Grit control for Bluesbreaker

[Mark Hammer]

I was looking over the schem for the Marshall Bluesbreaker from Tonepad the other day, and realized that the qualityof the clipping could be altered with one convenient and easy to install control.  The mod to the circuit (original is at Tonepad) can be seen here: http://img.photobucket.com/albums/v474/mhammer/BB.png

Normally, the Bluesbreaker has a 2+2 diode combo in series with a 6k8 resistor to provide a higher threshold and solfter clipping.  That stage has a 220k feedback resistor and 10k input resistor to provide a gain of 22x.  In combination with the gain set by theprevious stage, that is easily enough boost to produce clipping from the 2+2 diode combo.

But what if you want harder OR softer clipping?

The illustrated mod uses 2 resistor changes and an extra 100k linear pot to accomplish that.  The 6k8 resistor is changed to 8k2, and the feedback resistor is changed from 220k to 150k.  Feel free to try other values.  The inserted pot has one leg in series with the feedback resistor, and the other leg in parallel with the resistor between the diodes and pin 6.

So, what does it do?  The leg in parallel with the 8k2 resistor will yield a series resistance of anywhere between zero ohms and 7k6.  That series resistance will allow for somewhat softer and harder clipping.  At the same time, as you rotate the added pot, you change the feedback resistance.  At one end of rotation, the feedback resistance is 150k (gain of 15x) and the diode series resistance is 7k6.  At the other end of rotation, the gain is 25x and the series resistance is 0R.  The 8k2 value was chosen so that (hypothetically, because everything has tolerances) it would be possible to return to stock parameters.  In principle, when the pot is set to add 70k (out of 100k) in series with the 150k feedback resistor, the remaining 30k is in parallel with the 8k2 resistor, yielding 6k4, which is pretty close.

Kinda busy with other stuff this weekend so I haven't installed it.  In theory, it should add a little bit of variation to the BB.  As well, in principle it can also be inserted (presumably with appropriate component-value changes) in a number of other clippers using the feedback loop.

[Roobin]

Wow, this sounds like a real development. I always thought there was a little to much bite/grit to the BB (but was too dumb to figure out what to do!).

However, me trying this mod may be alittle hard, as currently my BB has hardly any gain at all, and when it does, it gives 'wolfy notes' liek someone is breathing relly hard in the background (that's the best description I can give!).

BTW Mark, the link isn't working for me.

Thanks,

[Mark Hammer]

Try this:

[Gus]

once the diodes start to conduct the 8.2K with the diodes overpowers the rest of the feedback.

The 150K of the stock is to keep some feedback control until the diodes conduct.

[Roobin]

Merci, Mark.

Sorry, slightly OT, but I've always wondered why the tone and volume are referenced to 4.5v, not ground. Does this have any audible effect?.

[Mark Hammer]

once the diodes start to conduct the 8.2K with the diodes overpowers the rest of the feedback.

The 150K of the stock is to keep some feedback control until the diodes conduct.

Not rejecting it or anything, but I'm not sure what your point is.

[Gus]

grit I think edges

Your idea looks good maybe a cap inserted in the leg of the 100kB that goes to the node of the 8.2k and the diodes

Your circuit will do something interesting with the 2nd stage gain

[Mark Hammer]

That could be interesting.  I was think about a relatively non-invasive modification of the basic Bluesbreaker that would allow those who like its gentler nuanced clipping to be a little more aggressive when they wanted it.  At a certain point, though, it stops being a Bluesbreaker and starts becoming something else....a bit like those hot-rodded Tube Screamers that strive to be heavy metal pedals!

[Roobin]

In my attempt to be 'inventive' i stuck a spare 220pf cap across the diodes. It didn't really change anything. Am I correct in saying that for higher freq. this allows the signal to bypass the diodes.

Wow idea: You know about splitting signals into bands eg mids, well how about a different clipping section for each band, eg LEDs for highs, MOSFETs for mids, or 2 types of clipping, and be able to blend them together.

That was hideously OT, but I had the brainwave, and I often forget them!

[brett]

Hi
Nice concept! 
How much different is the result to wiring a pot as a variable resistor in series with the 220k resistor in the feedback loop?

The max gain of the standard version seems low considering the dual diode system.  A nice wide range of gain might be cool in the second stage of the BB.  Perhaps 20 to 50?  A fixed resistor of 220k and a 250k pot would almost cover that, and work upwards from where the BB is now.
thanks Mark 

[shanter]

ok, i dont know if anyone else tried this mod, but it didnt seem to do much.

i was expecting this grit control to sort of clean up the upper sparkle the pedal had but it really didnt do much. from one side of the pot to the other, the pot gave the pedal a little bit more body and very very lightly affected the grit on the bluesbreaker.

http://www.rzuser.uni-heidelberg.de/~tpfluege/bluesbreaker.html

i also found this website and tested out the mods there aswell, and they all seemed to have very little or no affect at all.

I was sort of looking for a way to adjust the compression on the pedal and also shift the tone control's frequency range because its pretty much set up at sparkle to TOO much sparkle. And when the tone is way down, the pedal looses alot of volume and bite.

anyone have any ideas?

[Mark Hammer]

I find this pedal a little shrill too, and only really find an acceptable sound when the tone is turned fully off and the gain is set to medium.  Even then, it only really sounds decent on the neck pickup to my ears.

The shrillness can be tamed by sticking a small cap in the feedback loop of the second op-amp stage, in parallel with the 220k resistor. A value of 100-150pf seems about right.  Changing the .01uf (10nf) cap to ground/Vb in the tone control to something like 15nf will probably help as well.

One of the things about the "grit" mod is that, like a number of such mods, it will depend on your pick attack to be audible.  Same thing goes for stuff like switching between 2 vs 3 diodes.

[shanter]

mark, can you explain why/how adding the 100-150pf cap will tame the shrillness? i dont just wanna do the mod, i'd like to understand it aswell. 

[KerryF]

Anyone else try this mod?  I did it to my second BluesBreaker.  It adds to versitility and is pretty cool  .  I have it all finished with a box ready to be drilled, but im lazy 

[Mark Hammer]

mark, can you explain why/how adding the 100-150pf cap will tame the shrillness? i dont just wanna do the mod, i'd like to understand it aswell. 

Hmmm, I thought I had already responded to that one.

The cap in the feedback loop provides a low-impedance path for negative feedback from the output to dampen/reduce the gain of the op-amp.  Because the cap only provides a lower-impedance path for higher frequencies, it results in a treble-cutting action.  As the cap value goes up, more treble is cut.  WHERE the treble is cut is determined by the joint product of the feedback resistance and the cap, according to the formula: Freq (where rolloff starts) = 1 / [2 * pi * R-in-megs * C-in-uf]

[allan_belt]

JHS Morning Glory overdrive has 470pF switchable ... very good idea..

[stm]

AS for the grit control, I would simply try a 10k-lin pot instead of the 6k8 resistor.

[GGBB]

Very, very, very old thread, and the JHS idea post is somewhat off topic.

[stm]

LOL--I never look at the thread age!

[mimmotronics]

Merci, Mark.

Sorry, slightly OT, but I've always wondered why the tone and volume are referenced to 4.5v, not ground. Does this have any audible effect?.

The 4.5V bias is chosen for the pot because the other side of the pot is biased to 4.5V as well. If you have one side biased to ground and the other biased to 4.5V then you would have a fixed DC voltage at the wiper. As you turn the pot, the wiper sees a change in DC voltage which ultimately manifests as audible noise as the pot is turned.