The Pete Cornish G-2 is a Big Muff variant: germanium diodes, double buffered, and sporting a low-pass filter instead of the usual Big Muff tone stack.
That being said, this is not your standard Big Muff - there's little resemblance in sound, largely due to the lack of scooped mid-range provided by a standard Big Muff tone control, and the germanium diodes change the clipping profile quite a lot. Do not expect this to sound like a Big Muff.
There are so many opinions on how this should be built - but keep in mind that the same topology is used for any variation. The main complaint is that it's too dark-sounding using stock values, so builders try to address this by making various changes.
Now we have some new traces from Aion, which probably puts all of this to bed.
Some people change the diodes too, but once you get away from germanium diodes, you may as well make a Pete Cornish P-2. Pete Cornish uses the same PCB for both; one uses silicon diodes and is supposedly based on a Rams Head-era BMP. The other is closer to a BMP from the former Soviet Union but with germanium diodes (or perhaps glass-encased Schottky).
The layout below has the same values as the Dirk Hendrik trace; alternate values are all listed below. I'll update shortly to the Aion trace, bit for the moment, you can work it out or head on over to Dirt Box layouts.
This needs the standard Cornish buffer in front, and should be wired as buffered bypass if you want to stick to the Cornish program. If you don't plan on using the buffer, it's probably a good idea to drop that 220n cap on the input back to 100n.
NOTE: It does have one buffer on the board, as the G-2 is double-buffered.
PETE CORNISH G-2 - VERO LAYOUT
PETE CORNISH G-2 BUILD PICS
PETE CORNISH G-2 - SUGGESTED ALTERNATE VALUES (BY OTHERS, NOT ME)
- 10n cap on the input buffer - reduce to 1n (brightens the signal from the buffer)
- 220n caps down to 100n
- Collector and emitter resistors on input boost stage, change to 18k and 100ohms
- Reduce 4u7 caps down to 2u2
- All 1nf caps down to the standard 470pf Big Muff values
Bit of a belated comment, but I've begun to suspect that the G-2 doesn't use germanium diodes. I've measured about 20 different types of germanium diode, and all of them at least 12 times more leakage than I would want for a germanium diode in a transistor feedback loop. Cornish seemingly doesn't have that issue, so I went back and looked at Dirk's original tracing thread for the Cornish G-2. Looking at the trace in turn led me to wonder whether the G-2 might actually use a DO-35 or similar glass package Schottky diode, perhaps something like the HP2835. As your post noted, Dirk was not able to determine what model of diode was used, but the pictures show a glass package diode and measurements showed a Vf of roughly .22 volts. That Vf measurement is on the low side for a germanium diode, but it's well in spec for a Schottky diode. Moreover, using a Schottky would eliminate the leakage issue that is a fundamental trait of seemingly every germanium diode that I've encountered I know that Cornish describes the G-2 as having "warm Germanium qualities," but as Kevin at Aion and others have documented, Cornish also has a track record of exaggerating and making what I would say are misleading statements about the circuitry inside of his pedals. I'd be curious to hear what you think on that point.
ReplyDeleteHey PedalBuilder,
DeleteI think you make some very valid points, thank you for sharing - you are quite possibly right about all of that.
I've not played with a G2 in ages, largely because they never sounded as good as the demos. I'll have to have another look.