Chop Shop with MMBFJ201

[psb962]

I went the SMD + TO92 board route with this one as I read that J201 idss is critical for getting the FETs biased. I ordered 10 MMBFJ201 from Tayda at 42 cents each and 9 TO92 boards from Osh at $1.40 total (including shipping !!).

Soldering SMD was a new experience so it took a while but after nearly 2 hours I had 8 FETs mounted and leads added. I tested them using my new electronics tester (one of these):



It identified all the FETS as FETS (two needed remedial soldering) but idss range was 99uA-300uA, which was no good for the Chop Shop. I figured that the problem might be the tester so I built the simple test circuit (above) where you measure voltage across 100R resistor then multiply it by 100 to get idss in amps (for J201 you can also use 1k and multiply by 1k). Note that with that circuit if you only want idss then you can leave out the larger resistor and the switch and just have the drain and gate connected to the 100R. Measuring voltages across the 100R and doing the 100x math gave me a bunch in 600-700uA range and a few under 600uA. That allowed me to select a 650uA one for Q1 and a 550uA one for Q2.

Aside from all that the pedal was an easy build and the bias came in at 6V no problem. Here are the results.



I played it into a plexi-like amp last night; the sag worked well throughout the range of the knob and delivered an effect similar to old recordings of guitar from the 60s where you can hear the amps buckling under the load - nice! The drive is ok, nothing too special there, but I expect it would sound better into a clean Fender or similar.

I hooked it up to an 18V supply to see if that made a difference and it did - sag did nothing until almost full on, and the drive seemed weaker also. As you might expect, I guess. I will use it at 9V.

 

[CharlieTuna]

Very nice! Thanks for the write-up. Have you purchased a more accurate tester since doing the build? I haven't built my first pedal yet but it sounds testing components beforehand is important versus just trusting what comes in a build kit.

 

[psb962]

Very nice! Thanks for the write-up. Have you purchased a more accurate tester since doing the build? I haven't built my first pedal yet but it sounds testing components beforehand is important versus just trusting what comes in a build kit.

I think the low idss numbers might be due to the tester.using only 4v but I'm not sure about that.

 

[psb962]

I've noticed the pedal sometimes produces a faint hiss and occasionally a pop or two. I tested the connections and pots, and noticed that Sag is noisy. This is probably because it is a voltage divider, and might be the source of the noise. Would be interested to know if others hear that intermittent hiss/pop too or if its just my build.

 

[Lorok]

I don't understand the need to select j201s based on idss value seeing as how you are using the trimmers to bias the jfets. Selection is critical when the biasing resistors are fixed (like in fairfield's production run), but not when you're biasing with trimmers.

Let me know if I'm wrong

 

[airbud]

I don't understand the need to select j201s based on idss value seeing as how you are using the trimmers to bias the jfets. Selection is critical when the biasing resistors are fixed (like in fairfield's production run), but not when you're biasing with trimmers.

Let me know if I'm wrong

this was my understanding as well, but i never measure components and bias by ear. feels more "craft" to me (coming from a craft beer brewer). definitely not what one "should" do, but learning about the history of pedals has taught me that it's ok if part values fluctuate based on availability. again it's not "ideal."

 

[psb962]

I don't understand the need to select j201s based on idss value seeing as how you are using the trimmers to bias the jfets. Selection is critical when the biasing resistors are fixed (like in fairfield's production run), but not when you're biasing with trimmers.

Let me know if I'm wrong

This thread has all the good info on that.

 

[Brett]

Selection is critical when the biasing resistors are fixed (like in fairfield's production run), but not when you're biasing with trimmers.

Let me know if I'm wrong

Certain circuits might be okay with a wide range of Idss, like a boost pedal or something of that sort, but many circuits rely on JFETs being within a range of Idss (and/or Vp) in order to bias or perform correctly, even with trimmers. JFETs are notoriously inconsistent and benefit from being evaluated prior to being plugged into a circuit. Two JFETs, even from the same batch at the factory, can have wildly different characteristics. SMD JFETs seem to be slightly more consistent, but each individual transistor should be tested prior to use.

 

[Lorok]

I guess this is turning into a philosophical question. For completeness, here is what the author of this circuit had to say in an fsb thread:

right, it's easier to tune the resistors to the JFET. Just pick a J201 and try different resistors in the 7.5k to 12kohm range. You should be able to get somewhere between 6-7 V. No need to be more precise, it'll sound good.

There's no real right way to build this, I chose a set of components to get a certain repeatability.

Back to @psb962 's question about the hiss/random popping... Have you tried jiggling the wires & components with a pencil while connected to guitar & amp to reproduce the popping? Might be a bad connection somewhere.

 

[psb962]

I guess this is turning into a philosophical question. For completeness, here is what the author of this circuit had to say in an fsb thread:



Back to @psb962 's question about the hiss/random popping... Have you tried jiggling the wires & components with a pencil while connected to guitar & amp to reproduce the popping? Might be a bad connection somewhere.

I think it's interference from my internet which runs through the power wires.