For each of the 330's two pickups there are two knobs that control tone and volume. However in 1961,, Electronic History of Rickenbacker Guitars a fifth "blend" knob was added to the guitar to expand the possible adjustment of the guitar's tone. This knob has several different functions, such as acting as a tone blender between pickups and as a general EQ between the bass and treble tones of the guitar. Its primary function, however, is to serve as a bass pickup volume equalizer, allowing for the generally louder neck pickup to be reduced to comparable levels of volume with the bridge pickup. The knob is a potentiometer wired in as a variable resistor between the neck pickup and the neck pickup's volume knob.http://www.rickenbacker.com/pdfs/19511.pdf Since it is only wired into the neck pickup side and not in the circuit of the bridge pickup, it only affects the volume of the neck pickup in both the neck and middle switch positions and has no effect in the bridge position. Turning the knob up (clockwise) increases the resistance — more resistance is more volume reduction prior to the signal reaching the volume knob. The potentiometer value on older models was 500k offering variable resistance from 0 - 500kΩ. More recent productions have a 330k potentiometer which offers a variable resistance from 0 - 330kΩ.
