Marshall BB-2 Bluesbreaker Mods

[rmjlmartin]

Hi everyone, and thanks in advance for your help.

I have a Marshall BB-2 Bluesbreaker that I've had for awhile but rarely done much with, and I've recently decided that maybe I could try modding it to make it into a more usable pedal.

What I'd like to do is get the tone somewhere in the ball park of the original Bluesbreaker, or at least a bit closer than it is now. It has a lot more gain than the original, and I'm ok with that (I can turn the gain down, after all...), but it's a much warmer-sounding pedal than I'd like it to be.

I emailed Customer Service at Marshall, and they sent me the schematic (also available here- http://thebin.free.fr/FSTBX/bb2.pdf), and there are numerous schematics floating around for the original, or modded Bluesbreakers, like the Analogman King of Tone, JHS Morning Glory, etc. I was hoping that having the two schematics would help me figure out how to tweak the tone-shaping components, but it's a fairly different circuit, at least different enough that it's a bit over my head to try to figure out what exactly would need to be changed.

I have already removed the LED clipping diodes and installed sockets to be able to try different options there. I also notice that the original Bluesbreaker uses a TL072 op amp, while the BB-2 uses a 4558. Would socketing that op amp and trying a TL072 move me in the right direction, or would it not make that much difference? What are the tone-shaping components (caps and resistors, I assume?) that would affect the bass, middle, and treble.

[rmjlmartin]

Oops, I guess I kind of forgot about this post. And it doesn't look like anyone had any particular ideas, either.

Let me ask it this way- Does anyone know of some good, useful mods of any kind for this pedal?

[Rockhorst]

For the original bb, running it at 18V made all the difference for me. Check if the electrolytic caps are rated high enough and give it a try.

[Brach]

You can always change the clipping diodes for an easy mod.  Try making them asymmetrical...try replacing LED1 with 2 silicon diodes (like 1n914 or similar).  The red LED's Vf is usually around 2V, and the 1n914 is around .7 (1.4V as a pair) so you'll get around .6V difference between the positive and negative half of the signal.  This should cause it to break up a bit earlier.  You can also try changing both LEDs to silicon or germanium
(Ge) diodes for a completely different break up point...both will break up much sooner than the LEDs....especially the Ge diodes.
The original BB used 2 pairs of silicon diodes (if i remember correctly), so it must have had less voltage driving them.  To make the BB2 more like the BB1 you can reduce gain by lowering R10 (to lower the gain of the first opamp stage) and/or lower R13 to shelve off some more signal before it hits the next opamp stage.
I hope this is helpful.
-Brach

[sjaustin]

As far as the op amp goes, I'd leave that for last. Removing them is kind of a pain, and it's not likely to make much difference IMO. Try the other mods first.

[Rockhorst]

As far as the op amp goes, I'd leave that for last. Removing them is kind of a pain, and it's not likely to make much difference IMO. Try the other mods first.

That depends on the board. If it is single sided like the original then removing the IC is only a minutes work to do cleanly. It's with double sided boards that it gets annoying because there's always solder left over at the joint.

[sjaustin]

As far as the op amp goes, I'd leave that for last. Removing them is kind of a pain, and it's not likely to make much difference IMO. Try the other mods first.

That depends on the board. If it is single sided like the original then removing the IC is only a minutes work to do cleanly. It's with double sided boards that it gets annoying because there's always solder left over at the joint.

Great point, hadn't considered that!

[rmjlmartin]

Hi again, and thanks for all your help.

I've replaced the led clipping diodes with two pairs of silicon diodes like the original. I'm not 100% sure what they are- I harvested them from some old electronics, but they look like some 1N914s that I bought recently.

I also removed C15, and socketed R10 and R13 to be able to easily try different values, and ended up with 47k for R10 and 10k for R13.

The pcb is single-sided, so I did go ahead and socket the op amp. I tried a TL072 like the original, and I think it sounds a lot better. Or maybe it's psychosomatic? 

Sadly, though, most of the caps are 16v, so I can't run it at 18v without replacing them, and I didn't go to that trouble yet. Maybe I will at some point, though I don't have an 18v power supply at the moment.

I don't have a real Bluesbreaker to use as a point of reference. I do have a Mooer Blues Crab, which sounds very similar to a BYOC British Blues Overdrive kit that a friend has. The BB-2 is still a little warmer than the Blues Crab, and has a bit more gain, but it's definitely much better than it was. And the gain is no problem- I can always turn it down.

So I'd say the mods were a success. Thanks again to all of you that took the time to offer advice! If I decide to go further, I'll post the results here later. (Maybe upgrade the caps and build a charge pump?)