Home Forums General DIY Pedal Discussion Bias a circuit

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    I’m building a pedal with the Boogie 57 and the PURP.  The build doc for the PURP says to use a 9v battery as the power source.  The Boogie 57 build doc just says add power to the circuit.  It does not specify that it has to be a 9v battery.  I don’t plan on having a battery in the finished product.  Do I need to use the battery to bias the PURP or will it work okay if I just plug the pedal into the wall?

    I had the Boogie 57 plugged into the wall and was able to get Q1, Q2 and Q4 very close to 5v using the trimmers, but with Q3 I couldn’t get it below 8.1v.  Will that be a problem and why do you think that’s happening?  Is it a bad trimmer?



    The only reason to mention a battery in the build document is so that people do not use a depleted battery when biasing. Honestly it should say “Do not ever use a battery”.

    On the Boogie are you sure you are biasing the correct matching pair (trimmer / transistor).

    Also how does it sound?


    Okay. Thanks for clarifying that you should not use a battery when biasing.

    On the Boogie, I am sure I’m biasing the correct matching pair because I can see the voltage change when I rotate the trimmer. It just seems weird that when I turn it as far as it will go, it won’t drop below 8.1 volts even though the other three will.

    It sounds really good. My metal head friend, the guy I’m building it for loves it. I just wonder if it would sound even better if the voltage on Q3 were correct, around 5v.


    Do you think the trimmer is defective since it won’t let me adjust the voltage below 8.1v?


    You can check the trimmer adjusts resistance

    It’s wired as a variable resistor, with no power to the circuit check the resistance between legs 1 and 3 the two legs next to each other, turn it fully CCW then CW and see if the trimmer resistance changes

    You could also try swapping transistors if you’ve socketed them to see if it’s potentially a problem with Q3 transistor

    Just swap any Q that biased ok with Q3 and see if you can get it to bias


    Thanks Billy.  I tested the trimmer and it shows about 5k ohms completely turned CCW and about 40k ohms turned completed CW.  So I guess that would mean the trimmer works correctly?

    Also, I put the circuit in a different enclosure and now the voltage on Q3 won’t go below 6.3v.  I swapped out Q3 with three other transistors and I get the same result.  When I had it in the previous enclosure it sounded pretty good even with Q3 at 8.3v.  I assume it will sound at least as good and maybe even better now that it’s closer to the 5v level the build document says it’s supposed to be.

    I have it with the PURP and a 4pdt Easy Order Switching Board but I haven’t been able to get it to work yet so I don’t know how it sounds with Q3 at 6.3v.


    Hey, I just had a tranny or 2 that wouldn’t come down past 8!!!. I had a bad B1M pot. Once the gain pot was replaces, all was good with the biasing.



    Thanks for posting that Robert. I went back to test the pots and found that one of the leads from the B1M gain pot was not soldered onto the board. I soldered that on and it works now. I tested the pots anyway, just to see, and found that the A100k bass pot was at 50.1k and the gain pot was only .732M, but when I removed them and tested them again they were within spec. So I soldered them back onto the leads and they were back to .723M and 50.1k, but all the rest of the pots are where they should be and Q3 still won’t go below 8.32v. Any ideas why that might be?


    You will probably not get a correct resistance measurement with the component wired into the circuit.  It will be measuring other resistances in series or parallel with the resistance you are measuring.  You must isolate the component before measuring.

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