Hands-On Review:THD Boutique Amps and Hot Plates

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THD Boutique Amps and Hot Plates

Welcome to the house of tone

By Benjamin Hansen

The pursuit of ultimate tone often takes guitarists to some funny places. Stock guitars will not do. The pickups have to be switched for aftermarket or even custom-wound upgrades, and the factory pots and capacitors swapped for fresh ones with different values. One amp is not enough unless you've got an amp modeler to run through it as a preamp, and the stock presets most likely won't be getting any stage time at all. Other times it's as though all the stars align over a particular piece of gear and it's perfect right out of the box. How does that happen?


THD Electronics: Welcome to the house of tone Although you might not be able to stop chasing the perfect guitar, your search for the perfect amplifier is over thanks to THD Electronics. The pursuit of ultimate tone led Andy Marshall, music equipment and electronics wizard extraordinare, to found THD in the mid 80s. Since 1987 the Seattle-based outfit has been hand-building phenomenal guitar amplifiers that make guitarists weak in the knees—no modeling necessary, no mystery involved.


Meet the family
THD now makes three amplifier heads: the UniValve, the BiValve-30, and the Flexi-50, plus the Hot Plate, which is a power attenuator for guitar amps. The latter lets you play your amp full blast at volume levels that won't cheese off the neighbors, wake up the kids, or irritate your spouse. You simply place the Hot Plate between your amp and speaker cabinet and wail away, letting it absorb a big chunk of your amp's power output while passing along all the hot tone to your cab. The UniValve, BiValve-30, and Flexi-50 are all-tube guitar heads incredibly responsive to control or guitar changes and amazingly touch sensitive. Plus, they're so thick with features and sounds you'll think you're dreaming. If you can't find your personal ultimate tone with THD, your ears might be broken.


The UniValve is the smallest member of the THD clan, but in wattage only. It has all the big tone and features of the BiValve-30 and Flexi-50. It has a single-ended Class A circuit (that's right, only one power tube) that cranks out a muscular 15 watts. You get two inputs: one for high gain and one for low gain, as well as Volume, Treble, Bass, and Attitude knobs to shape your sound. The EQ controls are very responsive and seem to be at the center of the perfect frequency ranges for guitar. The Attitude knob controls the response of the driver tube and can take the overall attitude (what else?) of the amp from sweet to snarly with one full rotation. Of course, there are plenty of other moods in between. There's also a selectable noise-reduction circuit to keep things quiet, and a transformer-isolated line out with a level control for going straight to the board or to another amp or power amp. As a small gig amp, the UniValve is a lot of fun. For recording at home or the studio, however, the UniValve is second to none.


Click to Enlarge The BiValve-30 is the middle child, and it's got the look-at-me attitude to prove it, via a lavishly designed and artistically etched front panel with a marine-type theme (BiValve, geddit?). With a nearly identical feature set as the UniValve, what sets it apart is its two-tube power amp section (that BiValve thing again) that bumps the Class A wattage up to 30 and a treble cut switch next to the inputs. With an extremely loud output level it excels as a small-to-medium-venue gigging amp, especially when combined with one of THD's 2x12" or 4x12" speaker cabinets.


The Flexi-50 is the amplifier Andy Marshall designed to steal every rock guitarists' heart. THD has perfected the venerable 50W, single-channel guitar head, crafting a sonic weapon that elevates every aspect of the unit to high art. Unlike the UniValve and BiValve, the Flexi-50 has Class AB output, to better capture the feel of the classic ‘60s Californian and English amplifiers. It also has a footswitchable overdrive/boost function, a footswitchable master volume, and enough punch to rip the ears off the most battle-hardened guitarists. It's the perfect blend of robust American clean tones and the best British overdrive. It has the same sensitivity found in the UniValve and BiValve, allowing you to switch from raw, gain-y overdrive to loud, round clean tones by just rolling down the volume knob on your guitar. The Flexi-50's natural distortion will make rock and fusion players squeal like little girls at a slumber party. For the chunky, de-tuned metal riffer: bring a pedal. That's okay, though, because the Flexi-50 LOVES pedals, and if you want that level of aggression, there are lots of pedals that will push this puppy over the top.


Click to Enlarge Variable valve vitality
The central feature which makes the THD amps so versatile and absolutely freakin' killer is their ability to tube swap at the drop of a hat. You can swap any preamp or power amp tube for a different one and get a fresh sound immediately. On a normal tube amplifier this is a complicated process involving extensive biasing and testing, and you're limited to a small number of tube options. Biasing refers to the amount of voltage regulated to the grids of the tubes. Too much or too little can do very bad things to your amp, not to mention your tone, which is why it's usually a job for amp techs only. THD has made this process as simple as it possibly can be. With the UniValve and BiValve amps, you can swap any power amp or preamp tube for any other power amp or preamp tube without biasing at all. Genius, eh? And the Flexi-50 has easy-as-pie bias controls right on its chassis, so swapping tubes on the Flexi-50 only takes about a minute with a voltmeter.


