New mini-breadboard

[PedalBuilder]

I was thinking temperature stabilized. Q1 is leakage biased, so the bias drifts as the temperature changes. I've toyed with the idea of doing a Benson-style "temperature controlled" Mk II circuit (there's an unused half of the dual op amp in that circuit, so the potential is there), but something just feels wrong about a fuzz circuit that needs 200mA. The servo bias approach from the Mechanic is a much more elegant solution.

 

[Chuck D. Bones]

That was my thinking. Wrapping a control loop around the parameter we want to control: emitter current, makes a helluva lot more sense than controlling temperature in an effort to stabilize the emitter current.

Temperature stabilization has it's place, such as precision crystal oscillators that cannot be stabilized by any other means.

 

[PedalBuilder]

That was my thinking. Wrapping a control loop around the parameter we want to control: emitter current, makes a helluva lot more sense than controlling temperature in an effort to stabilize the emitter current.

Temperature stabilization has it's place, such as precision crystal oscillators that cannot be stabilized by any other means.

Yep. I'm just not sure how to implement a control loop around the emitter of Q1 in a Mk II, as it's so primitive—just a collector resistor, a base-ground resistor, and a grounded emitter.

 

[Chuck D. Bones]

You don't have to regulate emitter current like we do in the FF, you can regulate collector voltage. Which brings me back to the BMP 1st stage. Set the emitter resistor to 100Ω, use something like 47K from base to GND and pick a resistor between B & C that gives a collector voltage around 3V. It's a lot simpler than an opamp integrator and it does a good enough job stabilizing the bias.

 

[PedalBuilder]

So I tried the BMP approach, and while some iterations sounded decent, none of them sounded like a Mk II. The distortion sounded too synthy, with sub-octave notes, and it cleaned up too quickly with the volume knob. It actually reminded me a lot of a Harmonic Percolator. I used a 100Ω emitter resistor, 47kΩ base-ground, 150kΩ B-C resistor, a 10k collector resistor, and a variety of B-C caps. The transistor was a BC109 with an hFE around 250 and the collector voltage was just over 3v.

I also experimented with running the Tone Bender gain stage and the BMP gain stage straight into the amp. The Tone Bender stage was very dark and had an almost farty distortion. Even with a 4.7nF B-C capacitor, the BMP stage was brighter, and it never distorted.

 

[Chuck D. Bones]

Sounds like the 1st stage has too much gain. Try a larger emitter resistor. 470Ω or 1K.

 

[PedalBuilder]

Sounds like the 1st stage has too much gain. Try a larger emitter resistor. 470Ω or 1K.

That occurred to me, but wouldn't fix the issues with the brighter tone and lack of distortion.

 

[PedalBuilder]

Update: I tried 470Ω and 1k, along with a 2.2n B-C capacitor. I reduced the B-C resistor to 100k to keep the collector voltage down. It sounded better, but not very much like a Mk II. Both 470Ω and 1k were less synthy with the volume knob all the way up, especially on high notes which was an improvement. I'm still getting synthy artifacts on low notes, though, and it cleans up even more quickly with the volume knob.

 

[Falterthethomist]

Yes, but it's not needed for a Rangemaster. The Rangemaster's bias should be stable enough on its own.

Thanks for your feedback. I was thinking of something along the lines of the Benson Germanium Boost. Benson said at one point that he is using a servo to bias the transistor. Maybe I'll just play around on the breadboard and see what it does...