Taqueria biasing questions

Song Naga

Song Naga

Member
I'm not able to get my Taqueria pedal into the suggested biasing ranges for test points 2 and 3:

Test Point 1 was easy to set up, the Test Point in "taco" mode is at 7.53V and "bolt" is at 9.37V.

The lowest I can get Test Point 2 is about 11.14V, which is too high for both "taco" and "bolt" settings (both about 9.5V). I double checked R5 to confirm it's 2.7K and am not sure what else in the circuit might affect this test point. It seems like it's just the Bias trim pot, the J201 and the current limiting resistor.

The lowest I can get Test Point 3 is about 2.96V, which is still too high for the for the "taco" setting (0.75V). I can set this in the "bolt" range but don't know how I can crank it down to 0.75V. I double checked R7 to confirm it is 330R, and again am not sure what else in the circuit might affect this test point.

Output on the pedal is also pretty low (with all pots dimed it is about half the volume level as when the pedal is bypassed). I'm guessing this may be due to the biasing issues, but worth mentioning in the event that it's an indicator of something else.

Any tips on getting these test points into the suggested range? Thanks for any help!
 
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Hi again, I still haven't fixed this so I'm taking another stab! I'm focusing on the 2nd JFET for now in hopes I can figure out how to get it to bias properly and then apply what I've learned to the 3rd JFET as well.

I can measure 0.17V at the gate pin for the 2nd JFET in my circuit, and I modeled this part of the circuit in Falstad to see what values it would output at the test point in a simulation. I'm no Falstad wizard, so please let me know if I'm using it totally wrong or something. Also, the JFET model in the simulation is generic, I don't know if the J201 would behave much differently or not.

If the screengrab below looks correct, does this mean I should be able to get the test point to output a voltage range between ~0.5V and ~17V depending on the position of the 100k trim pot that's between the +18V and the drain pin?

1767750037640.png

If I can't get that sort of range when I adjust the trim pot, what do I do to get the test point into range? I'm supposed to set this test point to 9.5V, but in my circuit I can only get it down to 11.14V. I've verified that I've got +18V at the voltage source, a 100k trim at the drain pin, and a 2.7K resistor in place on the source pin. Are there other things in the circuit I should check? Is it maybe time to swap out the J201 and see if anything changes? Thank you again for any help!
 
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I swapped the 2.7K resistor out for 1K and can bias the 2nd JFET now.

The pedal's output is still SUPER low (barely audible when all pots are dimed), so I'm working on the 3rd JFET now since something is still not right.

I can measure ~17.2V going into the trims that feed this JFET, but I couldn't set the Test Point to the voltages recommended using the trims. I can currently bias the test point in a range between 17V and 8.5V for one trim and between 17V and 3.4V for the other trim. I'm not sure why these ranges are so different since they're the same type of 100k trim trim feeding into the same circuit.

I tried the same trick used on JFET 2 and replaced the Drain resistor with a lower value (330R got replaced with a 100R), but this didn't seem to get things into range.

Also, another weird thing is that the voltage on the Gate pin on this JFET is a NEGATIVE 2V, when the other JFETs have next to no voltage at their gates. This seems suspicious so I figured it was worth mentioning.

Any further advice on getting this 3rd JFET to work properly and finally get this pedal finished? Thanks again!
 
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