Bluesbreaker mod question

[Blacklake]

Hi everyone!

Im new here and also pretty new to the art of stompbox building. Love it, though!  A fantastic hobby! The satisfaction of doing a gig with a pedal on your board that you have built yourself and that sounds great is a wonderful feeling! 

I have a question for all you skilled builders and moders out there:

I have a bluesbreaker that I have built (GGG) and after some tweaking I love it! Sounds fantastic, great to just add some grit and breakup to the amp. The only thing I feel Id like to improve is that when I crank the drive to its maximum the highs are also boosted. It only happens at full drive. The EQ profile of the pedal is great, transparent and full until you get beyond 4 a clock (5 being full) on the drive knob. Then there is an increase of the higher frequencies that makes the pedal sound hard.

My guess is that this has something to do with the amount of signal being fed into the diodes (4002?s) and opamp (2 stacked 4558?s) ?

What would be a good way to address this problem? Should I try to put a cap i parallel with the clipping diodes? If so, what value would you suggest and does it matter on what diode I put it on? Do I put it on the last one, the one where the signal leaves the diodes, or on the one where the signal enters? Do I put it on only one or should I put it on a pair of diodes?

Is there a better way to get to this? I?m not the best with schematics, but it seems to me that resistors R9, R10 and R11 (referring to the layout on GGG?s site) would be involved when it comes to controlling the amount of drive that this pedal has. (am I completely wrong?) I don?t need more drive out of it, but maybe changing values of any of these would effect the gain structure i a way that could be helpful in this matter?

Should I try to put a cap in somewhere between the diode section and the opamp? Before the diodes?

Should I try to change the value of one of the existing caps in the circuit?

Help!   

Thanks a lot! 

//G

[GibsonGM]

I would (and have done this!) place a cap between the input and wiper of the gain pot.  Just grab pretty much any cap (.1u, .68, etc) and pop it in there with jumpers temporarily.   Play guitar, rotate drive control and see if you like the result.  Different values will allow more or less highs thru.  The 100p cap is doing this, but you might need 'more'.   Running the cap to the wiper allows it to be 'variable'.  In the end, you should wind up w/having similar highs thruout the range of the pot.

It's called a treble bypass cap - you find them on guitar volume knobs to allow highs to pass as the impedance increases when you turn down the volume.   It works for gain knobs, too.
I had the same issue you're describing, and it works pretty well!!! Assuming you want consistence and not just to kill the highs...

Now, if you want to REMOVE some highs when the diodes really clip, you can try adding a cap in parallel with the 4 diodes....something like 100p and up, could even try .1u.  That might smooth things out a bit when they clip.
You might end up doing both of these things to improve the circuit's performance, it's a matter of taste.

[Blacklake]

Hi.

Thanks a lot! I will try this right away! Just a couple of questions...

1. The "wiper" of the gain pot, would that be the centre lug?

2. To connect the cap in parallel with the diodes, how would I go about that, considering there are four diodes?  Do I solder one leg of the cap to one leg of the first Diode and the other leg of the cap to one leg of the last diode?

Sorry for being such a newbe...   

Thanks a lot for helping out!

[GibsonGM]

1) Yup!  Center lug.
2)  You'd straddle the diodes, such as by connecting the cap to the inner end of the 6.8K resistor and the inner end of the 220K....

We're all new at some point!   )

[Blacklake]

Ah, great, got it!
...I think...    Inner end of the resistors being the end facing the middle of the pcb, not the end pointing to the edge of it, right?

Thanks so much for helping out! I will get to it and post results when I´m done!

//G

[Blacklake]

Hi guys!

I finally got my BB clone right! No more harsh high end!  
I put a 47pf cap across the clipping diodes, like GibsonGM suggested (put on the legs of the diodes though, not on the resistors before/after them)
and then I solderd a 47pf and a 150pf in series and put them on the outer lugs of the volume pot. That really di smooth the highs out perfectly!

The pedal now sounds amazing with the other mods I did to it! It´s the "always on" pedal on my board now!

Thanks a lot for helping out everyone!

//G

[GibsonGM]

You're welcome ) Glad it worked! It's a good pedal...

[Blacklake]

Hi again!
Got another question about the BB and I figured it would be better to continue on this thread rather then starting a new one, since it is regarding the same pedal...?

Anyway, here goes;

I´m thinking about trying out asymmetrical clipping on the BB and making it an option available on a switch. My question is, where is the best place to add the diode? D2,D3,D4 or D5?
Does it matter where I put it? If so, why?

Thanks!

//G