Switching between PNP Germanium Transistors in the same pedal

[Bluto-61]

Hi,

Thanks for letting me into the group I've been working on and building tube amps for many years but I have shy'd away from pedals due to a lack of understanding and working knowledge of transistors, chips, op amps etc....  I'm going to build my first non-tube pedal doggonnit!

The question I have for this forum is a simple one I'm sure, but oblivious to me.  If I want to switch between 2 different PNP transistors (oc-44 and oc-71) what is the most elegant way of switching from one to the other? in other words, if both PNP's are wired in parallel, how do I turn one off while simultaneously turning the other on without causing a pop or some other unexpected issue?

I know for a tube triode (most similar devise I can think of) you would simply either 1) detach the cathode from ground, 2) detach the signal from the grid or, 3) cut voltage to the anode, in that order of preference due to common quirks that might arise by switching off the anode voltage or cutting the signal mili-voltage.   all other contacts could remain in parallel between the 2 triodes if one connection is switched from one triode to another without terribly negative affects,  the current would stop flowing in one triode and begin flowing in the next.  there may be a "pop" heard by switching anode or grid voltages but that can be corrected by the proper resistor or capacitor to ground.

it seems to me that switching off one base of 2 parallel connected PNP's while switching the other base on would achieve a fairly transparent selection between the oc-44 and oc-71 but I really don't KNOW... ya know?

Any Thoughts?

Bluto-61

[PRR]

Welcome!

Getting ahead of yourself. Build a pedal. Don't complicate. Multiple transistor switching is NOT a common thing.

[iainpunk]

multiple transistor switching is always going to give a pop, its really hard to not do that, since every transistor will bias different with the same resistor and base voltage, so a pop in unavoidable.

but i think that using a 2pdt to switch the base and emitters is the cleanest way of doing it.

cheers, Iain

[willienillie]

If I want to switch between 2 different PNP transistors (oc-44 and oc-71) what is the most elegant way of switching from one to the other?

socket

[R.G.]

You're getting good advice. It's possible to do, but you might want to consider the end objective. If it's just selecting between a number of transistors, use a socket. If it's on-the-fly switching, you will probably need some kind of mute during the switching because its going to pop very loudly. For just the switching, a DPDT switch will work, commons connected to the base and connector connections of the circuit, and the throws connected to the two transistors' base and collector. Wire the emitters together.

Here's a copy of my reply to you from another forum:
===
Electricity flows in circles (circuits...), so if any part has only one pin connected, no electricity flows through it. (note... This is a simplification, but a very useful one.)
To switch between two three-leaded devices, you need to switch at least two of the pins. You can leave the third pins tied solid. I would switch the connections to the base and collector, leaving the emitter connected.

This is probably not a switch you want to flip while it's plugged into a live amplifier. There is likely to be a big ( BIG ) pop as the DC levels re-equalize.
===

[toneman]

There\s  always a passive (or active) mini mixer(??)

[idy]

If you want an A/B for real life use, the answer is build two identical circuits with different transistors and A/B them.

[Mark Hammer]

The perceived difference between two transistors of the same family/class in what is essentially the same circuit is largely a result of how the surrounding passive components set the bias and gain.  It's far easier, and less problematic to simply switch/vary some of those surrounding components than to switch between transistors.

[pinkjimiphoton]

shit, all ya really have to "switch" is the base. i've done it a lot of times. may not be perfect, but it works. have also switched collector and base, as well as all three connections.
have done this to switch between si and ge, yeah, minimal popping on some things, but never that bad. perhaps on  the fly switching live may be an issue, but....


just sayin'

[iainpunk]

shit, all ya really have to "switch" is the base. i've done it a lot of times. may not be perfect, but it works....

only switching the base can work, but it will make both sides sound different than not having the doubled leakage there.
i guess its the least pop-prone, but it will change the sound.

i recommend making the circuit twice and switch the output (after teh DC coupling cap).

cheers

[ThermionicScott]

Welcome, Bluto!

If you want an A/B for real life use, the answer is build two identical circuits with different transistors and A/B them.

I feel like this is the most logical, well-travelled route.  Assuming this is some kind of vintage treble boost or fuzz circuit, your parts count still won't be very high if you just build two whole copies of the circuit and switch between them.  You could even use ganged pots so that your adjustments are the same.

I know for a tube triode (most similar devise I can think of) you would simply either 1) detach the cathode from ground, 2) detach the signal from the grid or, 3) cut voltage to the anode, in that order of preference due to common quirks that might arise by switching off the anode voltage or cutting the signal mili-voltage.   all other contacts could remain in parallel between the 2 triodes if one connection is switched from one triode to another without terribly negative affects,  the current would stop flowing in one triode and begin flowing in the next.  there may be a "pop" heard by switching anode or grid voltages but that can be corrected by the proper resistor or capacitor to ground.

To be frank, I wouldn't do any of those things to a tube, either.  Simply disconnecting the cathode or plate of a warm tube and letting it flap in the breeze isn't good practice, and you could wear it out prematurely.  Cutting the grid connection risks oscillation unless you are careful to maintain a ground reference.

Here again, re-routing the signal between complete circuits is a much better plan than just trying to disable the device itself.  Have some sympathy for these things.