Madbean Fatpants 2012 biasing problem...

[CheapChip]

Hi everyone!
I just finished my build of the Madbean Fatpants 2012 vero layout and I can't bias Q2 correctly:
though I have a 18v voltage between the trimmer (set to zero) and the 3k3 resistor, I only have a ~3.8v between the resistor and Q2 drain.
I didn't have any 3k3 resistor and put a 3k instead but I don't think it is a problem..?

Any idea?

[induction]

 

CheapChip wrote

though I have a 18v voltage between the trimmer (set to zero) and the 3k3 resistor, I only have a ~3.8v between the resistor and Q2 drain.

Have you tried  increasing the trimmer setting? You shouldn't have 18V at the node between the trimmer and the 3k3. You should have ~18V at the node between Q1 drain and the timmer wiper. Some of that voltage should drop across the trimmer. What is the full range of voltages you get on the Q2 drain for both extremes of trimmer settings?

Also, I assume you're measuring these voltages relative to ground and not simply across the components themselves, right? The way you phrased it is a little ambiguous, so I want to make sure I understand exactly what you measured.

Finally, are you using J201's? Do you have any extras that you can swap into the circuit and see if they bias correctly? There are many reports of fakes being sold on the internet, and J201's have extremely wide tolerances even when they're real.

[CheapChip]

Thanks for your answer,
and sorry for not being clear...

I saw I wrote an error reading your answer: it's when the trimmer is maxed and not set to zero that I can read ~3.8v on my DMM...
About my j201, I already had a doubt about their authenticity and they are all from the same source.
I'm planning to test some MMBFJ201 to see if it makes any difference.
What would be a good equivalent tranny for this circuit?
What about MPF102?

Thanks again for your help!

[induction]

CheapChip wrote

I saw I wrote an error reading your answer: it's when the trimmer is maxed and not set to zero that I can read ~3.8v on my DMM...

And what do you get when the trimmer is set to zero?

About my j201, I already had a doubt about their authenticity and they are all from the same source.

Don't discount all of them just because one isn't working out. Like I said, the tolerances on J201's are huge. If one doesn't bias correctly, try another one.

I'm planning to test some MMBFJ201 to see if it makes any difference.
What would be a good equivalent tranny for this circuit?
What about MPF102?

As far as I know, there is no close equivalent to J201, except for the SMD J201's like the MMBFJ201 you mentioned. This is not to say that other jfets won't work. You can always just try them and see if they bias correctly and if you like the sound.

[CheapChip]

Hello Induction!

In the morning, I have swapped both of the 2 J201's with MMBFJ201's on adapter boards and I have to say that for now, my pedal works correctly.
I tested all my batch of J201 before, and always with same (bad) results...

I definitely think my J201's are fakes...

Thanks again for your help!

[Beaker]

I'm glad you got your pedal sorted. Many of us have been stung by fake JFETs, so you are not alone. SMD JFETs are definately the way to go, at least for J201 and 2N5457, which seem to be the most commonly used in pedals, and the most commonly faked too. I made the switch after buying a batch of fake J201 from China, and would not go back now. Much greater consistency for just a little more effort.

Just be aware that you may need to use bigger value trimmers to get them to bias correctly.

Some tips on SMD soldering here:

http://guitar-fx-layouts.42897.x6.nabble.com/SMD-Jfet-soldering-tips-tp25899.html

There are a few other threads regarding SMD, just use the search bar above.