Home Forums GuitarPCB Build Support Plexi Plus Biasing issue (solved)

Viewing 6 posts - 1 through 6 (of 6 total)
  • Author
    Posts
  • #14824
    Chris
    Participant

    Hi, I have built 2 plexi plus circuits with the tone tweaker board.  The second one has a drastically lower output than the first, and when I bias Q1,3&5 to 5v I get almost no volume.  In the build document the q1 voltages are all 0.  When i bias Q1-D to 0, Q2,3&5 all fall in the correct voltages but Q4 is very low (.4ish).  I am wondering if my Q4 could be bad, as I know J201s can be iffy these days.  Any ideas?

    Can add pictures if needed, but the voltages currently are: Q1:0v, Q2:5v, Q3:9.23v, Q4:.43v Q5:4.73v

     

    Thanks – Chris

     

    #14825
    Billy
    Moderator

    It looks to me that Q1, 2 and 5 drain should be set to around 5v

    I haven’t built this circuit but it appears it may be a typo looking at the important bias information

    I’d post pictures anyway

    #14826
    Chris
    Participant

    When I set q1 to 5v, I had NO volume until I was “turned to 11”.
    I think the problem is in the Norm channel, as isolated it doesn’t seem correct.  Q4 is outside the expected values though so I’m still suspicious of that.

    here are some pictures…it’s in a “pedalboard build”.

    #14828
    Billy
    Moderator

    Chris If you haven’t already check the voltages on your working circuit for comparison

    I don’t see anything amiss can’t see R2 and 13

    Q4 drain should be the same as Q3 source looking at the schematic what do you get on Q3 gate

     

     

    #14844
    wilkie1
    Moderator

    Set the voltage for Q1, Q2 and Q5 drains at 5V or as close as you can.  The voltage for Q3 should be close to 9V.  Q4 drain will be the same as Q3 sourcel  Then do an audio test.

    The voltages shown on the build doc list are just samples for the format of how the voltages should be reported.  The instructions to set the voltages of Q1, Q2 and Q5 to 5V are correct.

    #15061
    Chris
    Participant

    Thanks everyone!  I was able to reset the biases and get the pedal working, albeit not at the 5v.  I think the J201 transistors i got were a little out of spec so I had to bias them a bit cold for the desired tone.  @wilkie1 & @billy – thanks for the explanation on the Q3-4 source-drain relationship!

Viewing 6 posts - 1 through 6 (of 6 total)
  • You must be logged in to reply to this topic.