Probably a stupid question- switching transistors?

[Killer Queen]

Hi, I'm wanting to get into the whole DIY stompbox thing, and intending to build the Tonepad Brian May booster as my firist build. I want to build this one in particular because I can use both Silicon and Germanium transistors with it- I was originally intending to buy a bunch of pnp Ge and Se transistors from www.musikding.de , and try them all out by manually swapping them, using a transistor socket.

However, I just figured that one might aswell try and modify the design so you have two (or more) transistors that that you can switch between- a Ge and a Se, hopefully, to give a bit more versatility.

Anyway, does anybody have any idea how I could do this? I think I can see how I could do it using 3 2-way switches, but that seems a bit impractical- is there any simpler way of doing it? Are there any components/switches you can buy especially for switching trannys? IS it possible to just have one switch on the box I can flick to change between Ge and Si?

[Khas Evets]

Use a 3PDT switch.

[toneless]

Switching trans isn't very versatile with a switch. :?
While bypassing transistors you will have lot's of noiz or damage to the unit :?
Maybe using a socket is the way to go!

[Outlaws]

Switching trans isn't very versatile with a switch. :?
While bypassing transistors you will have lot's of noiz or damage to the unit :?
Maybe using a socket is the way to go!

That wouldn't be the case though if it was only switched while off correct?  With a toggle rather than a footswitch.

[Peter Snowberg]

Welcome to the forum.

Switching transistors is indeed a good idea for an option if you ask me.

Different transistors have different internal resistances so along with the transistor, you also need to switch at least one bias resistor.

You may find that after taste testing several, there is really one that you prefer the tone of. Maybe the best is to build multiple devices.

You can use either a DP3T or a rotary switch to change things. You'll have to switch the base, output cap, and either the collector or emitter (depending on the circuit).

[Hal]

yea, with a circuit that simple, you might as well just build a couple of them, and use what would be like an a/b to switch the entire circuit in and out.

[Mark Hammer]

For changing diodes, I would concur with others that it can be easily done with toggle or even stompswitches.

Of course, there is a difference between being ABLE to do it, and whether it is a wise choice to do so on a given board.  After all, you can't always put switches where they need to be to keep paths short and noise low.  In those instances, use of a FET to switch things where they are can be a smart approach to pursue.  

There is a scan of an old article from POLYPHONY (the magazine that morphed into Electronic Musician in 1984 or so) from a former DOD engineer on my website (http://hammer.ampage.org) that shows and explains the FET switching circuit that became standard on their pedals.  The author explains choices of switching FETs and how to implement more complex switching than is shown in the schematic.  Well worth a read.  FWIW, DOD uses/used J113 FETs because of their very low "on" resistance.

[Paul Perry (Frostwave)]

Remember you only have to switch TWO of the legs (the other one can remain connected at all times).

[zachary vex]

Paul's right... you can just use a DPDT to switch two of the leads and leave one from each transistor permanently connected.  that should make the setup much easier.

[col]

The Brian May booster uses a BC182L transistor. Be very careful as the pin-outs are different to most other transistors and they can't be simply rotated.

[Killer Queen]

Ah, ok, I see where you're all coming from now- seems simple enough to use an xPxT switch to change between them. I'll add in a toggle on/off to cut out any noise/popping/whatever it might make.  (Is there any particular way I should do this?)

Come to think of it, I might use a rotary switch, to give me more flexibility to add more stuff later. Not that I'm really going to need more than 2 transistors to switch between, but it would be interesting to do.

Anyhow, thanks guys- i have a vague-ish sort of idea what I'm doing now.

[zachary vex]

just bypass the effect while switching transistors.  that will eliminate the most serious "pop".  there might be a quieter "pop" when you turn on the pedal as the bias settles down, but no more than when you first turn on your pedal.

[Killer Queen]

One last question- will I need to change the bias individually for each transistor I use?

The schematic has one transistor labeled as a "Rb1", and one as "Re". I'm guessing this has something to do with the biasing? (Resistor base, and Resistor emitter perhaps?)
Is it likely I'll need to chnage the value of these for different transistors (especially concidering I'll be using both ge and si?)
If so, I think I'll use a 4P3T rotary from Mouser to Change 2 legs of the transistor, and to switch both biasing resistors. (I'll bias each transistors' resistors individually, probabbly with trimpots.)

[Paul Perry (Frostwave)]

By the time you start to change bias arrangements as well as transistors, you might as well build 2 (or more!!) separate fuzz units on one board& either switch between the two, or have two blend controls & switch between the outputs of those!!
because, the cost of the resistors & caps is nothing compared to multipole switches & Ge transistors.

[Killer Queen]

Thats a point. It would have to be multible PCBs as I'm ordering them from tonepad etc for now, but theyre only a few quit each, which might come all up to less than all the extra switches etc. I like the idea of multiple blendable circuits- I could play with the input caps and perhaps have one more of a full-boost than trble boost. How would I wire it up though? If I were to have, say, 3 seperate (treble)boosters in the enclosure, how would I connect them all and blend them?