PNP "Positive Ground" to NPN "Negative Ground" Circuit Conversion
Many vintage pedals, particularly early germanium fuzz pedals, used PNP transistors and are wired to use positive ground. In the schematic for a positive ground build, you’ll usually see the build powered by -9V and GND. If a modern pedal is built to use battery power only, like those early fuzz pedals, wiring a pedal to use positive ground is simple. The positive battery lead can connect to GND, and the negative lead can be used for -9V. However, if a pedal is built to use a standard power supply, positive ground builds can present some issues. We discuss the issues in-depth and the various potential solutions in our PNP "Positive Ground" Pedal Considerations tech article. One of the simplest solutions which requires no additional parts is to simply build the pedal in a standard negative ground configuration using NPN transistors.
Most schematics for early germanium fuzz pedals are drawn showing a PNP positive ground configuration. To convert them to a negative ground configuration, there are three necessary steps:
- Change -9V to +9V
- Change the PNP transistors to NPN (and change NPN transistors to PNP, if there are any)
- Reverse any other polarized components (diodes, electrolytic/tantalum capacitors)
Figure 1: Dallas Rangemaster in original positive ground PNP format and the converted NPN negative ground circuit (changes highlighted)
Figure 1 is an example of the conversion of a germanium PNP circuit from positive ground to a negative ground NPN circuit. The original PNP circuit is the Dallas Rangemaster treble booster. The negative ground NPN version of this build can be wired normally using a standard 9V power supply and/or 9V battery, without any special concerns for what is powering it.
The Rangemaster treble booster is one of the simplest positive ground circuits out there, but regardless of how complex the circuit is, following the three simple steps listed above can convert any positive ground PNP circuit to a negative ground NPN circuit. Figure 2 is a Tonebender Mk2 converted to negative ground. This is a more complex 3-transistor fuzz circuit, but to make it a functioning negative ground NPN circuit, we simply change -9V to +9V, switch to NPN transistors, and reverse the electrolytic capacitors.
Figure 2: Tonebender Mk2 in original positive ground PNP format and the converted NPN negative ground circuit (changes highlighted)
Effects of the conversion
The converted NPN negative ground circuits will function the same way as their PNP positive ground counterparts. There is effectively no difference in how the two circuits perform. NPN transistors will be needed where PNP was used, which means that the original transistors cannot be used. The original types used in known pedal circuits tend to be very scarce and expensive these days, and alternate types are very commonly used already.
PNP transistors in a positive ground configuration were often used in germanium fuzzes, while NPN transistors in a negative ground configuration were often used in silicon fuzzes. Knowing how to convert a PNP positive ground fuzz can be useful for converting germanium builds to silicon, where NPN transistors are more common. However, converting a positive ground PNP germanium circuit to a negative ground NPN circuit does not mean that silicon transistors will work. Sometimes they will - the silicon Fuzz Face circuit is essentially the germanium Fuzz Face circuit converted to an NPN negative ground build (with one minor resistance value tweak). On the other hand, both the Tonebender Mk1 and Tonebender Mk2 require some amount of transistor leakage current for the first stage to work. Attempting to use silicon transistors, which have negligible leakage current compared to germanium, will result in the effect not working.
For more information about why germanium transistors are necessary for particular circuits as well as how to tweak the designs for silicon, see our article Silicon and Germanium Transistor Biasing - Part 1.