The Boot-Leg JBK-1 Jawbreaker is an overdrive pedal from Boot-Leg that the maker described as usable as an amp simulator. It covers what the circuit analysis on TagBoard Effects called a wide range, "from soft and delicate crunchy overdrive to rock-crushing hard overdrive," and it can be set as a hard distortion or backed down to a low-gain boost1. It sits in the same family as other overdrive pedals and distortion pedals, with a maker's description of "Heavy bottom. Rich harmonics. High tone which is not too peaky."1
How the circuit works
The Jawbreaker's published layout uses a quad op-amp in four stages. OPamp 1 is a unity gain buffer. OPamp 2 is the distorting stage, with diode clipping in the feedback loop, described as being like that in countless overdrives such as the SD-1. OPamp 3 is an amplifying stage that feeds the tone and volume controls, and OPamp 4 is unused1. The clipping is done with rectifier diodes, 1N4002 in the layout, and builders noted that any 1N400X rectifier can be used in their place1.
The controls
| Control | What it does |
|---|---|
| Break | Gain, a parallel variable resistor in the distorting stage's feedback loop; in other units this control is usually called Gain1 |
| Heavy | A simple tone control, fed by the third amplifying stage1 |
| Level | Output volume, after the tone control1 |
What builders found
The layout was traced and built on TagBoard Effects, a DIY pedal-building forum, in March 2013. The first version had a problem: "The vref network won't work as is, so i'm putting this on hold," the person tracing it wrote. The fix was to move the middle 10K resistor from the top row to ground, and once corrected the layout was confirmed working: "Just to confirm, this can definitely be tagged as verified"1.
Builders who got theirs working reported on how it behaves:
- Norwichbadger found the Level control very hot: beyond about 11 o'clock it is "way too loud," with Level at 8 roughly unity and 9 a good lead level1.
- Madferret found its best use at low gain into an already pushed amp: "For me this circuit shines most when boosting a tube amp, with the amp set for breakup and the Jawbreaker set for low ga[in]," adding body to the sound1.
- Early builds hit snags. Geiri reported "a very high pitch squeal when turning the 'Break' knob" with no signal and a Heavy control that seemed backwards, and dodger later had a build that "squeels really bad" after substituting a 100k pot for the specified 50k1.
Builders should stick to the specified 50k pot for the Break control. A 100k substitution was reported to cause bad squealing1.
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Questions
What clipping diodes does the Boot-Leg JBK-1 Jawbreaker use?
The Jawbreaker uses rectifier diodes for clipping, with 1N4002 diodes in the published layout. Builders on the layout thread noted that when using rectifier diodes like the 1N4002, any of the 1N400X series can be substituted, which gives builders some flexibility in sourcing parts.
What was the vref problem in the original Jawbreaker layout?
The first published layout had a faulty vref network, the reference voltage arrangement that biases the op-amp stages. The person who traced the circuit put the build on hold because of it, and the fix was to move the middle 10K resistor from the top row to ground. After that correction the layout was verified working by builders.
What does the Heavy knob do on the Jawbreaker?
Heavy is the tone control. It sits after the pedal's third amplifying stage and is described as a simple tone control. The maker's description of the pedal's sound mentions a heavy bottom, rich harmonics and a high tone that is not too peaky, which is what the Heavy control shapes.
Can the Jawbreaker be used as a clean boost?
Yes, at low gain settings. One builder found the circuit shines most when boosting a tube amp set for breakup, with the Jawbreaker set for low gain, adding body to the sound. The pedal covers a wide range from soft crunchy overdrive to hard overdrive, so low-gain boost is one end of its range.
Sources (1)
- Guitar FX Layouts: Boot-Leg JBK-1 Jawbreaker, tagboardeffects.blogspot.com, 2013-03-17