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Wren & Cuff - Phat Phuk B internal bias control?

Discussion in 'Effects [BG]' started by Grahams Groove, Oct 27, 2017.


  1. Grahams Groove

    Grahams Groove If it feels heavy, it's heavy. Supporting Member

    Apr 13, 2001
    Boulder, CO
    I've done some searching, but aside from brief mentions there is no real explanation of the internal (bias?) trim pot on the PPB.

    I just got one used and did not get a manual with it, nor can i find a pdf verison. I'm wondering if anyone can give me insight into how this internal trim pot functions?

    In the couple days I've had the PPB, I tested the internal pot on the extreme ends. Fully counter clockwise it seems like the signal going into the pedal is much hotter. I can have the external boost knob around 9:00 or so and it seems to be at unity gain. Likewise, a little digging in gives me some serious grit and breakup, especially on the lower notes.

    Conversely, turning the internal pot all the way clockwise seems to really choke the signal coming into the pedal. When i do this the tone seems to warm up and round out a bit more, though I have to put the external volume knob past 12:00 to find unity gain.

    This seemed counter-intuitive, as I would expect more output and breakup as I turn the internal trim pot 360-degrees clockwise.

    I'm hoping to explore the middle range of that trim pot if I have time tonight, but I'd be curious if anyone can explain what's actually happening and how this internal adjustment is meant to affect the tone and output of the PPB?
     
  2. gregmon79

    gregmon79 I did it for the muff... Supporting Member

    Dec 20, 2012
    Chicago IL
    It's almost like a gain control if you will. Its like you said, its hotter or colder one way or the other.
     
  3. gregmon79

    gregmon79 I did it for the muff... Supporting Member

    Dec 20, 2012
    Chicago IL
  4. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    AFAIK the 2K bias trimmer just biases the germanium transistor. as far as the CW/CCW rotation, since it's internal it doesn't really apply to which direction you turn it to have more or less 'gain'. sometimes it's just the design of the PCB that dictates the rotation.
     
  5. Grahams Groove

    Grahams Groove If it feels heavy, it's heavy. Supporting Member

    Apr 13, 2001
    Boulder, CO
    I haven't seen him around on TB, but I sent an email today. Just figured I'd ask for user insight as well, and see how much technical explanation I get from Matt.

    So I've read a bit about bias control for fuzz pedals, but I'm not sure that I followed. Any chance you could give me a slightly more layman's description of what's happening when you adjust/bias a transistor? Particularly a GE transistor?
     
  6. gregmon79

    gregmon79 I did it for the muff... Supporting Member

    Dec 20, 2012
    Chicago IL
    IME bias controls usually do stuff to the voltage stream of the pedal. So maybe it's either pumping it full of juice or sucking some away depending.
     
  7. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    all it does is adjust the voltage going to the emitter of the transistor to make operate at it's 'best' range of output.
     
    Grahams Groove likes this.
  8. TerribleLiar23

    TerribleLiar23 GNSRZRSKNVS Supporting Member

    Oct 11, 2011
    RTP
    It’s essentially starving the voltage, or “sagging”. Can sound gated, although it’s not.
     
    Grahams Groove likes this.
  9. Grahams Groove

    Grahams Groove If it feels heavy, it's heavy. Supporting Member

    Apr 13, 2001
    Boulder, CO
    Thanks guys. This is all completely in line with the email response I got from Matt today.

    So regarding the SAG or voltage adjustments, will each individual Germainium diode be unique in terms of where you set the bias knob to make it sound "best"? I guess I'm not familiar enough with this type of circuit to know if there's a more objective sweet spot or if it's totally based on your personal total preference.
     
  10. [EDIT: Got carried away listening to PPB demos and forgot to hit "post" earlier.

    Yeah, they're not very consistent, hence the need for a trimmer — so your friends PPB may have its trimmer at 2 o'clock, while yours sounds better at 9 o'clock

    The germanium in this instance refers to a transistor, not a diode. There is a diode in the circuit, but it's not a clipping diode. The first transistor in the circuit is not a Ge, just the second one that you're biasing.]







    Think of it like a carburetor adjustment on your car's engine. Higher altitude has less air density, therefore less fuel is needed — so you adjust the carb to run leaner. You can adjust the carb by ear so that the engine's not hesitating nor stalling nor racing at idle, just a smooth loping thumpety-thump. Both the adjustment screw and the accelerator pedal adjust how much gas is flowing through the carb, but the screw and pedal are not interchangeable.

    With the pedal, the volume knob is the gas pedal, the internal trimmer adjusts whether the pedal's engine runs lean or rich — adjust the carb trimmer, to suit your performance needs/preferences. May take a bit of fiddling to find your pedal's sweet spot, but just trust your ears.

    If you want to play mechanic and throw a diagnostics system on your pedal's engine then you'll need a digital multimeter (DMM).
    Use it to measure between ground and the collector of the Ge transistor. The collector voltage should be somewhere about half the input voltage, red test lead on the collector, black on ground. So I'm guessing that when your trimmer's fully CCW your DDM will read somewhere close to 6v ("hotter"), and trimmer full CW will be closer to 4.5v ("choked").


    Personally, I'd just adjust the volume to where I'd usually want it (some extra room to turn it up more), and then adjust the trimmer by ear to what my ear perceives to be the sweet spot.
     
    Last edited: Oct 27, 2017
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