Groove Tubes Soul-O Single

Soul-O Single
Soul-O Single

Groove Tubes founder Aspen Pittman is a tube connoisseur. For 24 years, he has listened to, tested, designed, and manufactured many variants of the familiar glass bottles that melt the hearts of guitar tone aficionados.

The latest outgrowth of Pittman’s passion for tubes and their many tones is his company’s new class A all-tube single-ended guitar amps designed for those of us who may be amp tweakers at heart, but don’t have the skills, time, or hardcore desire to build or buy amps that use different tube types.

The new member of GT’s family is the Soul-O Single. “Single” refers to the number of output tubes sported by the amp in its five configurations – a 1×10 combo, a 1×12 combo, a Top Cab, the LapTop model, with its two 4″ Auratone speakers in a top-grill baffleboard, and a 1×12 piggyback.

Capable of employing one of 25 output tubes ranging from EL84 to KT66 without having to modify or re-bias the amp, the Soul-O Single is a sweet studio or live amp. Its output, depending on the power tube used, ranges from seven to 15 watts. The head and combo share certain features, including black tolex cabinets with tan grillecloth, chrome chassis, black Bakelite knobs, and a chromed steel emblem.

Controls are also shared and include passive bass, mid, and high tone knobs, as well as presence, gain (with footswitchable pull boost), master volume, and an effects loop blend control. The tone section does not boost or cut frequencies. Rather, it filters them, which means a user can adjust tones based on what they hear, rather than what the knob is telling them.

GT says one major key to the amp’s tone and bass response – and its ability to use so many tubes without sacrificing tone – is its huge triple-tapped output transformer with an impedance selector (4/8/16-ohm). The Single’s OT is the physical size of one in a 50-watt Marshall.

The combos are equipped with a Jensen speakers, while the piggyback cab (which boasts a removable backside “plug”) is loaded with a 12″ Jensen Ceramic Classic Clone.

Soul-O Test-E
To get a grasp of the tones the Soul-O Single is capable of, we snared a triumvirate of classic guitars; a ’62 reissue Fender Strat, ’59 Fender Esquire, and a Gibson Gary Moore signature model Les Paul. We also brought an EH Holy Grail reverb unit for when we needed to wet things up a bit.

The “standard equipment” tube complement is Groove Tubes’ top-of-the-line GT6L6GE and two GTECC83 preamp tubes. Pittman told us the company considered shipping the amp with a “six-pack” of tubes, but decided against it, citing price and the complicated nature of personal tube preferences. Also, Pittman noted, GT’s recently introduced Silver Series of tubes will feature non-matched/non-rated tubes at a substantially lower price – a prime deal for Single owners.

For our test, GT sent seven power tubes (GTEL84S, GT6L6GE, GTEL34L, GT6V6R, GT6550, GT5881, GTKT66HP) and three preamp tubes (GT12AX7C, GT7025, GTECC83) to facilitate our Soul-O workout. Changing them was snap since the chrome-plated chassis is spartan, with plenty of room to get at its two tube sockets.

The first power tube we chose was the GTEL34L, which we ran through the 1×12 cab. This configuration gave us the best overdrive and a tone that could most closely be compared to a Marshall amp. So it made sense that to our ears, it sounded best with the backside plug installed, which tightened the low-end much like a 4×12 cab.

With the rear cabinet plug out and the GT6L6GE or GT6V6 installed, we were able to get a more round/Fender Deluxe-like sound. And in keeping with the spirit, if we pushed it too hard, we experienced a touch of authentic low-end fart-out… just like a Deluxe!

The GTKT66HP was the loudest power tube, and had the most personality. In the combo, it had a nice sparkle, with clean mids and a fair amount of overdrive. The GTEL84S, with its “928” pin adapter was the quietest and cleanest. With most of the tubes, we avoided using a lot preamp overdrive, opting instead to push the power tubes so they’d better reveal their differences.

The tone controls were transparent and natural-sounding, and worked best in the 10 o’clock to 2 o’clock positions. The effects loop was also very transparent, and the blend control allowed us to use the controls on the effects unit more effectively.

Either of these Soul-O Singles – and a handful of tubes – is capable of being a great workhorse in the studio, at a rehearsal, jam session, or a gig.

Groove Tubes Soul-O Single
Type of Amp: Single-ended Class A combo or head/cab piggyback.
Features: All-tube circuits, uses any of 25 output tubes and a variety of preamp tubes without re-biasing, effects loop with blend, classic styling, high-quality speakers.
Price: Combo, $1,099; piggyback head/cab, $1,498; gain-boost footswitch, $59.
Contact: Groove Tubes, 12866 Foothills Blvd., Sylmar, CA 91342, 818-361-4500,

This article originally appeared in VG‘s May ’03 issue. All copyrights are by the author and Vintage Guitar magazine. Unauthorized replication or use is strictly prohibited.

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