Rocktron ValveSonic Plexi

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Resulting from a partnership between Rocktron and Frank Lamara of LA Custom, the ValveSonic Plexi is an all-tube preamp designed to replicate the tones of British amps from the ’60s thorough the ’90s. To cover such a wide range of sounds, the 1U rackmount unit relies on a combination of switches to steer the tones toward particular eras of amplifiers. For example, the Tight 1-2 switch affects the input of the first tube stage, providing a modern-type response in the “1” setting, and warmer, vintage-style response in the “2” position. An accompanying switch labeled “Tight” firms up the sound when set to the up position. Following these is a 6-way rotary Bright switch that adjusts the overall sound from darker to brighter. The Gain control’s range is dramatically affected by the Boost and Boost + switches: The manual states that with Boost off the overall gain is lower, as per earlier amps, while activating it brings the Gain control’s range in line with models from the ’80s. The Boost + switch takes things into much higher amounts of distortion, which amps from the ’90s and later were designed to deliver. And if even more overdrive is needed, the Afterburner switch activates another gain stage post EQ, which is controlled by the Drive knob.

Tone shaping is accomplished by Bass, Middle, and Treble controls; a High Cut knob, and an EQ 1-2 switch that gives a choice of modern- or vintage-style response curves. Lastly, a Master Volume adjusts the final output of the preamp.

Hooked up to a Mesa/Boogie power amp driving Marshall 4x12 and 4x10 speaker cabinets, the ValveSonic Plexi was easy to dial in for everything from moderate crunch to mammoth sustain. In general, by keeping the Tight switch off and Tight 1-2 in the “2” position, leaving all the Boost and Afterburner functions off, and putting the EQ switch in its “2” setting made it possible to get tones that were in the camp of a mid-’60s tube-rectified Marshall JTM-50. Using the alternate setting on the Tight and EQ switches took things closer to an early ’70s JMP 50-watter, which sounded brighter and tighter in comparison. With all of the Plexi’s potential, I didn’t find it necessary to use anything more than the first Boost position, and with the Gain rolled down to around 10 ’o clock, the grind was similar to what you get using a booster pedal on an old Marshall, or what a JCM 800 gives at full tilt. North of that setting, the distortion and sustain increase tremendously, and let’s just say that if you’re into extreme levels of grind, switching to Boost + will deliver the goods.

Going at it a completely different way, the Afterburner function can be dialed for sounds that are reminiscent of overdriving the output section of a tube amp, and by using a lower Gain setting before the EQ and a higher Drive setting at this post-EQ stage, the grind was delivered with a splintering attack and the kind of touch responsiveness that comes when the amp’s power tubes are really sweating and contributing their own harmonics—as opposed to the more compressed thing that happens when they’re being fed tons of distortion from the front end. I think this addition to the Plexi’s many functions was a smart one.

The ValveSonic Plexi is very capable at delivering Brit-voiced amp tones over an exceeding wide range of gain and suffice to say that if you’re into a rack setup, and want the ability to use any power amp that fits the need, the Plexi is definitely worth investigating.


Product ValveSonic Plexi
Price $799 street
Channnels 1
Controls Tight and Tight 1-2 switches, 6-position Bright switch, Gain, Boost and Boost + switches, Bass Middle, Treble, Afterburner switch, Drive control, EQ 1-2 switch, High Cut, Master, on-off switch. [Rear panel] Line and instrument outputs (unbalanced), secondary Input jack.
Tubes Four 12AX7
Weight 5.5 lbs
Built Korea
Kudos Extremely wide range of British-voiced overdrive tones
Concerns No effects loop.