Home Forums GuitarPCB Build Support Biassing PUFF

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  • #1747
    Anonymous
    Inactive

    Hi All,

    Yesterday I assembled the PUFF. I tried setting the bias voltage on the collector of Q2 following the method described in the documentation. It says to set it around 4.5 – 4,8 V. In my case I don’t get it lower than 5.9 volts.

    Is this a problem and if so, what can/must I do to bias it proberly.

    With regards,

    Auke

    #1752
    Barry
    Keymaster

    If it sounds good at 5.9v that is fine but if you want to further adjust it you will likely have to Mod it since no transistor is equal and some can be really far off now days since they have been majorly cherry picked in the last 15 years leaving a lot more “off” ones than ones that are within a tight spec. (this is also covered in the doc since this is really the case) That said if it sounds great I would not worry. If you want to proceed with modding to get the bias a little lower it will require removing a resistor if the trimmer does not have enough swing to counter the variance in the transistor being used.

    We will need photos and voltage readings throughout if you wish to pursue any mods so we can be sure the problem is not tied to anything else because a 20k (trimmer) provides a great deal of variance. That said make sure it is a 20k or Code 203 and you might want to try another transistor if you have one and hopefully it is socketed for ease of testing.

    Here is the build doc link for reference: https://guitarpcb.webs.com/PDF%20Files/PUFF_v2.1.pdf

    #1758
    Anonymous
    Inactive

    Hi Barry,

    Thank you so much for your reply.

    I would like to pursuit this further. I bought the kit from Musikding and it came with all the parts. Setting the trimpot at it’s lowest setting results in the voltage mentioned. Could you please inform me witch voltages I would have to take?

    With regards,

    Auke

    #1760
    Barry
    Keymaster

    Hi we would need input voltage from supply tested. We need all three leads of the transistor. Then all 8 pins of the IC. Be sure to label the IC in it’s proper order as well as the named leads for each transistor leg so we know exactly what is going on. Google the datasheet for the transistor so you know the pin out names and supply us with the voltages for each.

    Please see our Guides page if you have any questions. It can be accessed easily from the Home Page by hitting the Guides button or it is also here in the forum. The Crash Course explains voltages. Voltages take a lot of guess work out of the equation.

    Please supply photos as well showing the build.

    Finally a description of how it currently sounds and what transistor type was used would also be helpful.

    #1761
    Anonymous
    Inactive

    Hi Barry,

    Thanks again for the fast response.

    I will take the measurements etc. like you said. I look into the Guides as well. Hopefully I will have everything by tomorrow evening.

    With regards,

    Auke

    #1766
    Anonymous
    Inactive

    Hi Barry,

    Turns out there was a little trace of solder connecting something that wasn’t suppose to connect I’m embarrassed to say….

    All’s ok now.

    It sound good, perhaps a little bit on the dark side.

    With regards,

    Auke

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