Home Forums GuitarPCB Build Support K.O.T.B FET Bias?

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  • #16696
    Dan
    Participant

    Hey all,

    Just wondering if someone could give me approximate bias measurements for the FETs in a K.O.T.B? (v3, r2)

    Had put 2N547s in mine, (socketed thankfully,) which I purchased from Pedal Parts Australia, so they hopefully should be right for the pedal. Bias voltages seemed very high, over 7v and up to more than the input (power supply) voltage at around 9.7v on the highest. Pedal sounded very harsh.

    Have swapped them out for some 2N5484s which sound really quite nice in it but the voltages seem a bit low, from around 2.8v, up to something more like what I had thought to expect at around 4.6v on the highest

    Just hoping someone might be able to tell me what I should be aiming for in terms of bias voltages before I go messing around with it any further.

    Cheers,

    Dan

    #16697
    Dan
    Participant

    Sorry, just looked closer at my board and apparently it’s just a v3, not v3 r2

    Bias readings are as follows, seems I misremembered them somewhat, have just checked them again:

    With 2N5457

    Q1: 7.85v

    Q2: 7.87v

    Q3: 8.18v

    Q4: 9.19v

    Q5: 7.95v

    Q6: 7.66v

    With 2N5484

    Q1: 3.14v

    Q2: 2.88v

    Q3: 4.67v

    Q4: 9.19v

    Q5: 3.95v

    Q6: 3.17v

    Any help on what the bias readings should be for this pedal would be greatly appreciated

    Cheers again,

    Dan

    #16698
    Barry
    Keymaster

    Please provide a clear picture of the Board top so that we can see the resistors and the associated color bands.

    Changes for v3r2 were purely cosmetic and minor.

    Also can you post a clear photo of the 2N5457 face just so we can see the logo imprint.

    Your voltage readings should be half the supply voltage however the readings on the 2nd set of transistors are in the right range of functionality for that particular transistor which is why they sound good. There are many reasons why the bias could be off which we would have to explore.

    #16703
    Dan
    Participant

    Here is a shot of the board. I test all the components I use with a component tester to ensure all are as close to spec as possible. A bit over zealous maybe but just the way I roll lol

    I have 2N5457s from a couple of different sources, don’t know which this is but they all look like this. All of them read in the same range according to my component tester regardless

    Cheers,

    Dan

    #16704
    Barry
    Keymaster

    I looked over the important resistors which seem to be fine so if it were me I would load the board up with all 2N5484 transistors except for the Q1 and then Swap out from your lot of 2N2457 transistors till you find one between 3 to 6.8 volt range.

    Once you have that then do the same thing again with Q2, Q3 and Q5.

    Then go ahead and replace Q2 and Q5.

    Reason is that in these days of Obsolete transistors some are going to be out of spec or to the furthest degree of it. Another reason to always socket.

    Or honestly just use the 2N5484 transistors since they are nearly identical and are biasing well for you.

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