DIY Guitar Pedals Oil Tanker Fuzz User manual

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Oil Tanker Fuzz
Design By Erik Vincent
Sometimes, bass players want something to pull them out of the mix and do something different. Sometimes the guitar
player wants to play clean or solos and there isn’t a rhythm guitarist to back him up, leaving the bass player to fend for
himself. The Oil Tanker Fuzz fits the bill!
If you want nice beefy explosive fuzz that makes your bass sound like an oil tanker scraping up on concrete, give this
pedal a shot. This pedal is based around several fuzz pedal ideas and focuses on bass delivery for stage and recording
mix scenarios.
This also works great as a “refined” fuzz for guitar players alike.
The pedal uses 3 pot controls: Volume, Tone, and Fuzz. Beginner friendly; be sure to watch the build video for the Oil
Tanker to get an idea of how simple this build can be.
The PCB itself will fit snug into a 1590B enclosure.


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Bill of Materials, Stock Oil Tanker Fuzz
Capacitor Resistor
C1 22nF (film) R1 1M
C2 22μF (Electrolytic) R2 100K
C3 100nF (film) R3 100K
C4 33nF (film) R4 1K
C5 220nF (film) R5 5.6K
Diode Potentiometer
D1 1N4001 Fuzz 1kb (16mm)
Volume 500ka (16mm)
Transistor/MOSFET Tone 10kb (16mm)
Q1 2N5088
Q2 BS170
Q3 IRF520 (TO-220)
Q4 IRF520 (TO-220)

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REV -
REV A
PCB Spacing
The Oil Tanker Fuzz PCB is spaced for 1590B sized enclosures or larger
Pot Spacing
The Oil Tanker Fuzz PCB mounted potentiometers are spaced for Alpha 16mm potentiometers.

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1. Soldering Order.
When soldering things to the PCB, the idea is to solder things on from lowest profile to tallest.
For the Oil Tanker Fuzz, the best order would be: resistors, transistors, film capacitors, Power MOSFETs, electrolytic
capacitors, wiring, and then potentiometers.
1.1 Resistors.
Resistors are small passive components designed to create a resistance of passage of an electric current.
For this pedal we will be using 1/4 Watt resistors. These can either be 5% tolerance carbon resistors, or 1% tolerance
metal film resistors. Orientation of “which way is up” doesn’t matter, so you can install them either way. After
installation and soldering, do not forget to clip the remaining legs from the PCB.

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1.2 Transistors/FETs (silicon).
These semiconductor devices come in a few categories, such as BJT, JFET, MOSFET, and IGBT and are used for a variety
of functions
These devices typically only install one way, but pinouts can differ from different part numbers, so if using a different
part number transistor than the one called out in the bill of materials will require that you check the datasheet of the
transistor and check which legs are what pins for it to function properly.
After installation and soldering, do not forget to clip the remaining legs from the PCB.
1.3 Capacitors (film).
Film capacitors are small passive components designed to hold a small amount of charge in a circuit.
Orientation of “which way is up” doesn’t matter, so you can install them either way. After installation and soldering, do
not forget to clip the remaining legs from the PCB.

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1.4 Power MOSFETs.
Power MOSFETs can be used for switching, but also have an internal diode which can be used for clipping as well.
The silks screen will indicate which way to fold the MOSFET if there isn’t room to have it stand straight up. After
installation and soldering, do not forget to clip the remaining legs from the PCB.
1.5 Capacitors (electrolytic).
Electrolytic capacitors are small passive components designed to hold a small amount of charge in a circuit.
Electrolytic capacitors are typically polarized, so orientation will matter.
After installation and soldering, do not forget to clip the remaining legs from the PCB.

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1.6 Wiring.
Wires used for the pedal are for delivering power over the hot and ground wires as well as signal for the input and
output.
These can be installed at the very end, but in some situations, installing them before potentiometers are soldered in
place can be advantageous. Colored wire doesn’t change the properties, but using color codes for hot and ground wires,
like red being hot, and black being ground, are common place. Typically, stranded hook-up wire, AWG 24 or 22 is used
for this task. Using wire strippers, strip away about 1/8” (3mm) of the wire from either end and then using a soldering
iron, tin the exposed tips with solder before installing into the PCB.

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1.7 Potentiometers.
Potentiometers are variable resistors that are used for controlling aspects of the pedal.
This pedal can utilize 16mm pots. These are typically installed on the backside of the PCB and uses the included washer
and jam-nut to mechanically secure the PCB to the enclosure via a strategically drilled hole on the enclosure. Orientation
of potentiometer is preferred to line up the knob on the silk screen with the knob of the potentiometer.

