There are several ways to control the tone - the balance of frequencies throughout the audible range - of your guitar.
If you have a guitar with two pickups and separate volume and tone controls you can get all the tonal variation you're likely to need. I frequently adjust the tonal balance of my guitar just by mixing the front and back pickups in varying amounts, especially when I'm playing through a DI straight to the PA.
Tone controls come into their own when you need to adjust your sound to better fit the mix or the room. There are several kinds of tone controls.
- Amplifiers designed for nominally-flat response (e.g. acoustic guitar amps, jazz guitar amps, PAs and mixing boards) have a combination of high- and low-frequency shelving EQ and zero or more (but typically no more than two) midrange controls. The midrange controls may have fixed or variable frequency and bandwidth.
- Amplifiers designed to shape the response of the instrument (i.e. guitar amps) typically have a scooped response curve. The response dip may be at a center frequency in the range of 200 to 1,000 Hz, depending upon the design of the amplifier. The tone controls are extremely interactive; the midrange dip moves around a bit depending upon the settings of the bass and treble tone controls.
- Some amplifiers may use a "graphic EQ", either on its own or in combination with one of the other kinds of tone controls. A graphic EQ is characterized by some number of frequency ranges, typically (but not always) spaced at a fixed musical interval such as an octave.
The use of a graphic EQ as the sole means of tone control is rather unusual. The Henriksen jazz guitar amplifiers are one such example. Since this is an unusual configration, I'll be discussing Henriksen's claims regarding this configuration.
With all due respect to Henriksen, he's trying to sell his amp's unique feature - the graphic EQ - as a benefit. Please take my following comments with a large grain of salt, as I've never owned (or played) a Henriksen. The Henriksen amps may indeed be just the thing you're seeking. However, I don't agree with certain of Henriksen's claims.
The problem with all of this is that all of the controls are "eating" part of your signal. In the jazz world we experience this by finding that we have to choose between higher notes that are "thin" or lower notes that are "boomy". Our ear tells us that neither compromise is acceptable. So we spend the gig fine tuning the bass/treble controls in search of the "sweet spot", only to go home unsatisfied. Removing the tone circuitry completely eliminates undue influence by the amplifier and leaves the establishment of tone up to the player and their instrument where it properly belongs.
This paragraph, taken verbatim from the Henriksen web site, mixes a little bit of factual information (that a shelving bass and treble tone stack alone is rarely adequate) with a lot of pseudoscientific nonsense (i.e. that standard tone controls "eat" your signal, and that a graphic EQ is somehow superior to other kinds of equalizers).
Then there's this:
Recording studios abandoned conventional tone controls in favor of graphic EQ decades ago. I saw the need to add a studio quality graphic equalizer to the amp. The purpose of the EQ is not so much to color the tone but to compensate for the environment the instrument is played in. Different venues carry sound in different ways. The EQ circuits on the JazzAmp are set up to increase or decrease 5 different frequency bands by +-10db. What makes a true EQ such as ours different is that is does not have any affect on frequencies outside its band. When you turn up the 300 Hz control on a JazzAmp you are affecting only frequencies between 150 Hz and 600 Hz. Turning it up actually amplifies the presence of that band in the signal path. Turning it down draws that frequency band closer to ground without having any effect on any of the other frequency bands. There is an entire paper on this web site dedicated to the use of our EQ controls.
This one really has me scratching my head. In my experience mixing boards have EQ on every channel on the board. The EQ is not graphic, but rather what is known as a parametric EQ. Parametric EQ combines the shelving bass and treble controls with one or more bands of midrange EQ. Each midrange band gives you controls for - at a minimum - the amount of boost or cut and the affected frequency. That much control is commonly referred to as "semi-parametric". You get "full" parametric control when you add a knob to adjust the "Q" - the "width" (loosely interpreted) of the affected frequencies.
Recording studios do use graphic EQ, but not usually on individual instruments. The most common use of a graphic EQ in a studio is to compensate for monitor frequency response and room acoustics. These graphic EQs usually have 1/3 octave bands, which means that you have over 30 sliders for mind-numbingly precise control.
Let me back up a bit and reiterate: I'm not in any way down on the Henriksen amp. For all I know it's probably an excellent amp. I've seen good reviews.
I have played amplifiers (other than the Henriksen) having 4, 5 and 6 band graphic EQs. I've found the use of "broadband" graphic EQ to be not as musically useful with a guitar as Henriksen's ad copy would have you believe. It's true that you get control over ranges of frequencies that you wouldn't be able to touch with bass and treble controls alone. But each graphic EQ control strongly affects its "center" frequency while increasingly higher and lower frequencies experience progressively less of a change in volume. Visualize a bell-shaped curve with its peak at the center frequency.
With the center frequencies an octave or more apart on wideband graphic EQs, it's impossible to affect those "in-between" frequencies without unnecessarily boosting or cutting unrelated frequencies.
I'm certain that Henriksen has a good argument for his choice of center frequencies and number of bands. His choices may in fact work in a broad range of playing situations (rooms, guitars, etc.); if so, that's all the really matters. But Henriksen's choice of EQ design is not inherently better than any other choice.
On to the next example...
I own a Fender Jazz King and like it a lot. The core EQ is semi-parametric. In other words it has bass and treble shelving EQ plus a midrange control with a variable center frequency. To me, the sweepable midrange frequency is a huge win since the guitar's "speaking voice" lives in the midrange frequencies. Being able to shift the midrange frequency by as little as a half-octave can make my guitar sound better in the mix when I'm playing with my trio.
Yes, it's important to have control over the bass and treble frequencies. These are important more for adjusting the amount of "thump" and "sparkle", respectively, heard from my instrument.
The bass and treble controls also become increasingly important when playing at higher volumes. Typically you need to reduce both to maintain a consistent sound as you increase the volume.
The really smart feature of the Jazz King, though, is the "EQ Tilt" control. Sometimes I'd like my guitar to sound just a bit "thicker" or "thinner". Rather than juggling a couple of different EQ controls to shift the overall balance, a quick change to the tilt control affects the entire range of frequencies. The tilt control works well in practice. This kind of overall response sculpting with a single knob is something that can't be done with either parametric or graphic EQ. Kudos to Fender for introducing this feature.
It's more important to have good EQ control when your guitar sound has a lot of energy in the low and low-middle frequencies. Those frequencies can really make your sound muddy, woofy or otherwise indistinct when overemphasized. If your rig has smaller speakers or less power it's probably not going to deliver as much low-frequency energy, allowing you to get by with simpler EQ.