Gated Tone Bender

[Gumby212]

Ive been playing around with germaniun tone benders for days (particularly the mk II) and im realizing how hard it is to get a consistent sound. Sometimes its spatty and gated on the decay, and other times its a nice, smooth tone with plenty of sustain, closer to a distortion. Both are great tones, but i prefer the non spatty/gated. Videos and other blogs only confuse me more. Are orginals supposed to be spatty and gated? From my experimentation it seems to be more transistor dependent and not affected by the bias as much, but i feel like i keep getting mixed results. Can anyone tell me whats causing the gating? Biasing the collector of Q3 between 4.5 and 8.5 (another topic of discussion) certainly affects the output level and fuzz tone, but not so much the gating. Im also unaware of any other biasing i should be doing, to Q1 or Q2, as i only mess with the Q3 collector resistor for biasing. Are there voltages i should be trying to reach with Q1 and Q2 when sticking to the 70 to 80 hfe, leakage under 300 range? Swapping different transistors seems to be the only way i get rid of the gating, but considering im using close to all the same hfe/leakage for each transistor every time i swap them out, my brain is spinning... Anyone care to help enlighten me?

[duck_arse]

how is the temperature there where you are, variable? constant?

[Electric Warrior]

You shouldn't need to bias on Q3 at all, as its collector voltage mostly depends on how Q2 is set up. Sola Sound set the bias on Q1's base and Q2's collector – that's all it takes. Vintage units either came with 10k on Q1B and 47k on Q2C when they used OC75s (which can be quite leaky. 500µA shouldn't be a problem here, but it may depend on other specs as well) or 100k in both spots for OC81Ds with much lower leakage. The voltages are in the same ballpark for either version.

Here are some measurements from my 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

Q1C certainly measures on the high side in this one, but the other voltages are close to average.

Q2 makes the biggest difference for gating. By adjusting its collector resistor you should be able to dial in more or less gating. Or just use a resistor in the right ballpark and use a transistor with a little less leakage if gating is not your thing.

Many vintage units do gate and it usually gets worse as the temperature increases, but when they're biased in the right ballpark this  can actually sound great. Mine is set up right on the verge of gating and there is a wide range of temperatures in which it's pretty much unpredictable wether it will gate or not. It sounds spectacular either way. Others will simply gate more as the temperature increases.

In my experience, hfe is not all that important as long as the voltages are in the right ballpark. I've tried some high gain OC75s that were extremely leaky, but they biased just fine with the same setup I used for the lower gain ones and the sound was hardly any different.

When auditioning transistors I usually start with a closely matched set and then listen for the difference when I swap them.