What is TCI?
Paul Reed Smith admits that part of his latest pickup and the concepts behind it came from an illustrious artist: “John Mayer came after my ass…” We find out more
TCI stands for Tuned Capacitance and Inductance. But what does that mean? The best way of getting a handle on these terms and their implications is to consider not just the pickup but the whole electric circuit, which ultimately includes your guitar lead and the input impedance of your amp and/or effects. As Paul Reed Smith tells us, while he “can’t control whether the player is using a long or a short lead, or the impedance of the input device”, there is plenty to consider onboard the guitar itself.
A pickup, of course, is basically an inductor. Inductance is measured in Henry units (H) and plenty of people believe it to be a lot more helpful in predicting the output of a pickup than the oft-used resistance (DCR), arguing it’s a better indicator of eventual sound. One reason for this is that while DCR measures the resistance of the coil wire, induction takes into account the whole pickup – the magnet, the baseplate,
“This TCI thing has just started. We named it before we understood it. But once we ran into it, it was like, ‘Whoa!’”
the polepiece material. Typically, the higher the inductance, as with a dual-coil humbucker, the bigger and darker the sound is compared with a single coil, which has a lower inductance.
A pickup coil has a capacitance, too, as does shielded hook-up cable, for example. In series, a capacitor blocks lower frequencies and allows higher ones to pass (which is the opposite of an inductor). That’s how a treble-bleed cap is typically wired on a volume control. A passive tone control, however, takes those selected high frequencies and passes them to ground. If you wired a capacitor with a value of .033 microfarads (like the tone cap used by PRS) in parallel between the hot and ground without the pot (a variable resistor), then you’d get a very dark sound. If you did the same but used a much smaller value
cap, such as the 180 picofarads (0.00018 microfarads) that PRS uses for its treblebleed circuit, then it would only pass very high frequencies to ground.
A similarly subtle high-frequency roll-off occurs via the hook-up cable that runs from your pickups and between your controls, switches and output jack.
“Everyone knows the [longer hookup] cable in a Les Paul changes the tone because the SG, with its switch down by the controls, is brighter,” illustrates Paul.
Then there are the actual volume and tone pots (variable resistors), which load the pickup. Of the commonly used values, a 1meg-ohm pot is known as the brightest, 500kohm pots are commonly used with humbuckers, and 250k for single coils. Again, to simplify, the whole circuit acts like a low-pass filter that has a resonant frequency or peak, and that, along with the height or strength of the peak (the amplitude), is what we’re interested in here.
Tuning The Tone
“Pickups have a sound,” says Paul as he begins to explain the thinking behind the TCI concept. “It’s kind of like a tuned note on a Neve console or something. The TCI thing is about tuning these coils, and the pickup as a whole, to exactly the ‘note’ we want it to be tuned to. Now, if the note is too loud, then all you hear is that note. But on a guitar you have lots of different frequencies going on, so it’s how loud that note is, and what is that note? Well, we’ve got our hands around that. The whole TCI thing, more and more, is becoming something that’s standard in this building.
“I want you to imagine I have a whistle,” continues Paul as he plays a single note, a high C# on the 14th fret. “Can you hear the whistle note [the upper harmonics that colour the fundamental]? If I turn the tone control down, that high whistle note is gone, yes?” Paul selects another pickup: “This whistle note is lower [he’s playing the same note, by the way]. So pickups are whistles and you can hear it in every note I play.
“It’s the same thing on a Neve console: you put it to 3.4kHz and you play with the amplitude. It’s a whistle. They were done with capacitors and inductors; you call it an inductive EQ. It’s known that a parametric equalizer is setting the frequency, the Q and the amplitude. That bass pickup on the guitar I was playing there has a capacitor on it that tunes it to the right note – it was too high, so we tuned it down. And it’s also about the amount we want it to be tuned. It’s not just the note; it’s how much of the note. See, I know the ‘whistle’ is a bizarre concept, but it’s because when we’re checking these pickups we’re looking at the resonant note and how loud is it: the resonant frequency and the amplitude.
“It’s like that 1meg-ohm pot on a [post-’67] Tele that took your head off because the tuned note was too loud. Everybody knows that putting a 250kohm pot in a Strat makes it better because it turns the amplitude of the ‘whistle’ down.
“The resistor is the winding; the capacitor is in the coil. You have an inductor into a capacitor and a resistor into the output and if you don’t like the note you can add another one [capacitor]. So here’s your inductor, capacitor and resistor all in series, then you can tune that down by putting a capacitor across that [in parallel]. A capacitor takes out bass, an inductor takes out treble, and the resistor changes the volume of the note.
“See, this TCI thing has just started. We named it before we completely understood it. But once we ran into it, it was like, ‘Whoa!’ It’s not just the type of magnet it has, what kind of wire it has, what the DC resistance is. I’m telling you, my measurements have nothing to do with any of that. I’m looking at the inductance and the capacitance. I went to my people and said, ‘Everything we’ve learned is wrong!’”