You're on stage and you're sweating. Your guitar feels heavy. You feel the eyes of the crowd on you, but worst of all, the most excruciating part of the whole ordeal is the fact that your guitar sounds like a bucket of mud coming out of of the backline amp they swore was a Marshall. No matter how the EQ is set, every note languishes out of the speakers. You futz with your tone knob, but it goes from slop to sludge. If only you could control the bass output right from your guitar...
Most people know Leo Fender as the guy that started Fender, invented the Strat & Tele, designed the Deluxe, Twin, & Super, and changed the face of music forever. For most people, his story ends in the mid 60s as he was phased out from the now CBS owned Fender corp. Leo was no slouch though. He kept working, designing, experimenting, and trying to push the technology behind the electric guitar to new frontiers.
During his time at his last venture, G&L, Leo designed an update to the passive treble cut tone circuit that he had made standard on guitars a couple decades earlier. By adding a second potentiometer and flipping the circuit, you can add a second, independently adjustable bass cut to your guitar. This can thin out your sound removing mud from the signal and polishing up the jangle on treble forward guitars. I sometimes refer to this as the "vintage" knob as it cuts the same frequencies that the overwhlemed engineers of the 50's & 60's would have cut (on the amp more often than not) to shoehorn a guitar into a mix.
How does it work? Well, first lets look at the standard tone control. Resistors RESIST a signal across the frequency spectrum and make it more difficult to pass through. Capacitors ALLOW HIGH FREQUENCIES to pass and block others. A potentiometer can be thought of as two resistors connected in series (one after the other). This is called a voltage divider. The benefit of a potentiometer is that we can select the ratio of the resistors in series. We can also use it as a single variable resistor by only connecting the first and second lug. A passive treble cut circuit connects the output to a capacitor to ground via 2 lugs on a potentiometer. By turning the knob, we can change how much resistance will go between the signal and the capacitor. The more resistance present, less of the high frequencies will be sent to ground via the capacitor. The capacitor allows us to select what range of frequencies we send to ground. A 68pf capacitor will send very little audible signal to ground resulting in a barely noticeable changes in tone. A 680nf capacitor will send nearly all of the signal to ground and leave us with only the lowest bass frequencies when the control is at a minimum. 47nf capacitors are standard in many guitars, but different pickups may require different choices in frequencies to be cut. Enough of the old shit, onto the new.
The passive bass control works in almost the opposite way. Instead of sending that capacitor to ground to discard the signal, we'll be sending that capacitor to the output to keep only the highest frequencies of the signal and to block the rest. We don't always want the bass cut though, so we can connect Lug 1 & 2 of a potentiometer to each side of the capacitor to make an adjustable full frequency bypass. With the setting at 10, there is no resistance between the lugs of the pot and the cap is completely bypassed. When the setting is at 0, we have maximum resistance in parallel with the capacitor preventing all frequencies from flowing past the cap and forcing only the high frequencies through the cap to the output. One bonus with these controls is that they won't passively change your signal in the way that a treble cut would. Lug 1 & 2 in a treble cut circuit still act as a resistor at maximum and send a small fraction of the high end to ground. Fenders for instance use a lower value pot in order to tame the bright pickups. With the bass cut at 10, we have a direct connection from the volume control to the output and the circuit is functionally disconnected. The tradeoff is that the value of the bass cut potentiometer determines the amount of bass content that passes when the knob is at minimum. Next, we'll talk about how we can use this circuit to replace the standard Jazzmaster rhythm circuit.
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