Blackstar HT-20R MK III 20-watt 1 x 12-inch Tube Combo Amp

When creating a series of amplifiers, the traditional approach would be to start with a preamp design, and then bolt this on to a range of power amplifiers of different power ratings. However, Blackstar acknowledges the interaction between preamp and power amp and the individual requirements that different amps need.

 

Therefore, although the preamps do share many similarities, each and every model has had the same individual care and attention during sonic testing and any required tweaking of the circuitry has been applied to ensure each model performs to Blackstar's high tonal expectations.

The MK III Clean channel is a very sophisticated circuit, albeit deceptively simple looking, that is extremely versatile for producing a range of classic clean and crunch tones. The two different voicings use both parts of an ECC83 (12AX7) tube exclusively for producing compression and crunch.

The HT MK III overdrive channels are the next in an evolution from Series One, HT MK I through to HT Venue MK III. The topology shares the same basic building blocks in the gain structure and tonal shaping and includes the cascaded gain stages of an ECC83 double triode valve. However, each amp has had the overdrive tones specifically tweaked, tuned, and refined during hours of listening tests, to optimize the tone for that model.

 

An important aspect of the Overdrive channels is how well the tone cleans up when the guitar volume is backed off. In conventional valve amp designs, the treble response of the overdriven sound is shaped by a series of cascaded first-order low-pass filters placed between each of the valve stages, which are being overdriven, which often leads to a droopy filter response and a dull or lifeless backed-off tone. In the HT MK III Overdrive channels, the majority of the high-frequency shaping of the distortion is done using a second-order low-pass filter. This has the advantage of removing the unpleasant high-frequency fizz without attenuating the signal below the cutoff frequency.

The conventional three-way guitar EQ network, or tone stack, is a key to the characteristic voice of traditional guitar amplifiers. Classic amplifier designs have their own versions of this network, which leads to their own tonal voicings.

 

The passive ISF control allows you to infinitely adjust the response of the tone control network between any of the traditional voicings in a way that has never before been possible, for everything from a classic American to vintage British tonal range. The ISF control is interactive, moves the whole tone stack at once, and is continuously variable.

The Voice 1 setting (LED off) has been based on a classic American clean sound, a mid '60s black-face Fender Twin Reverb. This has high headroom, and a bright clean sound, with solid low end. On the HT-20R MK III models, the power amp feedback is automatically adjusted to add presence and sparkle, as well as tightness to the full bottom end.

 

The Voice 2 setting (LED on) has been based on a much loved vintage Class-A British tone (Vox AC-30). The gain structure is reconfigured to now put the EQ shaping after both valve stages and the power amp is set to have no negative feedback. These, and other carefully voiced parts of the circuit, result in a warm tone with a chimelike top end and relaxed resonant bottom.

Switching to Voice 2 (LED on) on the Overdrive channel will add more gain for the overdrive and reduce power-amp damping, giving livelier highs and more resonant lows. Overall this changes the character of the overdrive from a classic tone to a higher gain, more contemporary tone.

While a traditional, possibly old fashioned, spring reverb is often a choice for tube amps and can be great for Fender-style clean tones, Blackstar felt this wasn't the best choice for modern overdrive tones. This is why they developed their own digital reverbs specifically voiced for both clean and OD tones.

These exclusive Blackstar designs respond well to both the dynamic nature of clean sounds and the more compressed, harmonically rich textures of crunch and high-gain overdrive sounds. In addition, the Reverb Time can be adjusted for Clean and OD channels independently using Blackstar’s Architect software. This level of choice and flexibility simply would not be possible with a mechanical spring reverb.