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DIY walkthrough: You don’t necessarily need to perform the steps in this order—it’s just one method.
Since I have no idea how your guitar is currently wired, I’ve started with a fresh set of pots. My demo guitar is a three-knob Hamer 20th Anniversary, which has its pickup selector in the control cavity alongside the pots [Photo 1a.]
In Photo 1b I’ve completed the ground connections as described above, though I’ve connected the output jack’s ground to the back of the treble pot. The white wire is the pickup selector output, connected here to lug 3 of the treble pot. (Remember: When viewing pots from the back, lugs down, lug 3 is on the left, and lug 1 is on the right.) This wire usually connects to lug 3 of the volume pot, but this circuit routes the signal through the bass pot first.
In Photo 1c I’ve added the treble-cut capacitor between lug 2 of the treble pot and ground. I used a .022 µF (also known as a 223). For more cut, try a larger value, such as .033 µF (333) or .047 µF (473). The larger the cap, the greater the cut.
Photo 1d adds the bass-cut components. Since the output from the pickup selector must feed both tone pots, I’ve run a wire from lug 3 of the treble pot to lug 3 of the bass pot. I’ve added a .0015 µF (152) cap between lugs 3 and 2. Here, pot values work in the opposite direction: the smaller the cap, the greater the bass cut. If the .0015 µF sounds too extreme, try stepping up to a .0022 µF (222). I’ve added a wire to the bass pot’s lug 2, which connects to the volume pot’s lug 3. Connect the volume pot’s lug 2 to the output jack, and you’re done.
The four-knob version. The procedure is similar for four-knob guitars. The only difference: Since the volume pots are upstream from the pickup selector, the signal runs directly from lug 2 of the bass pot to the output jack, as opposed to being routed through a volume pot.
A reverse-log bass pot? While you can get fine results using your guitar’s extant pots, the original G&L circuit calls for a 1M reverse-log pot (the “C” in C1M signifies reverse-log). With a standard audio-taper pot, the effect comes on quickly near the top of the pot’s range. With a reverse-log pot, you get a gradual onset of the bass cut that may be easier to fine-tune. The problem is, it’s almost impossible to find a C1M pot in a standard 24 mm format. You can get a 16 mm version from stompbox parts suppliers, but it won’t work in Les Pauls requiring long pot shafts. After experimenting with various options, I’ve gone back to a standard 500k pot, because when I reach for that control, I usually want the lows to evaporate quickly.