Before we begin and talk about Jazzmaster controls, we should point out that this will be for the traditional layout as there have been a few specialized models over the years such as the Squier Vintage Modified Jazzmaster Special and the Fender Blacktop Jazzmaster HS, not to mention artist series like the Lee Ranaldo and Thurston Moore models.
When the guitar was introduced in 1958 as Fender’s new top-of-the-line instrument, one of its several notable firsts was an elaborate dual-circuit design with separate volume and tone controls. The “lead circuit” had fairly standard controls for a Fender guitar—single master volume and master tone control knobs on the lower bout, and a three-position pickup selector toggle switch on the same side of the guitar on the upper bout. The Jazzmaster, however, also featured a warmer, darker-sounding second circuit—the “rhythm” circuit—with its own separate volume and tone controls on the upper horn in the form of two inset control wheels. A small slider switch on the upper horn let the player choose between the lead and rhythm circuits.
The Jazzmaster, then, had a lot of controls. Traditional models have kept this same design all along, as seen today in the Fender American Vintage ’65 Jazzmaster and Classic Player Jazzmaster Special, and the Squier J Mascis Jazzmaster and Vintage Modified Jazzmaster.
The Lead Circuit
The Jazzmaster’s lead circuit is pretty straightforward. As noted, there’s one master volume knob and one master tone knob on the lower bout. The pickup selector switch gives you the neck pickup alone, both pickups together or the bridge pickup alone.
The lead circuit is activated by setting the upper horn slide switch in the down position; that is, toward the neck.
The Rhythm Circuit
Moving the upper-bout slider switch to the up position—that is, away from the neck—activates the Jazzmaster’s rhythm circuit, which is entirely separate from the lead circuit.
In the rhythm setting, the bridge pickup is deactivated and only the neck pickup is operational. More than once, an unaware guitarist has mistaken a Jazzmaster in rhythm mode for one with a faulty bridge pickup. So let’s be crystal clear about this—when the guitar’s slider switch is in the rhythm (up) position, only the neck pickup is on.
The rhythm circuit has its own volume control and passive tone control on the upper horn—the two inset wheels next to the slider switch. The wheel nearest the slider switch is the volume control. Further, in rhythm mode, the pickup selector switch and lower-bout volume and tone control knobs become non-functional.
There are two ways, then, to set a Jazzmaster so that only the neck pickup is heard. First, in lead mode, you can set the pickup selector switch to neck-only. Second, you can switch the guitar into rhythm mode, which is neck-pickup-only by design (the bridge pickup becomes non-functional in this mode, remember).
Here’s the thing, though—you might suppose that the lead circuit neck-pickup-only tone and the rhythm circuit neck-pickup-only tone are the same, but they’re not. The rhythm circuit tone—which is neck-pickup-only, remember—is noticeably darker than the tone you get when the Jazzmaster is in lead-circuit mode and the pickup selector switch is set to the neck pickup only.
This is because the Jazzmaster doesn’t use identical potentiometers for its two onboard tone controls. The rhythm circuit tone control on the upper horn uses a potentiometer with a different electrical value than the one used for the lead circuit tone control on the lower bout. The non-identical electrical values of the two potentiometers is what accounts for the differing neck-pickup tones.