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1.8 Off Board Wiring Diagram.
Potentiometers are variable resistors that are used for controlling aspects of the pedal. Using a non-switched miniature
DC Jack and 2 Mono Jacks

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Oil Tanker Fuzz Circuit Analysis for modifying purposes.
2. Oil Tanker Fuzz Circuit.
The Oil Tanker Fuzz schematic can be broken down into some simpler blocks: Power Supply, Input Stage, Feedback
Network, Tone Control, and Output Stage.
The circuit is designed around a BJT-MOSFET pair for gain. After that, a small low-pass filter tone control and some hard
clipping with soft-knees via more MOSFETs.
The input impedance on the Oil Tanker Fuzz is close to 3.4K Ω, which is very low and will load guitar pickups. A
recommendation would be to put this pedal first on the pedal chain, just after the guitar.

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3. Power Supply.
The Power Supply Stage provides the electrical power to all the circuitry, the whole power consumption is low and
estimated around 1mA:
- The diode D1 protects the pedal against adapter reverse polarity connections.
For component economy, the power supply does not include any capacitors to remove ripple from the power
line which is something common in raw fuzz pedals. The usual solution in guitar pedals is to add some power
filtering by placing 47~100uF cap together with a 100nF from the +9v to ground.

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4. Input Stage.
The input stage is a Common Emitter NPN amplifier. It provides a high voltage gain with low input impedance and high
output impedance. It is not the ideal input stage for signal integrity but the best for simplicity and fast high gain.
- The 1MΩ R1 resistor from the input to ground is an anti-pop/bleeder resistor, it will avoid abrupt pop sounds
when the effect is engaged.
- The 22nF C1 capacitor is a film capacitor used to couple the input of the incoming guitar signal and the rest of
the circuit.
- The Q1 transistor just needs to be a low-noise/high-gain transistor (β = 90-700).
- The 100KΩ R3 resistor is a simple pull up resistor for the Q1 transistor.
4.1 Input Impedance.
Is equal to the input impedance of a common emitter stage. It can be calculated as:
Zin = Qin + (1 / ((1 / R1) + (1 / (β ⋅ re))))
Assuming the β (gain) of the Q1 transistor is 100, which are typical of the 2N5088 in this circuit and that the emitter leg
signal resistance is 25mV / 1mA or 25Ω. VT is the thermal voltage of a transistor, at room temperature the value is
approximately 25mV. The minimum impedance for the 2N5088, per the datasheet, at 1kHz is 1,000.
Zin = 1,000 + (1 / ((1 / 1,000,000) + (1 / (100 ⋅ 25))))
Zin = 1,000 + (1 / ((1 / 1,000,000) + (1 / 2,500)))
Zin = 1,000 + (1 / (0.000001 + 0.0004))
Zin = 1,000 + (1 / 0.000401)
Zin = 1,000 + 2,494
Zin = 3,493Ω(3.5K) @ 1 kHz
For this math calculation the feedback network is ignored but in practice, it will lower the input impedance closer to
3.4KΩ. The Oil Tanker Fuzz has a very low input impedance that will change with the position of the RFUZZ potentiometer.
So the feedback network has a big impact on this parameter.

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As a rule of thumb, Zin should be at least 1 MΩ. In other pedals with similar input stages like the one in the Big Muff Pi a
series resistor is placed at the input in order to higher the impedance (at the cost of creating a voltage divider that
reduces the available input signal).
The Oil Tanker Fuzz low input impedance will load the guitar pickups. This is the reason why they do not respond well
when they are placed after other pedals, it is best to place it first, or before them, in the pedal chain. However, due to
this property, it responds to the guitars volume knob very well.
4.2 Voltage Gain of the Input Stage.
In a Common Emitter transistor the voltage gain does require a bit of math to calculate and requires some assumed data
beforehand.
• First, we need to assume our pedal is being powered by 9V.
• Second, we need to know the thermal voltage of the transistor, which is approximately 25mV (sometimes
expressed at 26mV, depending on assumed temperature)
• Third, we need to know what the collector voltage for Q1 is at when no signal is going into the pedal. Typically,
this is found to be at 2.54V when using 2N5088 transistors on this circuit. They can range from 2.5V – 2.7V
depending on tolerances.
With this information, we can now calculate the gain of the first stage. First, we need to calculate the IE, which is the DC
emitter current. To calculate, we use the following formula:
IE= (VCC - VC) / R3 = (9V – 2.54V) / 100,000 = 0.0000646A = 0.0646mA
Next, we need to get the gm, or measure of conductance of the transistor in this state. To calculate, we use the following
formula:
gm = IE/ VT = 0.0646mA / 25mV = 0.002584
Lastly, we can now calculate voltage gain. To calculate, we use the following formula:
AV = −gm ⋅ RC = −gm ⋅ R3 = 0.002584 ⋅ 100K = 258.4 (48.2dB)
In the real life, the input stage will not reach 48dB of gain, the feedback network will reduce this levels to 47 dB approx,
assuming R4 is 100K and the 1K fuzz knob is maxed. It reduces the first stage gain to 36 dB when the 1K fuzz knob is at 0.
4.3 Input Capacitor Frequency Response.
The C1 22nF input cap creates a high pass filter together with the input pedal impedance (5KΩ approx.), removing
dangerous DC levels, hum and overloading bass.
fc = 1 / (2πRC) = 1 / (2π ⋅ Zin ⋅ C1) = 1 / (2π⋅ 41K ⋅ 0.022uF) = 176Hz
All harmonics below 176Hz will have 6dB/oct of attenuation

