JHS Superbolt / Supreaux Circuit analysis

JHS is not one of the most honest builders on the planet, and he is known for his ability to steal circuit designs from other builders. The Thunderbolt is no exception, and is in fact a really slightly modded Run Off Groove Supreaux. Run Off Groove is a community creating circuits for DIY people, and making it public through Creative Common license. There are many nice circuits there, take a look! JHS did not bother with the license and just copy it and started to sell it.

The circuit is inspired by an old class A amplifier, the Supro 16T. It is a small 5W amplifier from the late 60s. It became quite famous, because like many other class A amplifier, if you push the volume really loud, the amp begins to distort in a really nice way. It was used in studio by many guitarists, especially by Jimmy Page who recorded several Led Zeppelin songs with it.

Here is the schematic of the ROG Supreaux:

And here is the JHS Superbolt schematic:

You can see that it is basically the same circuit, with an added voltage doubler (top part of the circuit), and a switch adding a 120k resistor connected to the ground before Q3.

Lets divide it in several sections as usual:

You can see that it looks a lot like an amplifier! It is really inspired by an amplifier schematic, except that tubes are replaced with JFET transistors. Which is really appropriate because JFET transistors (Junction Field Effect Transistors) use a technology that is really close to tubes, and they sound quite "tube-like" when saturating: warm, creamy overdrive compared to the aggressive gritty saturation of silicon transistors.
All the saturation of this circuit is based on the saturation of these transistors: you can see that there are no diodes in the circuit. It is different from what we are used to with the Big Muff or the Jan Ray (Timmy).

To sum up, it works like this: the first preamp section amplifies the signal, and will drive the second transistor. The saturated signal will be slightly modified in tone by the tone stack section, and a third transistor will set up the final volume. Like I said, it really works like an amplifier!

Let's see every part of the circuit in detail.

Power supply section

First, there is a protection against polarity inversion with a 1N4001 diode, and voltage is stabilized with two capacitors. More information about voltage stabilization and polarity protection here.
A MAX1044 IC is then used to double the voltage, in order to have an 18 volts bus that will be provided to the JFET transistors. Having 18 volts instead of 9 gives a bit more headroom and clarity to the circuit, and this had been used in the Supreaux Deux version of the circuit. It uses the example circuit that you can find in the datasheet:

It is a very classical layout that you will find in most pedals using a voltage doubler (Klon centaur for instance).


Preamp section 1: gain

First, there are two 1M pulldown resistors. Pulldown resistors prevent loud popping noises when you turn the effect on. There is a 22nF coupling cap that prevents any parasitic DC current from your guitar to go through the circuit.

The signal is amplified by the first transistor (Q1). You can see that the overall gain of the transistor can be adjusted with a 100k trimpot that will adjust the DC current on the drain of the transistor. That way, you can set what the maximum gain of the pedal will be. The maximum possible gain is set also by the resistor R3 (1k5).

Then, the signal is filtered a bit by different means: a 470 pF transistor connected to ground let high frequencies going to ground, thus eliminating a bit of high frequencies. To eliminate more high frequencies, value of this capacitor can be changed to a bigger value like 560 pF for instance. A 470K resistor connected to ground will set the bass response of the unit, and will eliminate a bit of bass. A potentiometer wired as a variable resistor will then set how much signal will go to ground. Thus, it defines how much the signal is amplified, and how much signal will drive the second transistor: it is the gain potentiometer!


Preamp section 2: tonestack

The amplified signal of the first section will then make the second transistor (Q2) saturate. The amplification of Q2 is also set by a 100k trimpot and R5 (2k7 resistor), and can be modulate to have more or less saturation.

There is a 10 nF coupling cap to eliminate parasitic DC current remaining from the 18V bus, and then the signal is modified by the tone potentiometer. It is wired as a variable resistor connected to ground, and will eliminate more or less bass frequencies. An optional switch also allows you to connect a 120k resistor to ground, which will diminish overall gain (a part of the signal will go to ground), and bass response (more bass will go to ground than high frequencies with a 120k resistor). This allows you to simulate the two High and Low inputs of the Supro amplifiers.

Again, a 470K set the maximum gain of this part of the circuit.


Amplifier: master volume section

This part of the circuit simply amplifies the final signal. The amplification is set by the 220R resistor and a trimpot (again). Residual DC current is then filtered by a 10nF capacitor.
Final volume is set by a potentiometer wired as a variable resistor connected to ground, like we have already seen in many many circuit, like the Fuzz Face for instance. The higher the value of this resistor will be, the less signal will go to ground and the bigger the output signal will be: more volume!

Here it is!
I hope that everything is clear, do not hesitate to ask question if some stuff is still unclear by posting a comment!
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To go further:
Run Off Groove Supreaux webpage
Run Off Groove Supreaux Deux webpage
MAX1044 datasheet.