Q-Filter

The Q-filter is a noisefree inductor with exceptionally low resistance, encased in a 1” cube for easy installation, that is used on the tone control to reduce the impedance of the pickup overall or only in the highs. A traditional tone control uses a capacitor to preserve lower frequencies while steeply rolling off the response above. The Q-filter can add substantial versatility by lowering the highend volume while preserving and extending the range of response, like using a lower inductance pickup for your highs.

This is especially effective with a bridge pickup, since reducing the upper midrange exaggerated by the pickup location lets you keep the consistent lowend character with a well balanced highend free of harshness or edginess that often are the result of higher pickup inductances and higher capacitance cables. You can also use the filter by itself with the neck pickup to get the full range of sweet highs and even response out, regardless of pickup inductance or cable capacitance.

Bill had used both the Q-Tone (“EQ filter”) and L-Tone (inductor only) wirings since the ’60s and highly recommended using them with humbuckers over “coil splitting” for greater versatility and noisefree performance. They were famously used in the guitar he designed for Gibson back in the 70s -- the L6S. 

Inductance: 1.8H, 3.0H Bass

See here for more in depth information and wiring diagrams for the Q-filter.

Some comparisons between a conventional tone and a Q-filter wiring:

Including comparison to a neck pickup blend option: