Home GuitarPCB Forum GuitarPCB Build Support Bass OD v2 strange potentiometers behaviour?

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  • #25202
    JPLesage
    Participant

    Hi!

    I assembled a Bass OD v2. It works, except for the potentiometers having a strange behaviour.

    When the Drive knob is at 0, there’s no output. Odd, but I can live with that. I expected the input to go through with no/minimal gain (like most other overdrives I have). As soon as the Drive knob is turned the output goes through, but there is no fine-grained control; there’s always too much gain. I have to use the volume pot on the bass to have some sort of control.

    The volume pot doesn’t act like a volume pot. When the volume is at 0, output is still going through. Turning the knob have minimal effect on the overall volume and I cannot adjust the output volume according to the gain and it’s always too loud.

    I changed both potentiometers for known working ones, rechecked all solder joints, triple-checked the bias on the transistors (all 3 are between 5.3V and 5.7V), and made sure there are no shorts, but the same behaviour is still there. I’m pretty sure it’s not supposed to behave like that, and I’m not sure what else to check. Any help would be greatly appreciated.

    Thanks!

    #25203
    JPLesage
    Participant

    I did some digging on the diagram in the documentation, and pin 3 of the volume potentiometer goes through R11 before reaching ground. This is why the volume knob didn’t do much. Replacing R11 with a jumper solved that problem.

    Now that the volume works as it should there is a tad more control on the gain, but it is still generally quite high. I suppose it could be tweaked by adjusting the biases to lower voltages.

    #25206
    Barry
    Keymaster

    To be clear R11 is there to prevent attenuation. Yes it can be adjusted down to a jumper.

    I am guessing that you are testing with an Active Bass.

    I only tested this circuit with a Passive Bass and it works well and cleans up nicely with the guitar Volume knob.

    I will update the build documentation to make a suggestion for Active pickup guitars.

    Yes you can lower the voltage to clean things up more or even better add a Buff N Blend circuit to it for the best of both worlds.

    #25208
    JPLesage
    Participant

    I tested with my passive P-Bass, and with the amount of gain there is I didn’t want to risk plugin my active bass in until I could control the volume more accurately (I usually keep the instrument’s volume pot maxed out). After I replaced R11 with a jumper, I did play around with both the gain and volume knobs, as well as the instrument’s volume knob and I quite like the tones I can get with it.

    I’ll play around with the voltages and see what I can achieve. Adding a Buff N Blend is a good idea! Hopefully they’ll be back in stock soon.

    Thanks!

    #25216
    mybud
    Moderator

    Please double-check your pot orientation. Normally the leftmost lug goes to the square pad, so that the pots hang in space, not over the board. You could test this by turning the pots in the opposite direction and hearing if they interact as intended.

    #25217
    mybud
    Moderator

    Sorry, I think you’re quite correct. I struggle to visualise how this might work if flipped over.

    #25218
    JPLesage
    Participant

    The documentation’s drill template shows the pots to be over the board, so I installed them like that. They do react correctly installed as they are though: more volume/gain when turning clockwise.

    Thanks!

    #25237
    mybud
    Moderator

    Ok, sorry about the confusion on my part. It’s not a circuit I know particularly well, but maybe check the usual suspects (components, offboard wiring, and so on). If those all check out, then maybe try providing some voltages. I’m not convinced the circuit should behave like that but a bit stumped to help you further at this point. The voltages may assist other participants to chime in.

    #25240
    JPLesage
    Participant

    I played around with the transistor voltages. I get a more manageable overdrive sound at ±3V. When the Drive knob is maxed out it feels like a fuzz, but the clean-up is much better from the bass’ volume pot. I can get a wider range of tones, from a very mellow “tube-like” overdrive to a really heavy, almost fuzz-like distortion. I tried with my J-Bass as well, and the lower-output pickups don’t make that much of a difference. Overall, I think it’s great.

    I’m thinking of changing the trimpots for actual potentiometers so I can adjust the voltages on-the-fly. I’m not entirely convinced it’s a good idea, but I’ll most likely get another PCB to experiment separately with.

    Thanks!

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