Switching out the tubes on an amplifier is akin to replacing the pickups on your guitar. Putting humbuckers in a Stratocaster won't make it sound like a Les Paul, but it won't sound like a traditional Strat anymore, either. And like pickups, tubes have an awful lot to do with the most famous and desirable tube amp sounds. It's a little (okay, a lot) more complicated than this, but each family of tubes has its own signature tone and feel due to its physical characteristics. Swapping tubes you'll discover different EQ curves, compression effects, overdrive characteristics, sensitivity, and feel.


Beyond that, individual tubes have their own characteristics because of the conditions under which they were manufactured. Which is why you'll hear tube junkies muttering in low, fevered tones about some brands of EL34s having more sparkle than regular ones, or 6L6s from certain factories being punchier than ones from other factories. With a THD amp you'll not only be able to hone in on exactly which tubes and combinations get you closest to your ideal tone, but also which brand of tubes work best for you, too.


Hot, hot heat
The second thing that makes THD's Class A amps go, go, go is a THD Hot Plate built right into the UniValve and BiValve-30. It ensures you can always get the right amount of breakup, bite, or total meltdown without the accompanying hearing damage. When you use the Hot Plate on your THD amp it operates between the amp and the speakers, absorbing the brunt of the power but none of the tone or sonic impact. So where does the excess power go? The Hot Plate turns it into heat (Hot Plate, geddit?), which is dissipated through the unit's circuitry and large heat sink. Part of the Hot Plate's magic is that it also includes passive noise reduction circuit that eliminates a lot of the hiss and hum that guitars and amps, especially loud guitars and amps, generate.


Click to Enlarge Thankfully the Hot Plate isn't just for THD amp owners. Anyone can use a standalone Hot Plate to get screamin' distortion at whisper volumes with their own tube amplifier. THD even makes variations engineered specifically to match up to the differing impedance level of the wide assortment of amps out there. By choosing from 2, 2.7, 4, 8, or 16-ohm versions, you can seamlessly match your specific amp and speaker setup. Each Hot Plate also gives you a Bright and Deep switch, dual speaker outputs, plus a line out with adjustable level.


Industrial-strength tone generator
But THD amps aren't just about versatility—they're also about quality and consistency, with each step of the signal chain designed for tone and sensitivity. Say you find your most favorite sound ever and you want to leave your THD amp set up that way forever. Not only will you have achieved your ultimate tone, but you'll have done it with an amp designed to outlive you, not the other way around. THD units are hand built with extreme care and consideration for longevity and all the parts used in them are chosen for their careful balance of great tone and long life.


Click to Enlarge THD uses FAA-approved assemblers, who are held to even more stringent standards than manufacturers who build to military specifications; the same used by Boeing. The circuit boards THD products are built on are twice as thick and have eight times as much copper on them as most manufacturers' designs. This affects not only what you hear, but how long it lasts. THD has the lowest failure rate of any tube guitar manufacturer, meaning you will play your THD a lifetime and only change the tubes. That should make some guitarists weak in the knees, too, especially those who have found a great amp they really liked only to have it crap out on them half a dozen times.


I got to check out the BiValve-30 and one of THD's 2x12" speaker cabinets. From the first time I touched them until I shipped them back I was impressed. How could I not be? Every aspect was perfect, from the cut of the cloth-covered grille on the cabinet to the plate personally signed by Andy Marshall on the head's rear panel. Of course, the main attraction here is the sound and feel, and the setup didn't disappoint. With the stock 12AX7/EL34 tubes the BiValve-30 supplied solid, round clean tones that were gorgeous and lush. Using the Volume, EQ, Attitude, and Hot Plate controls in conjunction with the dual inputs gave me more than enough hot tones to play with; from Aerosmith to ZZ Top, Robben Ford to Eddie Van Halen.


I fell in love with the big, slightly raw sound and soft, compressed attack I got through the More input by keeping the Volume and Attitude low and turning up the EQ a little. True to its specs, overdrive increased with the volume from my guitar, and cleaned up just by backing off my pick attack or decreasing the volume. It was great to finally have that much control over my sound in such a simple way. No matter which sound I dialed up the BiValve-30 remained touch sensitive with a very immediate, forward sound. I could hear each string clearly even with overdriven chords. With THD, it really is all about the tone. Great tone is not an accident and your personal ultimate tone is a lock with a THD amp.