Marshall Bluesbreaker II BB-2 mods and analysis

[Renegadrian]

Today I got myself a BB-2 (traded for a Caline chorus I had) and I want to experiment deeply with it...

now I got the schem here http://thebin.free.fr/FSTBX/bb2.pdf and I clearly see a typical Marshall tonestack, but only the highs are controlled by pot, mids and bass have fixed values. gonna try to have a regular 3 pots TS. about the TS, what are those extra parts?!
R33 and C14.
the first seems to brake bass a little, correct me if I'm wrong...
C14...dunno...

the boost mode sucks! I think I am gonna use that switch for diode swapping (leds/4148s)

Any other valuable mod?!

[Renegadrian]

(taking out r33 and c14) real marshall TS sounds good!!! easy and effective mod...even if I am not a big fan of the marshall TS actually, it sounds pretty good in that pedal...

[PRR]

> clearly see a typical Marshall tonestack

I see extra parts and odd values.

The ratio R20 5K to R23+R24 102K is very different from Fender/Marshall, which take a large loss here to get respectable "boost".

C14 has no direct counterpart in a F/M BMT.

Top pot value is far lower than a tube BMT, but C13 C21 are very near tube values.

This can't be modeled in Duncan TSC (no C14). And I am too sore to SPICE tonight.

Throwing rocks: full-up, the main effect must be C12 5nFd against 20K tone pot, bass-cut below 1,600Hz. Full-down, bass is cut somewhat and high treble is cut seriously. But the path through R20 C13 is significant and may throw a notch at low-mid.

Yes, removing C14 makes some sense.

[Renegadrian]

these are the tone stacks side by side. I learned that you should multiply caps by ten and divide res by ten, to avoid noises. so it makes sense. we have the same values, except for those two extra components. one common mod in marshall amps is to change the slope resistor value to 56k. so here is 5.6k.

Now my pedal has the standard marshall tone stack with all 3 controls, as per the right snippet, and it behaves as you would imagine.

[Lew-Dawg]

Bump 

Gents,

A friend of mine has so kindly dropped one of these off for me to fiddle with.

These things are made like a tank!

Is removing C14 the only "confirmed" modification for this pedal? I've been scouring the web for snag, but have failed to find any decent information, besides C14...

Any help would be appreciated!

Cheers

Lewis

[Steben]

Hi guys,

this thread leads me to a new project: modding the BB-2 to BB-1 circuit.
This will lead to less gain IMHO and some filtering mods.

[Renegadrian]

mmm afair it's not easy or doable, but I can be wrong on that one...why not start a new one altogheter!?

[Steben]

mmm afair it's not easy or doable, but I can be wrong on that one...why not start a new one altogheter!?

True. Looks like the bb2 has jfet stages though.