The Krampus Bass OverDrive is a bass overdrive by J. Michael Pedals, offered to DIY builders as a stompbox kit and project with a PCB through General Guitar Gadgets. According to the project page, "It is based on the Tube Screamer ™ OverDrive, but tuned for bass," which places it in the same family as the classic overdrive circuit many players know from guitar use, reworked to suit a bass signal1.
The Krampus Bass OverDrive is a bass-oriented overdrive built around a TL072 op amp at IC1, the chip that carries its gain stage, and switched by a heavy duty 3PDT foot switch (Sw1)1. Those two components define the circuit at its core: the TL072 sets the tone-shaping and gain the player hears, while the 3PDT switch gives true bypass control on a pedalboard.
The controls
The kit uses three potentiometers, each with a specified value and taper1:
| Control | Position | Value | Taper |
|---|---|---|---|
| Drive | R19 | 1M | Log (or Audio) Taper |
| Tone | R20 | 20k | EQ Taper (or substitute 25k linear) |
| Level | R21 | 100k | Linear Taper |
The Drive control sets the amount of overdrive through the op amp gain stage, the Tone shapes the voicing, and the Level sets output volume. For the Tone control, the bill of materials allows a substitution: a 25k linear taper pot can stand in for the 20k EQ taper if that is what the builder has on hand1.
Clipping and the core circuit
Where a standard Tube Screamer layout uses diodes for clipping, the Krampus instead uses a 2N5457 JFET as the clipping device. The project page instructs: "You will need to solder 2 legs of a JFET clipper together in then install that as your D1 clipping device." The amplification itself comes from a TL072 op amp at IC11.
D5, a 1N914 diode, is optional. It is there to prevent reverse polarity power from destroying the IC. If you decide not to install D5, leave the position blank with no jumper1.
Jacks, switch and power
The wiring is conventional for a DIY stompbox build1:
- Input: Stereo quarter-inch jack at J1, which doubles as the battery switch contact
- Output: Mono quarter-inch jack at J2
- Power: DC jack at J3
- Bypass switch: Heavy Duty 3PDT foot switch at Sw1
- Battery: 9 volt battery at B1, not included in the kit
Because the battery is not part of the kit, builders planning battery power need to supply one; the DC jack at J3 covers external power1.
Background
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Questions
Does the Krampus Bass OverDrive come as a kit?
Yes. General Guitar Gadgets offers the Krampus as a DIY stompbox kit and project with a PCB. The kit was among four new bass DIY kits released in July 2014, three of them classic pedal modifications courtesy of J. Michael Pedals. A 9 volt battery is not included in the kit, so builders who want battery power need to supply their own.
Does the Krampus kit include a 9V battery?
No. The bill of materials lists a 9 volt battery at position B1, but it is explicitly not included in the kit. Builders who want to run the pedal from a battery need to add one themselves; the project also has a DC jack at position J3 for external power.
What clipping does the Krampus Bass OverDrive use?
The Krampus uses a 2N5457 JFET as its clipping device at position D1. The builder solders two legs of the JFET clipper together and installs it in the D1 position. This replaces the diode clipping a standard Tube Screamer layout would use, as part of the circuit's tuning for bass.
Is the reverse polarity diode required in the Krampus build?
No. D5, a 1N914, is optional. It is there to prevent reverse polarity power from destroying the IC. If you decide not to install D5, you leave the position blank with no jumper. Installing it is a simple safeguard for builders who want protection against a wrongly wired power supply.
Sources (1)
- Krampus Bass Overdrive: DIY Kit & Schematic, generalguitargadgets.com, 2014-07-13