Transistor selector switch - possible with SPDT-switch?

Discussion in 'Burnt Fingers DIY Effects' started by parademe, Sep 28, 2017.

  1. parademe

    parademe Tele-Meister

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    Hi to everyone.

    I have a COT 50 clone that sounds great but also different with either a russian MP38 or a Texas Instruments 2n1304 transistor. So now I want to install a switch to dial in either of the transistors.
    I know that a 3PDT switch would be the smartest way to go. But the enclosure (Hammond 1590G) is calling for a SPDT switch.
    So here is the question: is connecting the emitters and collectors of both transistors to the board and either base to separate lugs of the switch and then back to the board a good idea? Or better: is that the way to go for engaging one transistor and bypassing the other?
    Or is it - for technical reasons I didn't consider - better to connect the emitters or collectors to the switch while the other two terminals go directly to the board?

    Thanks for your help!

    And in case it works - I'm going to call it the "Cold War 50". I mean c'mon, a Russian vs. an American NOS transistor?!
     
  2. matmosphere

    matmosphere Tele-Holic

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    Pretty sure that wouldn't work. If I understand transistors correctly the emitters and collectors were together whatever was active on the switch at that point would also transfer voltage to the other transistors. Things would get weird.

    Have you seen the lower profile 3dpt switches some vendors sell? Might work in a g size enclosure. Or you could use a submini toggle switch.
     
  3. tubejockey

    tubejockey Tele-Meister

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    That is such a simple circuit, why not build two COT 50's in the same enclosure, and use your switch to toggle between the outputs?
     
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  4. Old Tele man

    Old Tele man Friend of Leo's

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    IF you have the voltage available, that SPDT switch COULD drive the higher voltage to one transistor while simultaneously REMOVING it from the other: transistors, just like tubes, can be driven into "cutoff (sleep)" by swamping the grid bias down below its normal operating level.

    Exactly HOW it would be implemented would depend upon the circuit and transistors you're playing with.
     
  5. dsutton24

    dsutton24 Doctor of Teleocity Ad Free Member

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    Just at a glance, it think that switching transistors as you propose would make all sorts of unpleasant noises as it's being switched.

    I think my colleague tubejockey has the best suggestion, build two of 'em.
     
  6. reddesert

    reddesert Tele-Meister

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    In principle you could make this work by using a DPDT mini toggle switch (about same size as an SPDT mini toggle) and switching two terminals of the transistors, like collector and base (or base and emitter, etc). The third terminal, eg the emitters would both be connected all the time, but since the other two terminals would be switched, the unused transistor would have only the emitter attached and would essentially do nothing.

    In practice, I agree that this has the potential to make loud popping noises (the emitters might be the safest to switch since they have a resistive path to ground) and it might be better to build a dual effect.
     
  7. NotAnotherHobby

    NotAnotherHobby Tele-Afflicted

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    I would be concerned that the mechanical noise the switch makes would possibly fry some junctions. That "pop" those switches make is essentially an inductive spike.

    If you wanted to do transistor switching, I'd use JFETS as switches, and hook those to a manually triggered flip-flop circuit as you might see in a non-true-bypass pedal.
     
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  8. parademe

    parademe Tele-Meister

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    Been a while since I asked, work kind of forced me to put the project on hold. But thank you guys for your answers!

    I also considered building two separate circuits - but the point was to have both options in one 1590G enclosure. I tried, but I couldn't make two boards fit into that tiny housing.

    So eventually I went with a 3pdt switch: two rows were used for switching between the transistors. So I can switch base and collector of either transistor, each emitter stays connected to the board. The third row was used to switch colors of a 3-pin-LED.
    Yes, it "pops". Not much or very loud, but it makes a noise. Since I won't switch while playing it doesn't hurt me. It might hurt the transistors, though, as you guys said.

    Again, thanks a lot for your input!
     
  9. parademe

    parademe Tele-Meister

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    Update - I finished the pedal some time ago but gave it away for some test driving. So finally here are some pictures (doesn't really look a lot like a COT50, does it?) :

    DA 1.JPG

    DA 2.JPG

    USA is blue, USSR is red... kinda self explanatory.

    DA 3.JPG

    And that's what it looks like under the hood:

    P1060429.JPG

    Top left the voltage regulator (9 --> 6 Volts), next to it the main circuit, the red switch for choosing transistors plus LED positive leads and bottom left the transistors.
     
    jddub440 and zippofan like this.
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