Trying switchable transistors for the first time

[saison94]

So I would like to incorporate a Ge/Si toggle in my next build, which is based on the Os Mutantes fuzz schematic, a super-simple circuit. The original calls for pn2222s, and I was wondering which set of Ge transistors I should pick to compliment these, or what would add the best variation in tone.

I've never done this before, so my two main questions are 1) since I will presumably be using the same three thru-holes in the circuit board for each transistor (wired out to a switch), how do I go about properly compensating for bias differences between the two types? rather than place the biasing resistors directly into the board where they would traditionally go on the collector/emitter traces, do i instead solder the different resistors onto the switch and/or the transistors themselves, completely separate from the board? what's the neatest way to do this? and 2) is it best to switch between just one transistor stage or both? which will give the most variation in sound. from scouring the threads on this subject it seems Q1 sets the gain character and Q2 sets the clipping texture. is there a way to set it up so you could choose between the four possibilities with just one switch, like a rotary?

thanks in advance for any advice!

[phector2004]

i was looking into this earlier today... not sure if this is the best approach, but it makes sense to me to just use trimmers, bias each one within the circuit (on breadboard), then either solder the trimpots or put fixed resistors onto the socket/transistor and have the switch incorporate the entire transistor portion, as much as it can using the least amount of poles. dont know if an SPDT can do it, but a DPDT surely can, as you leave one common lead from the transistor going straight into your circuit board and the circuit will be open in the unused tranny. got a schematic?

i've also seen a 4pdt on smallbear, but it might not be a toggle... could use that to switch both Q1 and Q2 at the same time.

As for switching each one individually, would that require rebiasing for the other transistor in case of a mix? If you've got a breadboard, try it out!

Haven't dealt with rotaries, but I stumbled onto this a while ago, hope it helps:
http://info.ee.surrey.ac.uk/Teaching/Labs/misc/RotarySwitchHowTo.pdf

[dr.benway]

there is some good advice on beavis' audio site but apparently it's gone dark.
How many resistors are we talking about ? Could you just incorporate them into the wire running from your board's transistor footprint to the toggle ?

[phector2004]

of course, how could i forget?!

http://www.beavisaudio.com/projects/FuzzLab/

i know his fuzz lab has switchable transistors... if you check the last picture, you can see he's even mounted them directly to the toggle switches via sockets. not sure how he biases each one though

check out his site, tons of neat stuff!

EDIT -- here's another: http://www.beavisaudio.com/Projects/FuzzLab/FuzzCloneMods_V2.gif

[saison94]

Thanks, that's a really neat way to switch, just still wondering how the diagram compensates for bias differences. is it just that the bc109 and ac127 biased very similarly and do not need resistor variations?

[phector2004]

doubt it. i think they do need rebiasing when switched. look at the bottom of the second link. he actually has a section with an external pot wired to where the bias trimpot should be. also note how he's using a BYOC fuzz face clone circuit board so he doesn't have as much flexibility. If you make your own circuit on perfboard or something of the sort, you can easily integrate 2 sets of biasing resistors on your switch.


can't really help you much more as im a noob with biasing, but good luck!

[saison94]

thanks for pointing that out, i missed it the first time

does anyone recommend a good link or book for learning how to properly bias different types of transistors? how to know where to dial in their sweet spot?