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5. Tone Control.
The tone control of the circuit is made up of a low pass filter created by R5 and either C4 and C5 or C4 and C5 with a 10K
resistor between them, depending on how the blending tone 10K potentiometer is set.
As the 9V rail is just a rail, to calculate and understand the tone filtering going on, you can assume ground. So, to redraw
this to make it easier to understand, see the below figure:
RO is the calculation of R4, which is 1K in parallel to the maxed value of VR2, which is 500K. This would be calculated at:
1 / RO = (1 / R4) + (1 / RvolMAX)
1 / RO = (1 / 1,000) + (1 / 500,000)
1 / RO = 0.001 + 0.000002
1 / RO = 0.001002
RO = 1 / 0.001002
RO = 998.004 Ω

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So when the tone potentiometer is turned all the way counter-clockwise and the tone becomes its most dark. The
resistance of VR3 drops down from 10K to practically 0. C4 and C5 are now in parallel with each other, which means the
capacitances of C4 and C5 add together.
=
fc = 1 / (2πRC) = 1 / (2π ⋅ RO ⋅ (C5 + C4)) = 1 / (2π⋅ 998 ⋅ 0.253uF) = 630Hz
This means that with the tone knob down all the way, frequencies above 630Hz will be cut, giving a darker, wooly tone.
As the pot is turned clockwise, the tone pot becomes a 10K resistor between C4 and C5. This is when the tone becomes
its most bright.
Since C5 is much larger than C4 in capacitance, we can say C5 is a short to ground/rail when calculating the filter. Now
we have a two-pole low pass filter, so we will need to recalculate the value of RO as it will be RO in parallel with the 10K
Tone Pot. This would look like RO-OLD // 10K = 907Ω:
fc = 1 / (2πRC) = 1 / (2π ⋅ RO ⋅ C4) = 1 / (2π⋅ 907 ⋅ 0.033uF) = 5317Hz
This means that with the tone knob up all the way, frequencies above 5.3kHz will be cut, which is a much brighter tone
than when the other way. It still cuts highs, but mostly all the abrasively shrill highs, still giving a bit of a warm tone.

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6. Output Stage.
The output stage is an Enhancement N-Channel MOSFET Common Source Amplifier coupled with a variable source
degeneration resistor (RFUZZ=1KΩ).
- The 1KΩ R4 resistor is the drain resistor for the Q2 MOSFET which help sets the voltage gain, bias points, and
maximum drain current.
- The 5.6KΩ R5 resistor is the drain resistor for the Q2 MOSFET which help sets the voltage gain, bias points, and
maximum drain current.
- The 100nF C3 capacitor is coupler capacitor to send the AC signal out to the volume potentiometer without any
DC.
- The VR2 500K volume potentiometer is being used to control signal volume by sinking some of it to ground.
- The Q2 MOSFET is the core of this amplifier.
- The Q3 and Q4 power MOSFETs are being used for their internal diode to perform the role of subtle hard
clipping

