Bolt BTH100 3-Channel Amp Head

Item: XPBTH100HEAD

Despite the many advances in amplifier technology that continue to raise the bar for tone, there’s still room for improvement. For over two years, Bolt Amplification has been probing both the weak areas in amps, and the strengths of various technologies, to determine if a fresh approach could re-define the meaning of "tone" with guitar amps. The Bolt amp is the result of that research.

Vacuum Tubes: Pitfalls and Potential: Guitar players love tubes because no existing DSP can really deliver the totality of the "tube sound." With Bolt, we put tubes where the amp meets the outside world: The input preamp stage, and the output power stage. These parts of the signal chain are where tubes have the most importance, and contribute the most to not only the amp’s sound, but the "feel" as well. Having a tube at the input provides a load that interacts with the guitar, where the impedance changes as you play. This is something that simply can’t be "modeled" in software. Another essential element of the tube sound is an all-tube power amp, including the driver. The low damping factor of the output tubes, coupled with the output transformer, allow speakers to physically move more air. Unlike solid-state amps, this colors the sound in a way that guitarists prefer. Another advantage of tube output stages is that they generally cut through better at loud stage volumes than solid-state amps. There’s a solid technical reason for this: In addition to the natural coloration that adds brightness, and therefore gives a louder perceived sound, tube amps clip in a more desirable way when you push them into distortion. Solid-state amps, when clipped, give a harsher, less appealing sound, and therefore guitarists are reluctant to push them as hard as they push tube amps.

However, using tubes doesn’t guarantee you’ll get the tone you want. An amplifier represents a complex interaction among tube, speaker, gain, cabinet, and more. The settings where that interaction is at its best is called the "sweet spot"—the most desirable sound an amp can make. Unfortunately, this is generally at a high volume, and lowering or raising the level moves away from the sweet spot, degrading the tone. While amp modeling has come a long way in terms of sound quality, many (if not most) guitarists feel that the experience of playing through a tube amp at its "sweet spot" has not yet been achieved by modeling technology.

Our priority with the Bolt amp was to be able to hit the "sweet spot" tone at any volume. But, tubes don’t work that way by themselves; they have very specific ranges where they sound "right." Enter digital technology. The Bolt amp uses digital technology to monitor multiple parameters responsible for the "tube sound," including gain, damping, and other crucial elements. After identifying the Bolt amp’s natural sweet spot, the digital monitoring circuits compensate to attain the same tonal characteristics regardless of the level setting. Playing through a Bolt for the first time is uncanny: It sounds "right" at any volume. Of course, you can vary the sound in many ways—there are three channels, for clean, crunch, and distortion—but these variations always occur within the range that tubes prefer. A useful analogy is a car’s onboard computer. It doesn’t have anything to do with the moving parts that make a car go, but monitors the fuel injection, timing, carburation, and the like to produce the optimal efficiency whether going slowly down a country road, or opening up the throttle on the highway. This analogy applies to Bolt’s use of digital technology to ensure the tubes are continually operating in their "sweet spot."

After perfecting the input and output stages, the next step was to optimize everything in between. We wanted EQ that could "paint with a broad stroke," like classic passive EQ, but also be able to dial in precision sounds for the studio, or when playing in venues where you need to tweak your tone carefully to match changes in acoustics. After trying and discarding numerous approaches, we found that digital signal processing (DSP) was by far the best answer, for several reasons. Today’s DSP is incredibly powerful, but rather than using it to make lots of effects, we dedicated virtually all the power to tone and reverb. There is ample processing power to create precision, transparent EQ that complement the tube stages, as well as reverb that delivers the same type of spring sound as conventional guitar amps, along with studio-quality plate and hall sounds. Each type is selectable per channel for example, your clean channel can have a spring sound, but you can put a hall sound on the lead channel for a "big" solo sound. The DSP also provides "virtual tubes" within the EQ blocks. Unlike modeling, which usually refers to emulating an amp’s sound, these virtual tubes are simply algorithms that make the DSP behave the same way an ideal tube would. It’s the actual input tubes, output tubes, and output transformer that do the majority of the work in creating the Bolt’s tone.

Why 96kHz? A/D and D/A converters have reached maturity, with 24-bit/96kHz converters approaching theoretical minimums for noise and distortion. Only a few years ago, the quality of all but the most costly converters often caused guitars to have a "digital" sound. However, with today’s converters, digitizing a guitar signal is no longer an issue, particularly because the tube sections have so much influence over the sound. What really makes a difference, though, is using 96kHz sample rates. In recording, there’s a lot of debate about whether recording at 96kHz makes a difference on playback. We won’t comment on that controversy, but we believe virtually no one would deny that for live use with guitar amps, 96kHz makes a significant improvement in sound quality compared to 44.1kHz or 48kHz. It’s not because we can hear that high, but because the better transient and high frequency response makes a difference in the audible range in terms of faithfully reproducing a tube’s complex harmonic structure. This isn’t voodoo, many plug-ins for computers let you double the internal sampling rate for better sound, with the tradeoff being more processor load. The sonic difference is not subtle. Given the processor power in the Bolt, it’s well worth running the DSP sections at 96kHz so the sound quality can match that of the tube sections.

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