The “Memphis Science” of Volume and Tone Controls
The Historic Line from Gibson’s Memphis division not only brings legendary semi-hollowbody guitars to life, but also pays attention to a detail that few, if any, other guitar manufacturers have considered: how to take advantage of inherent differences among level and tone potentiometers (a potentiometer is a type of variable resistor).
The reality of resistance. Even quality potentiometers aren’t necessarily precision devices. Typical pots have a plus or minus 10% tolerance, so a 500,000 ohm pot can measure as little as 450,000 ohms (or even less if the pot is out of spec), which doesn’t make much difference when changing volume in a radio or TV—but can make a big difference with passive guitar pickups.
Passive pickups produce relatively low-level signals, and ideally, don’t want to connect to components with resistance (like pots) or capacitance (like cables). These can “load down” the pickup, reducing level and high frequency response—it’s like driving a car with the parking brake not quite fully released.
Ideally there wouldn’t be any loading, but not only is that not possible, with no loading the electronics can act almost like an antenna and pick up radio frequencies, “dirt” from computers, and other interference. So, the object is to choose level and tone controls that are low enough not to act as antennas, but high enough not to affect tone negatively.
The Memphis division found that with their magnetic pickup and wiring configurations, 500,000 ohms for the volume and tone pots is a “dividing line” for loading. While lower values can affect the sound, the loading from higher values is so slight most musicians won’t notice it. So, to insure that the pots provide minimum loading, Memphis specifies a custom-made 550,000 ohm pot (Fig. 1).
Fig. 1: Volume and tone pots (and "bumblebee" capacitors), ready to rock a 1959 ES-330 from the Historic Line.
But wait…there’s more. You might think the story would end there, but Memphis takes an additional step by measuring each pot’s resistance. This not only provides quality control to verify all pots are within spec, but Memphis classifies each pot as having a resistance more toward the higher or lower end of the specified range of values (Fig 2).
Fig. 2: Jim Lillard of Gibson Memphis preparing the pots for assembly.
Those higher resistances are used with the neck pickup, as that’s where you want maximum high frequency response to reduce muddiness. Those with lower resistances team up with the bridge pickup, which “sands off” the edge ever so slightly. The difference is very subtle, but can make a difference to those who are sensitive to tone.
So the next time you play a guitar from the Historic Line, if it seems that the pickups have an unusually good tone…now you know at least part of the reason why.
The Memphis Historic line consists of seven different re-creations of legendary guitars. For more information, go here.