivalent), this is far from perfection and, finally, as for the Guv’nor, the plastic jack welded directly on the circuit board are quite weak (cracked, do not take out the guitar jack etc..).
The quality of electronic circuit is not very suitable for mods (but it’s OK anyway, huh …) : the components have their legs bent copper side, and to desolder it’s much less convenient than the Japanese (Boss, Ibanez).
Stop criticizing and here are the schematics : the original, then that inspired the Analogman KOT.
Marshall Bluesbreaker schematic
Analogman KOT schematic
I begin by replacing the resistor R8 with a carbon resistor 1M (instead of 2.2M for the stock pedal). I also solder a 100uF electrolytic capacitor of the terminals of R6, R7, and a small MKT 0.1uF Wima of the terminals C6 (Vr = 4.5V approx).
Then I replaced C3 by a Styroflex 100pF, and R1 by a 27k and R2 with a 33k :
I must then add the 10k resistor in series with the gain pot. Tweak ! I desolder one leg of the pot and I place there the 10k resistor :
My stock Marshall Bluesbreaker pedal chip is a JRC072BD :
I replace the original chip … I’ll try with a Burr Brown OPA 2134 and other ICs ; I replace R3 (1k) by a carbon resistor (I would have replaced C4 and C7 of the Panasonic DMC-V film and R10 and R11 by carbon resistors, but I do not have the ingredients on hand …)
Now let’s face the big tweak of death : in soft clipping, we replace the 4 by 4 diodes original MA856 (that are just wonderful in my TS9DX modded) and in hard clipping (not provided in the original Bluesbreaker) 2 diodes placed 1S1588. The transition from one to another will be made with a DPDT switch.
The 4 MA856 diodes and the switch …
For the addition of diodes 1S1588 hard clipping, I connected directly to the pots, Vr is on the volume knob :
Then I mounted a 50k potentiometer (as a variable resistor) between C8 and volume pot (I moved C8 to solder it directly on the knob of presence) :
To finish the job, I replace C10 by a 1uF panasonic ECQ-V :
Firsts tests on my nice little clone of tweed field (I definitely prefer soft to hard clipping clipping, more harsh to my ears, already found on TS9DX modded). The guitar is a Squier Classic Vibe Telecaster Custom w/ Lollar Special-T pickups : Marshall bluesbreaker modded analogman KOT sample
Here are 2 samples with my Gibson Les Paul Custom on a AC30 CCH, cabinet Orange PPC 1×12 V30 :
Les Paul Gibson 2PUs Marshall Bluesbreaker KOT Modded
Les Paul Gibson PU Bridge Marshall Bluesbreaker KOT Modded