Swapping a GGG Bluesbreaker tone control for a BMT tone stack. Any issues?

[skiraly017]

Any foreseeable problems with swapping out the single tone control in a GGG Bluesbreaker build for a Bass/Middle/Treble type tone stack? Thanks.

[Mark Hammer]

Only one problem - major passive signal loss.

What dio you need this pedal to do for you?  Does it require the full 3-band thing?  or could you live with something simpler?

[skiraly017]

It's more a case of experimenting than anything. I've read through all the posted mods plus done some experimenting on my own and I'm 99% on the way to tonal nirvana but I guess I'm looking for the mojo that will just push the build over the edge. I was hoping to add some flexibilty to the tone shaping, even separate bass and treble controls would be nice. I just think the single tone control can be outdated in some applications. While I know the best application for the BB is pushing an amp at louder volumes, I'm trying to adapt this pedal to bedroom playing use. I'm very pleased with the amount of gain I get but there's something missing tonally and I'm think I'm close to getting what I want but it's escaping me. I need a hair more bass and a little less coloring. I want it to sound like my clean tone with grit if that makes sense. If you've got something simple in mind I'd love to hear about.

[Mark Hammer]

Well, first off, the schem PDF at GGG shows two .22uf/4k7 networks in series ( http://www.generalguitargadgets.com/pdf/ggg_mbb_sc2.pdf ).  You only need one of those pairs.

Second, somewhat like the Rat, though not quite as severe or intense, the C2/R3 network provides a bit more gain in the high end.  Consider inserting a make/break toggle to lift one end of that network out of circuit for a smoother tone.

Third, there are two lowpass filters on the output, one of them fixed (R12/C9) and the other variable (R16/C8).  Just for the hell of it, consider wiring up R16 like an SWTC, such that the outside tone pot lug goes to R12 instead of C8, and the wiper goes to C8 instead of R16.  This should provide more consistent output level with changes to tone.

Finally, the grit is a function of the clipping action of the diodes.  The 6k8 resistor in series with the diodes softens that effect.  Consider replacing the 6k8 resistor with a 2k2+10k pot in series, or maybe even just a 10k pot, such that the clipping action can be varied from softer (more series resistance) to harder (less R).

I built a BB but frankly it was not to my tastes.  I found it surprisingly harsh and really only usable within a very narrow range of control settings.  Maybe I should have done these mods myself.

[skiraly017]

Well, first off, the schem PDF at GGG shows two .22uf/4k7 networks in series ( http://www.generalguitargadgets.com/pdf/ggg_mbb_sc2.pdf ).  You only need one of those pairs.

The factory schematic shows the same setup.

Second, somewhat like the Rat, though not quite as severe or intense, the C2/R3 network provides a bit more gain in the high end.  Consider inserting a make/break toggle to lift one end of that network out of circuit for a smoother tone.

Will try.

Third, there are two lowpass filters on the output, one of them fixed (R12/C9) and the other variable (R16/C8).  Just for the hell of it, consider wiring up R16 like an SWTC, such that the outside tone pot lug goes to R12 instead of C8, and the wiper goes to C8 instead of R16.  This should provide more consistent output level with changes to tone.

Will try.

Finally, the grit is a function of the clipping action of the diodes.  The 6k8 resistor in series with the diodes softens that effect.  Consider replacing the 6k8 resistor with a 2k2+10k pot in series, or maybe even just a 10k pot, such that the clipping action can be varied from softer (more series resistance) to harder (less R).

Did that, like it.

I built a BB but frankly it was not to my tastes.  I found it surprisingly harsh and really only usable within a very narrow range of control settings.  Maybe I should have done these mods myself.

My build has been mucked with so much that I don't find it harsh at all, I just need to nudge something in the tone section. I need a little more...get ready for the cliché...clarity. I want to up the bass response a bit and slightly reduce the mids. Bahhh! Back to the drawing board.

[Mark Hammer]

Well, first off, the schem PDF at GGG shows two .22uf/4k7 networks in series ( http://www.generalguitargadgets.com/pdf/ggg_mbb_sc2.pdf ).  You only need one of those pairs.

The factory schematic shows the same setup.

