BSIAB2 - how to physically switch bewteen FET pairs?

[kevinabb]

Can't find this topic covered anywhere, so I'll ask:

Re: BSIAB2 (link to circuit diagram provided below): What is the best way(s) to incorporate an external physical switch (stomp, toggle, pushbutton, whatever) to swap out the Q3/Q4 FET pair that make up gain stage 2?

Goal:  Modded BSIAB2, with physical switch to choose between a pair of J201 or pair of 2N5457 for Q3+Q4.  Instead of building two pedals, I want to try cramming both options into one! But my analog design chops are waaaay rusty , so here are the questions:

Q1: If all else stays constant (i.e. cap values, biasing, etc.) where should my switch point(s) be?  How many switch points are required/recommended?  I'd love a nice simple approach.  But I found a 6PDT switch for $5.00, so I could literally switch all legs of each FET if needed.  (See Q2.)

Q2:  Will doing a complete switch-out (the 6PDT switch) provide any benefit over a more-modest approach?  Specifically, will one approach be quieter than another?  And will any approach be quiet enough to allow mid-song switching?  I don't need that luxury, but I'd gladly deal with a little extra hassle to get it.

Thanks for any help. Once the monster is done, I'll let you all know how it works out.

- Kevin

BSIAB2 CIRCUIT DIAGRAM:  http://www.generalguitargadgets.com/pdf/ggg_bsiab2_sc.pdf

[Mark Hammer]

I'm unclear as to why you need to switch out the entire front end and not just select between two preset Drive/Volume values.

I'm not saying that choice of FETs in the front end will not make a difference.  But if the amount of drive between stages has an equal, if not greater, impact on resulting tone, why go to the trouble of swapping out the entire front end?

[flo]

Q3 & Q4 form a minibooster. So you could make two miniboosters each one with the FETs you want to use or other changes to the miniboosters.
You can then select which minibooster you want to use.

There are two switch points in the schematic:
- Point 1 - minibooster input (just after the "drive" control): Gate of Q3, R19 wiper, C6
- Point 2 - minibooster output (after C12): C12, R14, Gate of Q5.
So you need a DoublePointDoubleThrow => DPDT switch.

Connect the DPDT as follows:
1A minibooster 1 In
1B Point 1
1C minibooster 2 In
2A minibooster 1 Out
2B Point 2
2C minibooster 2 Out

[Bucksears]

Given the numerous tones that I have found to be had in the BSIAB II (not counting using lower-gain-than-stock transistors), I have found it almost a necessity to have a separate (yet independent) gain2 control.
I wired up a 3PDT footswitch to lugs 3 and 2 so I can alternate between different gain levels.
Trying to find parts on the board to insert/remove on the fly was just too much work for me, plus it's adjustable with a 'gain2'.

[kevinabb]

I'm unclear as to why you need to switch out the entire front end and not just select between two preset Drive/Volume values.

I'm not saying that choice of FETs in the front end will not make a difference.  But if the amount of drive between stages has an equal, if not greater, impact on resulting tone, why go to the trouble of swapping out the entire front end?

Very fair question, Mark.  For those seeking mainly the Brown Sound (i.e. gain levels of high or higher) your solution - two preset drive values - is the way to go.  I started down that road but found that it didn't work out for low(er) gain sounds. It lets me choose "heavy", "heavier" or "heaviest", but nothing lower than "heavy".

I think the BSIAB2 has monster potential for lower-gain Marshall tones, i.e. Dickey Betts or Duane Allman. I love what it does as DRIVE approaches zero. But it is not optimized to easily dial in those tones; even at VOL=max. the output level is weak when DRIVE is lower than what I call "heavy". A different DRIVE pot value helps but doesn't get the job completely done.  Some good folks at The Gear Page (FR8_TRANE, Bucksears, others) have clued me in to the trick of replacing Q3&Q4 with lower-gain FETs, specifically another pair of 2N5457. The goal is a little more gain with a little less clipping. But that swap does reduce available max. gain, which takes away from ultimate EVH potential (and, more important to me, Eric Johnson potential) of the pedal.

I love the BSIAB2 circuit so much, I want to broaden it to deliver almost any overdriven Marshall tone.  But I still want to have an unadulterated BSIAB2 in there, available at the flick of a switch.  I will eventually incorporate your suggestion of multiple selectable settings, maybe four or five of them, into an oversized stomp box.  But at least two of the "go-to" settings I want just aren't readily accessible with the J201s in the circuit. 

BTW, I am also working on a combined pre/post EQ mod that optimizes the EJ/violin and EC/woman tones.  The circuit behaves fantastically with highs rolled off pre-gain, then mixed back in post-gain.  I want to come up with a one-button "womanizer" switch that will instantly EQ it from EVH to EJ, or from AC/DC to Cream.  So my questions about transistor-switching are part of what might eventually be a large-scale implementation of the circuit. Switchable FETs, multiple preset stomps, "womanizer" switch ... the whole deal. 

flo - thanks for the great suggestion. I think you've nailed what I was looking for. I knew that it could be done, but I long ago forgot how best to do it.  (Can you say, "Project Manager"?)    Your suggestion minimizes the switch wiring, and allows me to play a bit with R & C values to get even better differentiation between the two "miniboosters", as you label them. Also makes for easy swap-in/out if I want to experiment with even other "minibooster" modules.  Very cool idea - this is why I ask for help!  (And many thanks to Ed Guidry and everyone else who created and tweaked out the BSIAB2 to begin with!!)

Bucksears - we meet again!      Yes, your GAIN 2 idea turned out to be well-advised. I cannot find any single pot that will do what I want re: GAIN levels.  Ed Guidry, and Matt at CMATmods, each had some excellent suggestions, one of which my buddy is putting in his BSIAB2.  But I need more!! So not only switchable transistors, but also multiple gain pots.  Heck, it's a three-stage gain circuit and each stage affects the signal in a unique fashion. Why not harness some more of that? Adding two extra transistors, a switch, and an extra pot or two doesn't cost that much. About $15 extra, maybe another buck or two for a larger box. But man, each item seems to exponentially expand the device's usefulness.  If I was marketing these I'd have to charge a big premium, but I'm just building one for me and one for my buddy.  Why not go DeLuxe?   

[flo]

AMZ mini-booster from Jack Orman:
http://www.muzique.com/amz/mini.htm

[kevinabb]

AMZ mini-booster from Jack Orman:
http://www.muzique.com/amz/mini.htm

Got it. Always helps to go back to the roots!