Chuck D. Bones
Circuit Wizard
A friend of mine on these forums gave me an all-in-one pedal with a Golden Falk inside. It's a sweet pedal, but I was unable to set the NORMAL or TREBLE knobs low enough to play clean. So a couple of days ago I a ran some sims and built a breadboard. I figured I'd have to fiddle some resistors to reduce the gain in a couple of places. Turns out that the 2nd stage (Q3 & Q4) had a biasing problem. when the MODE switch is in the 1987 position (right-hand position on the schematic), Q4 is biased to run at Idss. Q3 & Q4 have the same DC drain current, so Q3 needs to have an Idss that equal to, or greater than Q4. JFETs with the same part # will be different, so there's a 50/50 chance that Q3's Idss would be greater than Q4's. There's an easy way to check that in-circuit (see below) and sure enough, Q3's Idss was lower than Q4's. Not good. This circuit was never going to run right. The JFETs are SMD in my pedal, so the rework is going to wait until all of the breadboard testing is completed.
I pulled out my J201 stash and started selecting JFETs for the breadboard. I did not install trimmers on the breadboard. I made TRIM1 = 10K, TRIM2 = 20K and TRIM3 = 10K, same as the trimmer setting in my pedal (actually, TRIM3 was 9K, but 10K is close enough). Then I selected J201s that got the drain voltages all between 5V and 5.5V That took care of Q1, Q2 & Q5. For Q3 and Q4, I measured their Idss and installed the one with the higher Idss in Q3's spot. Everything worked great. I could turn NORMAL & TREBLE up to 9:00 and play clean with hot humbuckers. Then I swapped Q3 & Q4. Now it distorted all of the time even with NORMAL & TREBLE at 8:00 and for all three positions of the MODE switch. I checked Q4's drain voltage with the MODE switch set to 1987. It was 4.3V. When I checked it on the pedal I got a similar reading. If Q3 & Q4 are biased correctly, Q4's DC drain voltage will be above 1/2 Vcc (4.5V). I'll need to swap Q3 and Q4 in my pedal.
If you play a guitar with high-output pickups and you want to be able to play clean with the guitar volume dimed, you have a couple of choices. You can run the pedal at 18V and adjust TRIM3 for 9V on Q5-D, or you can drop the gain of the 2nd stage by about 6dB. To accomplish the latter, change R12 and R13 to 33K and change C12 to 1nF. Another way to do it is to leave R12, R13 and C12 at the stock values and replace Q3 & Q4 with 2N5457. Both ways work equally well. I also increased R11 to 1.5K to get a little more difference in gain when switching the MODE between 1959 and 1987.
A note on trimmer settings
TRIM3 is part of the filter network following Q5. If you go above 15K, the tone will get darker. Better to set TRIM3 to 10K and pick a JFET that gives you the right drain voltage.
Parting shots
JFETs are not plug & play. Q1 & Q2 should be J201 (mmbf201) or similar. With Q3, Q4 and Q5, we have choices. We should keep Q4's Idss below 10mA to limit the power consumption in 1987 mode. Most of the gain in this circuit is in the 2nd stage and Q3 & Q4 have a major impact on the gain. J201 will provide high gain in this circuit. Most other part numbers I tried produced lower gain. R12 & R13 load the mu-amp and influence the gain. Making them larger than 100K won't increase the gain much, but lowering them will reduce the gain. C3 does not need to be 100uF; 10uF is plenty and rolls off the bass below 27Hz. All in all, a very nice circuit when it's working right.
Knobs (L-R): VOLUME - TONE - TREBLE - NORMAL
2nd row from the bottom, JFETs are spare, Q5, Q3, Q4, Q2 & Q1. The JFETs in the bottom row are "on deck."
I pulled out my J201 stash and started selecting JFETs for the breadboard. I did not install trimmers on the breadboard. I made TRIM1 = 10K, TRIM2 = 20K and TRIM3 = 10K, same as the trimmer setting in my pedal (actually, TRIM3 was 9K, but 10K is close enough). Then I selected J201s that got the drain voltages all between 5V and 5.5V That took care of Q1, Q2 & Q5. For Q3 and Q4, I measured their Idss and installed the one with the higher Idss in Q3's spot. Everything worked great. I could turn NORMAL & TREBLE up to 9:00 and play clean with hot humbuckers. Then I swapped Q3 & Q4. Now it distorted all of the time even with NORMAL & TREBLE at 8:00 and for all three positions of the MODE switch. I checked Q4's drain voltage with the MODE switch set to 1987. It was 4.3V. When I checked it on the pedal I got a similar reading. If Q3 & Q4 are biased correctly, Q4's DC drain voltage will be above 1/2 Vcc (4.5V). I'll need to swap Q3 and Q4 in my pedal.
If you play a guitar with high-output pickups and you want to be able to play clean with the guitar volume dimed, you have a couple of choices. You can run the pedal at 18V and adjust TRIM3 for 9V on Q5-D, or you can drop the gain of the 2nd stage by about 6dB. To accomplish the latter, change R12 and R13 to 33K and change C12 to 1nF. Another way to do it is to leave R12, R13 and C12 at the stock values and replace Q3 & Q4 with 2N5457. Both ways work equally well. I also increased R11 to 1.5K to get a little more difference in gain when switching the MODE between 1959 and 1987.
A note on trimmer settings
TRIM3 is part of the filter network following Q5. If you go above 15K, the tone will get darker. Better to set TRIM3 to 10K and pick a JFET that gives you the right drain voltage.
Parting shots
JFETs are not plug & play. Q1 & Q2 should be J201 (mmbf201) or similar. With Q3, Q4 and Q5, we have choices. We should keep Q4's Idss below 10mA to limit the power consumption in 1987 mode. Most of the gain in this circuit is in the 2nd stage and Q3 & Q4 have a major impact on the gain. J201 will provide high gain in this circuit. Most other part numbers I tried produced lower gain. R12 & R13 load the mu-amp and influence the gain. Making them larger than 100K won't increase the gain much, but lowering them will reduce the gain. C3 does not need to be 100uF; 10uF is plenty and rolls off the bass below 27Hz. All in all, a very nice circuit when it's working right.
Knobs (L-R): VOLUME - TONE - TREBLE - NORMAL
2nd row from the bottom, JFETs are spare, Q5, Q3, Q4, Q2 & Q1. The JFETs in the bottom row are "on deck."
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