Vox CoolTron Pedals

July 15, 2005

Solid-state products are also easier to mass-produce, and therefore cheaper to manufacture. So why then do we have this seemingly irrational obsession with tubes? Perhaps the best reason is that tubes impart their own unique musical character—a character that guitar players (and other tone connoisseurs) find irresistibly compelling and difficult to forsake for mere convenience and budgetary concerns.

Vox obviously understands our insatiable lust for tubes and tube tone, and how that passion can extend to stompboxes as well as amps. That’s why each new CoolTron pedal proudly sports a 12AU7A—which is highlighted in dramatic fashion by the cool glow of a blue LED.

Tube-loaded stompboxes have been around for decades, and while some have featured conventional high-voltage tube circuitry, and others operated the tubes under very low voltage “starved plate” conditions, their designs have always been tethered by the need for some form of AC power. But thanks to some clever design work—including a special low-current filament/heater “chopper” power supply—the CoolTrons can operate for about 15 hours powered by four AA batteries.

The CoolTron concept began with a circuit developed by Fumio Mieda, designer of the legendary Univox Uni-Vibe. Mieda had been experimenting with an innovative “servo bias” scheme that extracted more performance from a tube running at low voltages, and a more refined version of that circuit now forms the keystone of the CoolTron design. Veteran amp designer Steve Grindrod—who now heads the R&D team at Vox—designed the CoolTrons around this unique, battery-friendly, performance-enhancing approach. With the combined talents of two of the most experienced designers in the history of musical electronics, it’s natural to expect the CoolTrons to be something special. Indeed, the three pedals represent a significant advancement in the evolving art of stompbox design.

Made in Japan, the CoolTrons emanate a boutique pedal vibe with their chrome-plated steel enclosures and cool silk-screened graphics. They feel quite substantial, too, weighing about 2.25 lbs each. Measuring approximately 6" x 6w", however, they don’t hog as much space on your pedalboard as some tube-equipped stompboxes. Inside, the construction is crisp, clean, and modern, with three securely mounted PC boards that are peppered with an abundance of surface-mount components.

The Cooltrons’ 12AU7A dual-triode tube is better suited for low-voltage operation than the more familiar, though closely related, 12AX7. The tube’s two triode sections are tied together, so it functions as a single tone-enhancing gain stage. (For prolonged battery life, the tube’s plates, as well as its heaters, run at approximately 3 volts DC.) This tube stage “building block” is essentially identical in all three pedals, while the rest of the circuitry—which is a mixture of op amp ICs and discrete transistors—is completely different in each design.

Big Ben Overdrive

With Gain, Volume, and Tone controls, and a single footswitch, the Big Ben is the simplest of the CoolTron designs. Op amps are used for the Gain control and soft clipping stages, which are followed by the tube stage. Following the tube stage, discrete transistors buffer and isolate the passive Tone and Volume controls. This simple circuit gives Big Ben an organic and natural tube-flavored tone that never sounds buzzy, as well as a lively dynamic response that never sounds squashed or overly compressed.

With the Gain knob fully counter-clockwise and the Tone pot wide open, Big Ben sounds virtually indistinguishable from its bypass tone. This provides a perfect starting point for those who want to sneak in just a taste of overdriven texture by slowly increasing the Gain setting, while preserving their amp’s natural tone and dynamic response. On the other hand, wide-open gain settings still sound convincingly amp-like, and the simple, yet surprisingly effective Tone control keeps the higher frequencies smooth and mellow. The Big Ben is a great choice for old-school tone hounds who want to wrench a little more tube goodness from their vintage-style amps—especially when playing at lower volume levels. It even sounds great when used as a direct-recording device (as it conveys authentic tube-seasoned warmth and color without an amp), and it can be used as a clean boost pedal/preamp, as well. The Big Ben Overdrive is the CoolTron most likely to cause a stir with more traditional players who enjoy browsing the latest boutique pedal offerings.

Brit Boost

With its Gain, Volume, and Tone controls, the Brit Boost may appear to be similar to the Big Ben Overdrive, but it actually has more in common with the classic British-made treble boosters from the ’60s and ’70s. Like the legendary Dallas Rangemaster and the Hornby Skewes and Vox treble boosters, the Brit Boost’s first stage is built around a single transistor. (While some early boosters used germanium transistors, the Brit Boost uses a silicon type.) The Brit Boost actually has two of these circuits: one voiced with the classic “upward tilt,” and the other providing a more balanced full-range response. (The Treble-Boost/Full-Range footswitch selects between these two independent circuits.) Following the selected one-transistor boost stage, an op amp controls the Gain that’s fed to the tube, and the circuit finishes with another op amp driving the active Tone control, which is followed by a passive Volume control.

Even with the Gain knob set very low and the Full-Range mode selected, the Brit Boost exhibits a more aggressive voice than the Big Ben Overdrive. It also has a more distinctive and prominent midrange voice, and more grind and grit to its texture. Switch to Treble Boost mode, plug it into a Vox AC30’s Normal channel, and you’ll immediately relive Brian May’s glory years with Queen. This combination provides a tone bigger than Wembley Stadium, with a taut and percussive pick response, and tons of exciting top-end sizzle and ringing presence. The well-voiced Tone control is effective in both modes, but I found it especially useful for taming the Treble Boost mode’s highest frequencies. As this is one important feature that the vintage treble boosters lacked, why search eBay for an overpriced relic when you can get more tonal flexibility—and an overall smoother tube-enhanced sound—from the Brit Boost?

Bulldog Distortion

The most versatile, and also the most modern of the CoolTrons, the Bulldog Distortion offers two contrasting channels with independent Gain and Volume controls, shared Bass and Treble knobs, and an additional Voice control for the second channel. The first channel uses op amps in a soft-clip circuit that’s reminiscent of the Big Ben’s (though tailored for more distinctly modern tones), and the second channel also uses op amps, but in a circuit designed for more aggressive and hard-edged clipping. The output of the selected channel drives the tube stage, which drives a unique Treble and Bass tone control design that’s a combination of passive and active topologies. The second channel’s Voice control follows the tone controls. It features a passive tone-sculpting network, which is followed by separate passive Volume controls that are electronically engaged when the appropriate channel is selected.

The Bulldog’s first channel sounds more aggressive than the Big Ben, with more upper-midrange sizzle, and the potential for more penetrating top-end sting. The Bass control can add some impressive low-end girth for wallowing low-voiced riffing, and reduced Treble control settings can eliminate the cheesy-sounding top-end fizz that so frequently accompanies and plagues higher-gain tones. This channel also imparts its own distinctive character to the tone, which is clearly more modern than vintage. With its gravelly midrange grit and crushed-glass crunch, the Bulldog’s second channel provides the most modern tones of the CoolTron trio. The time-warping range of its mid-scooping Voice control is particularly impressive, as it can take you from Link Wray to Metallica with the twist of a knob. Covering an impressive range of tones, the Bulldog Distortion represents an astounding value for those seeking more modern distortion tones from a truly cool pedal.

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