The dual network is so that the switching approach used by Marshall (grounding the point between the first and second op-amps) results in less or no audible noise.  The challenge they try to meet is that when you have a circuit with the potential for generating so much gain (over 500x at max gain by my calculations), you want to suppress that risk of oscillations or boosted hiss bleeding to signal lines.  Grounding the point between the stages stops that.  If you are directly connecting the first and second stages without using Marshall's switching arrangement, you don't need the 2nd network because you don't need the C6/R8 network to shoulder the burden of controlling the current that the first stage is dumping to ground.  One of those networks will suffice to provide all the DC-blocking and gain-setting you require between stages.

So, if you intend to use Marshall's approach, keep both networks.  If your intent is to use the more standard input/output switching for true bypass, you can safely skip it.  Do make a point of maybe using shielded lines from switch to input/output, though.  I think we can safely assume that Marshall used the midpoint grounding for a reason.

Finally, note that the gain control Marshall uses does two things at once.  First, it reduces the feedback resistance in stage 1 (reducing gain).  Second, it reciprocally increases the overall input resistance for stage 2 (thereby reducing gain).  Peavey uses a variant of this in some of their solid-state amps, though in a slightly different way.  In the Peavey instance, a single pot is used to both adjust the amount of gain in an op-amp and the amount of attenuation on the output of that op-amp.  For instance, check out the Rage 158: http://www.schematicheaven.com/newamps/peavey_rage158.pdf

The wiper of VR6 goes to ground.  One half of VR6 is in parallel with R16 and sets the drive of that stage.  The other half of VR6 forms a resistive divider in tandem with R19.  As you turn the gain up (left half of VR6 gets smaller), there is less post-gain attenuation of the signal (right half of VR6 gets bigger), so there is a much hotter signal available at the junction of R19 and VR6.

Marshall's arrangement for the BB involves an analogously complementary effect by altering the gain of two stages at once.  One of the byproducts of that is that it introduces some low-end rolloff on the input of the 2nd stage as gain is increased.  Not so sure I like that.  If you don't like it either, consider wiring up the gain pot as simply a variable feedback resistance.  This way the gain and bandwidth of stage 2 remains fixed, independent of gain in stage 1.

[bean]

Great explanation Mark!

As far as the tone stack, you could also try using a second opamp and introducing an active control. Check out any number of the ones on Jack Orman's site. Maybe even a little bit of tone clipping? If you use a dual IC, then use the second half for a little recovery (although you probably won't need it). If you are sold on the BMT stack, try using a jFet stage before the output to get some signal back. You could also try a BMP, too.

I also suggest using an SPDT to take the clipping diodes out. You can get a nice dirty boost from this circuit without the diodes if you tweak a few things!

Last thing: try a small value resistor between the pins 2&3 of the gain stage. It will help control the volume drop as you turn the gain down. Suggest 2k2-4k7.

[Mark Hammer]

Thanks, Bean.  Much appreciated.

Note that, between the rather copious gain and high output level available from the use of a 2+2 diode complement and series resistor,  there should be plenty of signal to survive a tonestack.  The assorted circuit elements between the output of the clipping stage and the volume pot (two poles of passive lowpass filtering) already take away some signal passively, and replacing them with a tonestack is entirely feasible.  Whether they take away significantly less than a full t/m/b tonestack might is an empirical question, or at least beyond my abilities to calculate. 

Of course, even if they did take away significant level, the real question to be asked is whether what you are ultimately left with is sufficient for your needs.  Keep in mind that perhaps the tones you like from it come when the gain control is well below full tilt.  That will, of course, result in a lower amplitude signal reaching the tonestack, and a lower signal amplitude, again, reaching the volume pot.  It MAY be enough for your needs but you may also find yourself in the same quandry as folks using a Distortion+ too often find themselves in; where the volume/level you want for a solo or to push the amp hard enough comes at a distortion setting you don't want.  That is the corner I was trying to keep you from being backed into.  Of course, if your goal is not necessarily to stress the amp with the pedal, then it's no corner at all.

I guess another question that needs to be asked in order to determine viable suggestions is whether you have a need to make all of this work within the constraints of a GGG board/layout.

[skiraly017]

I spent a few hours playing with the circuit last night and I think I have it just about where I want it. The goal is to make a really good bedroom pedal. I have no doubts that at gig volume this thing will work very well but getting a pedal to get "that sound and feel" at lower volumes is what's keeping me busy these days. I'll try the resistor between pins 2 and 3 tonight as that sounds like it could be a good thing but I think I may have gone as far as I can with the base circuit.

Side note...JD's new style boards are great.