Please help - Biasing a Tonebender Professional mkii clone

[Governor]

I've build a dozen pedals now including a germanium fuzzface, I've just finished a tonebender build and have replaced the Q3 collector resistor with a 25k Pot as per http://tagboardeffects.blogspot.com.au/2012/02/sola-sound-tone-bender-professional.html with AC128 ge trannies:
Q1- Hfe 59, L 162uA
Q2- Hfe 69, L 124uA
Q3- Hfe 110, L 246uA

It sounds pretty sweet, has some farty decay and may need some more tweaking, the best sound is with the Q3 collector voltage (referenced to ground/positive) of 7.8V, but is ground the reference you use for the voltage reading??

After spending a lot of time searching, I just want to confirm where to take the voltage readings when measuring the Q3 collector voltages, does this voltage reference to ground or to negative?? or to the emitter or base? ie. one dmm probe on the collector pin, where does the other probe go?  I just want to check before I go replacing the Q2 100K resistor for a pot....

[thermionix]

Yes to ground.  (-)7.8v on Q3c is right around where commonly suggested for the MkII.  If it sounds good, it is good!

[Electric Warrior]

Don't use a trim pot at Q3's collector. If you must weak the bias, do it at Q1's base resistor and Q2's collector resistor. The latter one controls the bias of both, Q2 and Q3 at the same time. If Q2 is set right (around 0.16V-ish), Q3 should be in the right ballpark as well.

You should probably try a lower gain transistor for Q3. I like them fairly closely matched.

What are the other voltages? I guess with the amount of leakage your transistors have and 100ks on Q1B and Q2C you shouldn't be far off.

Here are voltages of a great sounding 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

This one has OC75s (which are generally rather leaky) and the 10k/47k bias setup.

[duck_arse]

as said, refer to ground. always black lead to ground, and if/when the meter indicates negative, write it down that way. and before antonis gets here, welcome to the forum.

[Governor]

Awesome, thanks for the replies!

Ok, so these are the voltage readings I've taken:

Q1 C -5.34V B -0.07V E 0V
Q2 C -0.05V B -0.04V E 0V
Q3 C -7.68V B -0.05V E -0.05V

Does this look right?

I've also tested the trannies, I live in a hotter climate in Australia to where these came from (and were tested), I got (measured the right way not with the DMM) more leakage:

Q1- Hfe 64, L 344uA
Q2- Hfe 76, L 267uA
Q3- Hfe 108, L 485uA

[Electric Warrior]

No, it looks like these are too leaky for your bias setup. 10k on Q1B and 47k on Q2C would probably be better after all.

[Governor]

No worries, so might try a 100k trim pot on the Q1 B not the collector?  And a 100K trip pot on the Q2 collector.

[Electric Warrior]

Yes, those are the resistors that were tweaked when they started using lower leakage OC81Ds.

If you need to put trim pots there you should probably limit their range with resistors.

[Governor]

Ok cool, so with 10k on Q1B I now have -8.3v at Q1C.  From the above readings from your vintage unit you have approx 0.6v difference.  I also currently have -8.1v on Q3C, but I can lower this via the Q2C and Q3C trimpots.  Battery is 9.2v.

What are your thoughts? Cheers.

[Electric Warrior]

That certainly is the right ballpark. The 10k sure did the trick. If you take the difference in supply voltage into consideration it only measures three percent lower than my vintage unit - which is more than just close enough.

How is it sounding now? I guess you can dial in as much or as little gating as you like via Q2's trim pot...

[Governor]

Yep, the 10K resistor definitely made all the difference and a noticeable improvement with the sustain, its amazing how much the collector voltages can vary with temperature, but on average I'm now getting:

Battery 9.2V
Q1 C -8.47V B -0.03V E 0V
Q2 C -0.16V B -0.06V E 0V
Q3 C -7.8V B -0.17V E -0.12V

Thanks for your help with this Electric Warrior, I now have a pretty nice sounding tone bender!