Supreaux Deux not biasing correctly

[Tuxedo3]

Hey guys, I built this ROG Supreaux Deux and the build couldn't have been easier. Seriously great PCB.
When it came time to bias it, though, I noticed that I could only get Q1 to 12v like the instructions say. I turned both other trimpots all the way up and could only get it down to 17v. I tested it and really did like the way it sounded, so I boxed it up and now i get a loud whine when I turn the pedal on. In addition to this, I believe my R11 (a 50k trimpot) might be busted. It has a smooth rotation for about 2/3 of it and then it locks up and is really tough to turn.

Why do you think I can't bias to the correct voltage for my other two transistors, what should I check? I'm going to go write the voltages down and post them in a few minutes.

[midwayfair]

All your voltages on all pins.

Measure the resistors on the sources of Q2 and Q3 with your multimeter.

[Tuxedo3]

Okay update. As I was going through this circuit writing the voltages down the top of the suspect trimpot just popped off, along with the wiper. So that one is shot. Luckily I'm building 2 of these pedals, so I wrote down the voltages from the other one.

According to the directions all test points should be 12v.
Test point 1: 15v (this is as close to 12v as it gets, the trimpot is all the way up)
Test point 2: 12.1v
Test point 3: 14.4v (this is as close to 12v as it gets, the trimpot is all the way up)

Q1 (2n5457):
Drain - 15v
Source - 0.1v
Gate - 0v

Q2 (J201):
Drain - 12.1v
Source - 0.4v
Gate - 0v

Q3 (2n5457):
Drain - 14.4v
Source - 0.1v
Gate - 0v

R3 read 980 ohms
R8 reads 375 ohms

Any help would be appreciated. Is 14v - 15v alright for this? It does seem pretty gainy at this voltage. Thanks guys.

[midwayfair]

Your sources should be more like a couple volts, not so close to ground. However, the more likely situation is that your 2N5457s are particularly high gain. If you short R3, does your voltage on Q1's drain change?

[Tuxedo3]

Yes, I just shorted it and it dropped to about 7v with the trimpot all the way up. I then adjusted the trimpot and got it to 12v. What does that say? Are those transistors just pretty gainy?

[playpunk]

I know what I'm doing to my supreaux deux tonight. I've had the same biasing issues.

[midwayfair]

Yes, I just shorted it and it dropped to about 7v with the trimpot all the way up. I then adjusted the trimpot and got it to 12v. What does that say? Are those transistors just pretty gainy?

Yeah, they're probably just unusually high gain for a 2N5457. In that case, and if you're getting 18V on the other side of the trims, 15V might be okay, because your gain is still 10000/390, which is a lot, though the wave might be a little more asymmetrical than intended. (If your supply is only 16V, then 15V would probably be a problem.) If you aren't getting any really strange sounds either leave it like it is, or try out some more 2N5457s.

[jubal81]

Where did you get your 5457s from?

[playpunk]

Where did you get your 5457s from?

I used smd 5457s and had the same result as OP.

[Tuxedo3]

Where did you get your 5457s from?

I believe ebay. I haven't had any issues with them at all. But they could definitely just be high gain.
Would it be worth just shorting out those 3 resistors?

[Frank_NH]

I happen to have an LTSpice model of the Supreaux Deux... 

The 2N5457 stages with the 390 ohm source resistors should bias (12V at drain) at around 4k ohms.  The J201 with the 1K source resistor should bias at about 22K ohms.  That's assuming "average" Idss/Vp characteristics for the JFETs.  Your 2N5457s likely are at the lower end of spec for Idss/Vp.  If you have some additional 2N5457 JFETs to use, I'd try them out and see if they work better.  Otherwise, your best course of action (outside of buying more JFETs) would be to replace the 10K trimmers with 20K or 50K.  That will allow more range to bias the JFETs.  Hope this helps...