THD Flexi-50

December 1, 2004

Tested By Terry Buddingh

Founded in 1987, THD was years ahead of the pack as guitar players began to embrace the tube-amp renaissance that would spawn dozens of boutique amp manufacturers in the 1990s. THD's first amp-a thoughtful redesign of the venerable '59 Fender Bassman-was intended to be a more reliable and lower-noise alternative to its vintage counterpart. THD continued to build its road-ready retro recreations until a new chapter in the company's history began in 2001 with the introduction of the distinctly styled UniValve.

While its perforated steel cage gave it a startlingly retro appearance reminiscent of a '50s-era P.A. amp, the single-output-tube UniValve was a fresh design with many hip features. Following the success of the UniValve, THD introduced the more powerful BiValve. Both models have become recording studio favorites, and now the similarly styled, but even more potent, Flexi-50 is poised to flaunt its virtues in full-fledged gigging environments.

Flexi Flyer
The Flexi-50 is a single-channel, class A/B, all-tube tone laboratory that can accommodate a wide variety of preamp and output tubes. And dig this: Its rear-panel biasing test-points accompany separate adjustment pots for each output tube, so you can properly bias mismatched tubes and even run different types of tubes in the amp's push/pull output stage. Very cool. You might surmise from its moniker and gold plexiglass faceplate that this newest addition to the THD "lunchbox" line is based on a '60s-era Marshall circuit. But, in fact, the Flexi-50's circuit-especially its preamp-has very little in common with vintage Marshall topology. For example, instead of using a cathode-biased first preamp stage ` la Marshall and many others, the Flexi-50 uses an ancient grid-leak bias scheme that hearkens back to the funky combos of the '40s and early '50s.

"I chose grid-leak biasing both for its very high input impedance and the 'feel' it provides," explains THD's Andy Marshall. "The higher impedance makes for a much more sensitive input, which, in turn, allows you to more easily discern the subtle differences between pickups, effects, and cables. The feel-factor of the grid-leak stage is due partly to the asymmetrical nature of its compression. It's quite lovely, and it makes for very fat-sounding clean tones and rich, complex overdrive."

The Flexi-50's tone control section is also quite different from a plexi Marshall. The Treble and Bass controls are derived from a circuit developed by British designer Peter Baxandall in the 1950s, and Andy Marshall adapted the unique Midrange control circuit from an old radio receiver design. The good news is that the Flexi-50's controls respond much more independently than the more interactive tone control circuits traditionally used by most tube amp builders. THD has always been a leading proponent of well-designed, high-quality, printed circuit board construction techniques, which Andy Marshall says helps reduce noise and insure consistency. Inside, the Flexi-50 looks more like an expensive piece of vintage test equipment than a guitar amp. Built to survive decades of industrial-grade abuse, the Flexi-50 serves as a textbook example of how a professional guitar amp should be built.

Flexi Tones

With its stock tubes, the Flexi-50 is indeed configured like a plexi Marshall, so I began my listening tests by comparing it with several '60s-era Marshalls. Despite the Flexi-50's markedly different preamp design, I was surprised at how similar it sounded to our reference amps-only better. In clean to mildly overdriven settings, the Flexi-50 delivered a sweeter and more refined midrange texture, along with enhanced high-resolution detail and complexity. With the Gain boost engaged, it sounded buttery smooth, yielding a gigantic low-end bloom that made our vintage Marshalls pale in comparison. While its effect was rather subtle at lower gain settings, I found the Cut control quite valuable for precisely sculpting the leading edge attack of overdriven tones.

Switching to 20-watt mode reduces the output tube's plate voltage from a studly 475 volts to a more tweed-like 325 volts. Still, this didn't provide the dramatic decrease in volume you might expect. Instead, I noticed a greater change in texture and dynamic response; the sound became smoother and more compressed, with a softer attack and more squeeze factor when playing single-note lines. Swapping a pair of GE 6L6GCs for the EL34s in this mode brought the Flexi-50 a lot closer to eliciting the deeply burnished textures you'd expect from a wide-panel Fender tweed.

This brief look at the Flexi-50 provides just a glimpse of the amp's vast sonic potential. If you're a tone tweaker with a good assortment of tubes, you'll have a blast experimenting with more unusual tube types and combinations in the quest to find a personalized sound that's as unique and fresh as the Flexi-50's design. This is an outstanding amp, and it wins an Editors' Pick Award.


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