trying to model a fuzzface

Discussion in 'The Projects Forum' started by stirling, Nov 29, 2011.

  1. stirling

    Thread Starter Member

    Mar 11, 2010
    52
    2
    Hi - There are many schematics on the web for a fuzzface guitar fuzzbox. I've attached one example. I've attempted to model one with LTSpice (attached) but don't seem to be getting any sort of useful/expected output. My input is close to "middle C" and I expected to get an approx square wave out but it's "not happenin' dude". Could it be because I don't have a model of an AC128 transistor or am I missing something else?

    Thanks
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    You have your battery (V1) polarity inverted. The circuit needs -9V.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Carl,
    Take a look at the transistors. It's a very old schematic, from when PNP transistors were in vogue, and negative was customarily shown towards the top of the schematic.

    I got started using PNP's mainly. I had a hard time changing over to the newer standard of using NPNs and having positive voltages on the upper part of the schematic.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Here, I threw an LTSpice simulation together myself.

    I found a generic germanium spice model on the web somewhere (forget where offhand) and used it in the simulation.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    His simulation works with the models he shows if you change V1 to -9V.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Whoops - yes, it does; I thought his attached simulation was the same as the image.

    I missed putting a ground between my V1/V2; the output from my simulation then changes to something very similar to Stirlings' output with the correct polarity supply.
     
  7. stirling

    Thread Starter Member

    Mar 11, 2010
    52
    2
    Nice one guys. Thanks for your help. I will write 200 times - check once, check twice, then check again.

    Thanks again.
     
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