K.O.T.B FET Bias?

[Anonymous]

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

[Anonymous]

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

[Barry]

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.

[Anonymous]

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

[Barry]

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.

[Author]

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