Transistor update! - AionFX Phobos Germanium Fuzz
The original transistors
The "new" transistors
|Their heatsink looks nice. Standard 2N3904 for comparison.|
|Turns out they fit just fine.|
What if the circuit is made for low-gain transistors and starts misbehaving with mid- to high-gain transistors?
For some reason, this thought did not occur to me when debugging the day before. It makes sense when you think of it: the first stage is a Darlington-pair of transistors to amplify the input signal, and the second stage is a saturation stage, where all the nice fuzz is generated. Too much gain on the first stage will already saturate the signal which will cause gating and clipping and all sorts of strange things to happen in the second stage if the output voltage of the previous stage is not within the range of what the circuit expects.
With that in mind, I re-tested all the TFK transistors I had to confirm their hFE values. Out of the five in the pack, I picked the two with the lowest value for Q1 and Q2, with an hFE of 61 and 64 respectively. Then I picked the hFE of 84 for Q3. Once they were neatly installed, I plugged everything back in and checked the voltages.
The bias of Q1/Q2 is a little high at 4V, but a quick turn on the trimpot lowers it at 3.502V. I monitor that for a few minutes, and it's not going anywhere, which is great. Next, the bias of Q3 is at 4V! This is also great. I never really saw a voltage on startup much higher than 1.8-1.9V for Q3 with the trimpot at 0Ω. I dial it in about half way and the bias is now at 2.2V. Voltage is still rising, albeit slowly (which means the heatsinks are working as they should).
I plugged in my trusty P-Bass, and started playing around: no more impromptu gating or strange clipping. The sound has the slight imperfections you'd expect from a circuit like this, and the fuzz sounds great. At maximum fuzz it's thick and dirty, and it dials back nicely with either the fuzz potentiometer or the instrument volume knob (the latter can have some unintended side effects depending on the potentiometer installed in your instrument, but let's not get into that). I very much like the noises that comes out of this pedal.