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6.1 Output EQ Curve.
In regards to the output capacitor of C3, changing the volume pot from a 500K resistance pot to a 100K resistance pot
changes the high pass filter response. For example, changing C3 from 100nF to 10nF and changing the pot to 100K will
give a much higher cut-off, making the sound brighter.
fc = 1 / (2πRC)
fc = 1 / (2π⋅ RvolMAX ⋅ C3)
fc = 1 / (2π⋅ 500K ⋅ 0.1uF)
fc = 1 / (2π⋅ 500,000 ⋅ 0.0000001)
fc = 3.2 Hz
This basically starts cutting out the low frequencies below 3.2Hz, which basically is just cutting out DC noise from leaving
the pedal and going into the next, which is a good thing. However, all the bass and sub frequency bass will leave the
pedal if above 3.2Hz
So, using a 100K volume pot and 10nF C3 cap:
fc = 1/(2πRC)
fc = 1/(2π⋅ RvolMAX ⋅ C3)
fc = 1/(2π⋅ 100K ⋅ 0.01uF)
fc = 1/(2π⋅ 100,000 ⋅ 0.00000001)
fc = 160 Hz
Now, frequencies get cut under 160Hz, still protecting the next pedal from low frequency DC noise, but also cuts a lot of
the bass out of the exit of the pedal.
The C2 capacitor shunts part of the signal to ground, but its value is so high (22uF) that in the worst case only signals
below 7Hz (and the audio spectrum) will be affected, so the contribution for the general frequency response can be
discarded.

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6.2 Output Impedance.
The value of the output impedance can be calculated using the formula:
Zout = Rvol Parallel to R4
Zout = 500K Parallel to 1,000 Ω = 998Ω
The output impedance is affected by the feedback network and has a real value of 1.88KΩ (measured at 1 KHz with
RVOL=500KΩ). This value varies with the volume control level and the fuzz control position. It is, however, well-under
10KΩ of resistance in most situations, so it still is on the upper end of ideal.

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6.3 Total Voltage Gain.
The source degeneration resistor RFUZZ creates a local negative feedback, making the second amplifier stage more
stable and immune to gain variations due to temperature, bias current and transistor intrinsic properties.
With this source resistance added, the Common Source N-Channel major parameters (ignoring by the moment the
feedback network) can be determined by the ratio between the drain resistors (R4 + R5) to and the source resistor (the
portion of RFUZZ not shorted to ground through the 22uF cap).
AV = RC / RE = (R4+R5) / Rpot1
AVmin = (1K + 5.6K) / 1K = 6.6 (16dB)
The voltage gain (AV) can go from 6.6 to as high as the transistor's basic internal gain (when RFUZZ is maxed out).
If we take into consideration the feedback network, once again the second stage will not reach values as 16dB. In this
case, the total voltage gain measured at Q2 source is around 19.5dB. Remember that the input stage had a gain of 18.6
dB, that leaves the second stage a total amount of 1dB of gain (19.5-18.6=0.9dB). The general amount of gain is
considerably reduced due to the feedback network.
But the output of the pedal is not directly taken from Q2 source, there is a voltage divider created by R4 and R5 (the
power supply is effectively at AC ground). This divider reduces the gain by a factor of R4 / (R4 + R5) = 1000 / (1000 +
5600) = 0.1515 (-16.4dB), so the real gain of the output stage is:
GVTOTAL = GVPEDAL - Attenuation of R4 / (R4 + R5) = 19.5 – 16.4 = 3.1dB
This voltage divider created by R4 and R5 will greatly reduce the output level. The value usually does not get as low as
3.1dB, the series resistor of the battery should be taken into consideration and will raise the output level.
It might look funny but it has a reason: the output signal is not much larger than the input signal to keep the huge
amount of signal available from over-driving the input of the pedal or amplifier following it. The fuzz is not designed to
overdrive the following system by level.
Oil Tanker Fuzz sounds different with different batteries and with the same battery as it gets run down. The internal
series resistance of the battery is added to the 1K Ω R4 resistor, modifying the value by a significant amount.
Any impedance between C2 source and ground (RFUZZ) will reduce the gain of the output stage, it is a form of local
negative feedback. Increasing this impedance will reduce the gain. If we are looking for high gain it is a common practice
to have part or all of the source resistor grounded with a bypass capacitor.
Capacitors present an impedance that decreases with frequency, the bias (DC) points will remain the same but high
guitar (AC) signals will get higher voltage gain. In terms of design, the bypass capacitor C2 should have a reactance, at
the lowest frequency you are interested to amplify, less than the value of RFUZZ. We can use the formula:
fc = 1 / (2πRC) = 1 / (2π⋅ Rpot1 ⋅ C2) = 1 / (2π⋅ 1K ⋅ 22uF) = 7.2Hz
All the frequencies over 7.2Hz get full amplification. The 22uF is so big that almost all the frequencies get full
amplification.