How to Add a boost mod just by modifying a distrotion's original circuit!

[zencafe]

Hi everybody, this is Douglas, I'm from Chile. I've been a DIY since last year. Well, since I heard that you can add a boost to a distortion pedal just with modifying it's own circuit, I've been wondering how does that work. How could I do it? I've seen this mod in many pedals, such as Sansamps GT-2, tube Screamers and also in tube preamps, such as the Matchless Hot Box.
Well, I don't really know how does this mod work, I guess you could get it with duplicating the buffer phase and changing some values of some components in order to get more gain than it would have originally had. And then connecting those two "buffers" to a switch, so that you could activate the original buffer, or the high gain one.

Am I right or completely clueless? I'd ñlike to know if some of has done it, and how... some support material such as schematics or layouts would also help a lot!

Thanks for your time, and sorry for my English, it may not be perfect, but tried to write as clear as I could. I apologize if this question has been already posted, I searched in the forum and in the FAQ section and didn't find an answer to my question.

[ubersam]

Hey Douglas - check this schematic out to see how one well known Tube Screamer derivative added a boost feature. My personal preference is to use a separate booster, which there are plenty of nice ones to choose from.

[Mark Hammer]

I'm sure he appreciates the effort you made, but my hunch is that is actually the opposite of what Douglas/zencafe asked about.  Or maybe I'm the one who got it wrong.

In any event, what ubersam linked you to provides for a gain boost by selecting between different value feedback resistances, but at the same time it changes the amount of clipping produced.  My sense is that the request was for something that can provide a gain boost, or at lest a boost in volume, without any change in the basic tone.  Sort of like an effect with two switch-selectable output levels: off-distortion-distortion-but-louder, as opposed to off-distortion-even-more-distortion.

The Bradshaw-designed MC-402 Boost/Overdrive from MXR sounds like the sort of design Douglas is looking for.  Accomplishing a more-gain-but-no-tonal-change requires situating the extra gain stage after the clipping takes place, rather than before.  Sticking more gain up front will nail you a harder clip, to be sure, but the output level will be limited by the act of clipping itself. 

I think an excellent way of implementing what you want is to use something like Jack Orman's MOsFet booster on the output stage.  The nice thing about it is that it can very easily substitute for the output transistor buffer on something like the TS-9 clone that ubersam linked to, and be configured for either buffering, or gain.

For me, one of the desirable features of that booster (or any similar FET-based booster where gain is determined by a variable resistance from source to ground) is that one can use a pot in parallel with a fixed resistor to switch back and forth between buffering or clean boost.  In Jack's Booster, the amount of gain depends on how efficiently the 100uf cap (C5) can bypass the 2k7 resistance to ground (R5).  When the resistance presented by R6 is high, then gain is determined primarily by R5.  As the path for AC signal through C5 gets easier and easier, gain goes up.  So, if one had a permanent 22kor even 47k resistor between ground and C5, a simple SPST footswitch could be used to place R6 in parallel and suddenly get a lower-resistance path to ground and voila! more clean gain and a huge volume boost to the tone you've already dialed in.  If you want to be able to know when the extra boost is on and off, then you can use a DPDT stompswitch to engage that pot and also turn on an LED.

[Ben N]

Mark:
I realize the gain on tap is more limited, but couldn't you do the same thing with a JFET?
Ben

[Mark Hammer]

Certainly.

[zencafe]

Wow! Thanks Very much guys! I'll experiment with my Sansamp!!

Greetings

D.