Help with Tonebender MKII build - bias control not working

[AndersonCouncil]

Hello all. I am working on building a Tonebender MKII using monkeyxx's vero layout here:


Link to his post: http://www.diystompboxes.com/smfforum/index.php?topic=92080.0

I'm using AC125s:
Q1: 81 hFE, leak .14
Q2: 89 hFE, leak .17
Q3: 108 hFE, leak .24

It's working fine for the most part but does not have much bass. I've never used a Tonebender before so I can't compare it to anything other than my Fuzz Face mini which has some nice lows. The bigger issue is that the Bias knob does not seem to be working as it should. When it's turned all the way up it's loud and sounds like there's a little less gain. When I turn it down to about 90% the volume drops dramatically and it becomes more saturated. Turning it down past that point just lowers the volume considerably. When it's at zero I can barely hear anything.

Here are the voltage readings of the Q3 collector with the bias knob turned up 100%, 50% and at 0%.

-8.05 100%
-6.72 50%
-5.58 0%

I'm using a 50K Linear pot per the vero layout. Most schematics call for an 8K2 resistor here. Would this be the issue? Should I try a 10K pot? 25K? On his build he used 2N404s so is that pot value better suited to those vs. my AC125s?

I've also read that a bias control on Q2 is more useful as an external knob since it biases both Q2 and Q3. If that's the case and you guys recommend doing that instead, should I just use a 100K pot in place of the R4 100K resistor. And then bias Q3 with an internal trimmer?

I'm very noobish with pedal builds if you couldn't already tell. Any guidance is greatly appreciated.

[Electric Warrior]

If you use a bias pot on Q2's collector you shouldn't need to tweak anything on Q3. The stock 8k2 and 470Ω (R6) should do just fine. Both collector voltages depend mostly on how Q2 is set up.

The bias pot pretty much behaves as expected. More resistance there will lower the output.

With the bias pot fully cranked (I assume that means 0 resistance, as the voltage should decrease with increased resistance) the collector voltage is alright. Could be a little higher even. But actually it should be in that ballpark or higher with more resistance on the collector.

Which brings us back to Q2: I bet its collector voltage is too high (resulting in a lower voltage on Q3C) - you could get more voltage readings to confirm this.
Using a 100k pot on the collector won't fix it. You need more resistance than that to bring down the voltage some more. But I'm not sure if you can go much higher. I've heard there may be impedance issues with the third transistor with resistances greater than 100k, but you could give it a try.

You could also try transistors with more leakage if you have any. The MKII variant with 100ks on Q1b and Q2c is set up for low leakage, but for some reason your transistors seem to bias as if they leak too little. I've had transistors in the same gain and leakage range bias alright with a 100k, but it appears appears other specs have an influence on how they bias as well.

When biased correctly it shouldn't be all that bass shy. But the low end should be a tighter than with a Fuzz Face.

Here are some voltages from my vintage unit for comparison:

Battery: 9.67V
Q1 C -9.02V B -0.03V E 0V
Q2 C -0.17V B -0.08V E 0V
Q3 C -8.44V B -0.17V E -0.11V

[AndersonCouncil]

Here are some voltages from my vintage unit for comparison:

Battery: 9.67V
Q1 C -9.02V B -0.03V E 0V
Q2 C -0.17V B -0.08V E 0V
Q3 C -8.44V B -0.17V E -0.11V

Hey thanks buddy. Turns out that I'm just an idiot. After checking everything for a third time I discovered that I never soldered in R2 to Q1s base. Used a 10K there and went with a 100K pot in series with a 20K resistor for Q2 bias and all my voltages are near identical with the ones you posted above.

thanks again

[Electric Warrior]

Cool, glad I could help!