Son Of Ben Q2 Bias Question - affected by gain knob?

[Lytt Effects]

Ok, I know there are a bunch of threads about biasing the Son Of Ben, and even more about just biasing in general, but I could not find an answer to this. I have built two separate pedals that need biasing (3-4 of each), one being the Son of Ben (Benson Preamp) and the other being the Taqueria (Superbolt/Tacobolt). I used a 9V power supply that came from a Strymon pedal that was reading at ~8.9V on my DMM, and all went well (except for when I accidentally used a 5K trim pot instead of a 50K on one...oops! I fixed it, don't worry).

Here is my question: does/should the gain knob have an effect on the bias of the transistors? Specifically, I'm looking at Q2 of the Son of Ben (schematic attached). I went to do a once over before packing up a pedal to ship out, and I was double checking the bias on all of the drains for Q1-Q3. Both Q1 and Q3 were where they should be, but Q2 was dramatically off, reading very low at below 2V. I thought that was odd, so I set the bias of Q2 to ~4V following usual procedure. DMM black clamped to ground of output jack, input jack tip connected to ground to short out signal, pedal plugged in with the 9V supply, and red probe of DMM to drain of Q2. All good.

I disconnected everything and then flipped the pedal over, and saw that the gain knob was fully CW, which sparked my curiosity. Did this have an effect on the bias of Q2? I turned the knob fully CCW and hooked everything back up. The voltage on Q2 drain was now reading well over 6V. I adjusted the gain knob back to noon, and it was closer to 5V, and then fully CW it was back to normal at 4V. I set the gain knob back to fully CCW, biased the drain to ~4V again, and then went full bore on the gain knob, and the drain then read ~1.2V.

Should this be happening? If not, oh boy, why is it happening and please tell me it's an easy fix? If so, why does it happen, and where should I be setting the gain knob when it comes to biasing transistors? Neither Q1 or Q3 were affected by this in the same way, so I'm very curious about this.

 

[Lytt Effects]

Please, sir. My crops, they are dying.

 

[Fama]

I'm sorry, the best I can do is "I think I read once in some instructions to bias the pedal with knobs turned to certain positions, one of which might have been a drive or fuzz with the gain turned up, but I'm not sure".

Hopefully someone can actually chime in with some actual knowledge.

 

[Lytt Effects]

I'm sorry, the best I can do is "I think I read once in some instructions to bias the pedal with knobs turned to certain positions, one of which might have been a drive or fuzz with the gain turned up, but I'm not sure".

Hopefully someone can actually chime in with some actual knowledge.

Appreciate it! I'm pretty sure I read something like that as well, but I can't seem to track it down. Something about "gain to max" and then bias. I gave the schematic another once over, and I think maybe I'm seeing what's going on, but I'm still a novice at this. Leg two of the "drive" pot is connected to a 470K resistor, which then connects to the gate (pin 3) of Q2 (J201). I don't quite understand how this affects the drain voltage though, or if it even should do so. Above my pay grade.

Obviously, biasing by ear is what we should be doing anyway, but I'd at least like the knowledge here if there is something to be gained from it. Very intriguing to me.

 

[lrgaraujo]

Disclaimer: it's entirely possible I'm wrong.

With that out of the way, I don't think it should matter as much as you're reporting. Jfet gates need a path to ground, but that's one of the functions of legs 2 and 1 of the gain potentiometer (the other is providing a voltage divider to set the amount of signal that reaches the gate of Q2).

Leg two of the "drive" pot is connected to a 470K resistor, which then connects to the gate (pin 3) of Q2 (J201). I don't quite understand how this affects the drain voltage though, or if it even should do so.

R7 and c4 are responsible for letting some treble pass when the gain is low, so they shouldn't affect drain voltage.

Simulating a similar stage with LTspice, we can see drain voltage is pretty stable with different gain settings (Parameter X in the simulation)

 

[giovanni]

Based on the schematic alone, the drive pot should not affect the bias on any transistor. It’s capacitively coupled to Q1 and connected to the gate of Q2 which itself is capacitively coupled (by FET design). So if in your pedal the drive knob is changing the bias I am afraid you may have a short somewhere which is causing the pot to interact with either the transistor or the power rail, or your transistor is somehow leaky (I’m not familiar with FET enough to determine whether this is a thing) and thus bias voltage is leaking into the drive pot and affecting the transistor voltage.

I would try a different transistor first and then by visual inspection check if there may be any shorts. Good luck!

 

[giovanni]

Also you can post pictures so we can help with the visual inspection.