Looking for Tone Bender advice

[thermionix]

I'm considering a Mk II build.  I have no experience with them, but have been reading up about them online.  It would be a Ge PNP unit with OC76 transistors.

The recommendations I have seen regarding transistor gains are hFE 70, 70, 100 for Q1-Q3, respectively.  The transistors I have on hand have gains of (approximately) 75, 107, and 175.  Leakages are all around 175~185.

My idea is to use the hFE 75 as Q2, 107 as Q3, and the 175 as Q1 BUT with a small resistor between emitter and ground to limit the gain.

Does this sound like a good, workable plan?  A dumb idea?  Any other advice?

Thanks.

[Electric Warrior]

I don't know who's coming up with these recommendations. It's not as easy as picking transistors with three particular gains. Bias doesn't even depend all that much on hfes in germanium fuzzes. They're all about the leakage.

Building a MKII with just three transistors to choose from is a bad idea. You want to swap them around to find trio that sounds good and behaves well.
The hfe range of your transistors is not all that important. It can sound good with high gainers as well as low gainers, but you ideally start out with three transistors in roughly the same gain range and then substitute and see how it affects the sound.

You should choose transistors that have enough leakage to bias right. Vintage units came with two different bias setups: 10k at Q1's base and 47k at Q2's collector for OC75s (high-ish leakage) and 100k for Q1's base and Q2's collector for OC81Ds (less leakage). If your voltages are off, these should be the resistors that need tweaking, as Q2's collector resistor biases Q3 as well.

Selecting the right transistor for Q2 is critical. If your pedal is hissy but cleans up well, you'll need more leakage here. If it's too gated, pick a transistor that leaks less.

Here are a couple of voltages from a great sounding vintage unit:

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

With your particular set of transistors, picking the highest gain for Q1 and the ones that have better matched hfes for Q2 and Q3 may be a good start, indeed. It may sound better with Q2 and Q3 swapped around.
Luckily, the leakage of Q1 is in the same ballpark as Q2 and Q3. I guess it should bias alright without any further modification. Not sure how combining such a high gain Q1 with lower gain Q2 and Q3 will sound, though.
Good luck with your build!

[a2music]

Learning to conquer the MKII is an important rite of passage for most pedal builders. The 70-70-100 convention comes from Fuzz Central and is really just a suggestion. It might make sense to buy a transistor set that has been tested and auditioned. Small Bear is a good source and I think there are some eBay sellers who have transistor sets that have been pre-tested for a specific circuit. Achieving the correct collector voltages is an important goal. I have found that adjusting the collector resistor on Q2 until I get an 8v reading on Q3 C is the fastest/easiest way to dial in this circuit. I use a 100k trimpot in series with a 33k resistor to bias Q2. It usually lands somewhere around .2v when Q3 is at .8

Have fun!


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[a2music]

That should have read "when Q3 is at 8v" not .8v.


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[thermionix]

Thanks for the input guys.  Still not sure if I'm going to attempt the Mk II, but good info if I do.