How to limit range of gain pot in Bluesbreaker clone

[odieux fonzie]

Hi everyone !

I use bluesbreaker clones for my overdive tone. (Mooer Blues Crabs)
I'd like to control gain pot with my foot while playing. I only use the pot from 12h to max.
I'm going to build a new bluesbreaker using the Muzikding kit.
How can I mod the gain to pot so when at zero it's like on 12h with no mod ? I don't want to change the maximum value.
I can solder and have made a few kits and two stripboards, but I'm still a newbie as I  can't really real a schematic... Thanks a lot for your help !!

[antonis]

You can't..

(at least, not via an easy passive way..)

[FiveseveN]

an easy passive way

Make the pot 50K and put a 47K resistor in series.

[ElectricDruid]

an easy passive way

Make the pot 50K and put a 47K resistor in series.

Yeah, my thoughts exactly. Should be simple enough.

[antonis]

Should I direct your attention to IC1a & IC1b combined gain topology..??

[ElectricDruid]

Should I direct your attention to IC1a & IC1b combined gain topology..??

No, I've already looked. If you're only using half the pot from 50% to 100%, then you can definitely replace it with 47K+50K pot and get the same effect. What was the middle of the pot becomes the new bottom, so the pot now runs from 50% gain up. Think about it, Antonis - there's really no problem.

[antonis]

Ouch... 

Yeaapp, but only for 47k resistor connected on GAIN pot lug 1..
(leftmost the feedback loop)

[Mark Hammer]

Nah.
The Gain pot on a Bluesbreaker adjusts the gain of two stages simultaneously, by increasing the feedback resistance in one inverting stage, and decreasing the input resistance on a second inverting stage.

Stock, at minimum setting the Gain pot makes the gain of stage 1 essentially unity, and the 2nd stage essentially 2x.  As the Gain pot is maxed, stage 1's gain goes up to roughly 22x, and stage 2 goes up to about 22x as well (depending on which schematic you're looking at), yielding a combined gain of just under 500x, which is more than enough to get the 2+2 diode s to clip.  At minimum gain, the pot "creates" an input resistance to the clipping stage of 100k.  By placing a resistor between the wiper of the pot and end of the Gain pot that feeds the 2nd stage stage, you can make the minimum gain still pretty hot. 

So, if we stuck a 10k resistor in parallel with that leg of the pot, it would set the minimum input resistance to stage 2 not at 100k but at 9k, making stage 2's minimum gain a bit under 25x.  Nudge the Gain put up a hair, and the slightest bit of increase in stage 1's gain now becomes a multiplier.  Let's say we nudge the Gain pot so that stage 1's feedback resistance is now 10k, with 90k on the other side of the Gain pot's wiper.  That will make stage 1's gain just over 3x, and stage 2's gain still around 25x, but 3 x 25 is a gain of 75x. 

Not exactly high gain territory.  Okay, let's use a 6k8 parallel resistor instead of 10k.  That will make stage 2's minimum gain around 34x.   Not startling, but again, adding just a bit of gain in stage 1 acts as a multiplier, such that a smidgen of gain in stage 1 easily turns that 34x in stage 2 to 100x and more.

The mod I'm suggesting has a few things to recommend it:
1) You don't have to remove anything.
2) The max gain remains the same. We're just altering how quickly the Gain pot moves you into the desired range.
3) It only involves "tacking" on a resistor to the Gain/Drive pot.
4) It is easily reversible if you feel the need to sell the pedal.

[Steben]

It is one of my favourite circuits, but below 50% the pedal is quite lame indeed. Makes sense. No biggie when using classic manual settings.

[antonis]

A booster on output should settle things up..