switching between 2 parallel wired PNP Transistors

[Bluto]

Hi,

I've been working on tube amps for a while 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 using PNP Transistors (a basic Rangemaster) and I would like to use one OC-44 and one OC-71 because I love the sound of both and want them in the same pedal if possible.

The question I have is probably a simple one, but is oblivious to me. QUESTION: 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 and connected to their respective Emitters,Bases and Collectors, how do I turn one off while simultaneously turning the other one on without causing noise or some other unexpected issue?

I know that for a tube triode (most similar devise I can think of) you would either 1) detach one cathode from ground, 2) detach one signal from the grid or, 3) cut voltage to one of the anodes... in that order of preference due to common quirks that might arise by switching off the anode voltage or cutting the signal. My experience is that all other connections could remain wired in parallel between the 2 triodes if one connection is switched from one triode to another without creating any problems that could not be addressed using some rightly placed caps or resistors. The current would simply stop flowing in one triode and begin flowing in the next without affecting the function of the operating tube.

Is it the same with PNP or NPN transistors? It seems to me that switching off one base of 2 parallel connected PNP's while switching the other base on should achieve a fairly transparent selection between the oc-44 and oc-71 in the same pedal but I really don't KNOW...

rangemaster schematic.gif

Any Thoughts?

Bluto-61

[pompeiisneaks]

Treat a transistor in this way like a triode.

If you cut signal to the the base that's like cutting signal to the grid. You could just take the signal into a switch that basically outputs to either base.

Another means could be to swap the anode(collector) b+ connection between the two so that one is off and the other is on

and another would be to swap the cathode(emitter) between ground connections. I don't know which is most optimal for noise/current draw/etc. I'm just saying you can do any of those three, and you'd get the result you want, but each may have different pros and cons.

~Phil

[Colossal]

Rather than trying to select between transistors, I would build both (complete) circuits in the same enclosure and just switch the input to either one. Use a 4PDT on/on switch to route the input to and from each. This way, you have each transistor biased correctly and you just switch between the circuit you want. The part count is pretty low.

[roberto]

Instead of switching, I'd treat it like a plexi double input stage.
Give the possibility to blend the signal of the two: you have both, use them!

[R.G.]

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.

[Bluto]

Treat a transistor in this way like a triode.

If you cut signal to the the base that's like cutting signal to the grid. You could just take the signal into a switch that basically outputs to either base.

Another means could be to swap the anode(collector) b+ connection between the two so that one is off and the other is on

and another would be to swap the cathode(emitter) between ground connections. I don't know which is most optimal for noise/current draw/etc. I'm just saying you can do any of those three, and you'd get the result you want, but each may have different pros and cons.

~Phil

Phil, Thank you so much for this confirmation! It's very helpful. I was getting the functions confused because I was seeing drawings of this same basic circuit with the PNP drawn "upside down" and the ground being negative. so, like a tube triode you can take the amplified signal from either the Collector (standard Anode tube connection) or the Emitter (Cathode follower type connection).

I've read different opinions about whether the Transistor sounds the same with negative or positive ground schemes. if it sounds anything like a tube there would be a huge difference no?

Don

[Bluto]

Rather than try to select between transistors, I would build both (complete) circuits in the same enclosure and just switch the input to either one. Use a 4PDT on/on switch to route the input to and from each. This way, you have each transistor biased correctly and you just switch between the circuit you want. The part count is pretty low.

This is a great idea! I considered this and may still go in this general direction. Thanks!

Don

[Bluto]

Instead of switching, I'd treat it like a plexi double input stage.
Give the possibility to blend the signal of the two: you have both, use them!

Now this is something I haven't considered Cool Idea! maybe I should order another OC-44? that's my favorite sound

Don

[Bluto]

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.

This is definitely the consideration that I had with my initial questions and comments. If what pompeiisneaks is saying is correct tho (a transistor working as an anode-grid-cathode like a tube), you should be able to stop the current flow in a PNP fairly silently by ungrounding one Emitter (cathode) and simultaneously grounding the Emitter on the 2nd PNP, no?

I've done this many times with tube triodes and pentodes with good results. However, I have no hands-on experience with these PNP devises other than simply changing out bad transistors on some old SS equipment. Perhaps I need to experiment but I don't want to blow up my PNP's as they are vintage Mullard and GE components and were kind-of pricey

Thanks! Don

[R.G.]

You run into problems with germanium transistors in particular. Germanium junctions leak just about 1000 times as much as silicon, so you have to take leakage into account. That's why you need to switch two out of three of the terminals. Silicon is very much off with the base open, so you might be able to switch only the bases of silicon devices. Germanium will still pollute things by leaking if it has a chance.

[Bluto]

Germanium junctions leak just about 1000 times as much as silicon, so you have to take leakage into account.

OK... This is exactly what I need to know! Thanks for taking the time to educate... it is very much appreciated

Don

[turbofeedus]

Must read/build for anything germanium transistors: http://www.geofex.com/article_folders/ffselect.htm

I'd recommend just building the rangemaster and swapping the OC44 and OC71 manually, see if it's even worth building the swapping circuit.
I sense you might be surprised how similar they will sound.

You run into problems with germanium transistors in particular. Germanium junctions leak just about 1000 times as much as silicon, so you have to take leakage into account. That's why you need to switch two out of three of the terminals. Silicon is very much off with the base open, so you might be able to switch only the bases of silicon devices. Germanium will still pollute things by leaking if it has a chance.

Especially true for and OC44 any OC71 still floating around.

[pompeiisneaks]

Must read/build for anything germanium transistors: http://www.geofex.com/article_folders/ffselect.htm

I'd recommend just building the rangemaster and swapping the OC44 and OC71 manually, see if it's even worth building the swapping circuit.
I sense you might be surprised how similar they will sound.

You run into problems with germanium transistors in particular. Germanium junctions leak just about 1000 times as much as silicon, so you have to take leakage into account. That's why you need to switch two out of three of the terminals. Silicon is very much off with the base open, so you might be able to switch only the bases of silicon devices. Germanium will still pollute things by leaking if it has a chance.

Especially true for and OC44 any OC71 still floating around.

Just so you're aware, geofex.com is written by R G Keen, who is R.G. a few posts back

~Phil

[turbofeedus]

Just so you're aware, geofex.com is written by R G Keen, who is R.G. a few posts back

~Phil

OH didn't read the usernames, small world.
I still use that circuit from time to time, even though I have a DCA75 now.