Review: THD 15W Univalve Class A Amplifier Head

THD Electronics is a manufacturer of vacuum-tube guitar amplifiers and amplifier accessories, and is located in Seattle, Washington. It was founded in 1986 by President and CEO Andrew Marshall.  The company is well known in the boutique amplifier market, and is often credited with the creation of this market in the United States. THD’s line of products has stayed relatively small despite its 22-year history as a company. Of its amplifiers, THD currently makes four: the Univalve, BiValve-30 and Flexi-50.

The Specs

The THD Univalve is a 15W Class A amplifier. The controls include volume, treble, and bass, along with an Attitude control knob that determines how the driver stage responds to signal and how it drives the power tube. There are also controls for the Hot Plate, including a hard-bypass switch. The noise reduction circuit in the Hot Plate features a light bulb fixture on the front panel that glows when you play. There is also an On/Off switch for the light bulb.

The single output tube can be switched at will with nearly any octal-based power tube for different tones without re-biasing the amp, and the 2 preamp tubes can be switched for any combination of 12AX7, 12AT7, 12AU7, 12AY7, or 12AZ7 tubes.

There are 2 inputs—one for high gain and one for low gain. With 12AX7 preamp tubes, the low gain input gives a wide range of sounds from clean and crisp to down and dirty. The low-gain input delivers a kind of Texas blues overdrive. The high-gain input can range from early British tones to modern high-gain metal.

On the back panel the Univalve has a transformer-isolated line out with an adjustable level control and a switch to choose line-level or instrument-level signal. The speaker outs on the Univalve can drive any speaker load from 2 ohms all the way up to 16 ohms. When there is no speaker plugged into the speaker jack, the amp uses its own built-in dummy load so no damage will occur, and it’s awesome for direct recording.

2×12 Ported Extension Cabinet

The THD 2-12 Extension Cabinet is perfectly balanced and matched to a T. The Ported Extension Cab is a bass-reflex design cabinet with a slot-port in the rear instead of the more common tube port in the front. THD cabinets are constructed from 3/4″ 9-ply birch with 1/2″ birch ply baffle and rear panels.

The Low Down

The THD Univalve is a great boutique amp with a wide range of tonal possibilities. I will admit that it takes a little bit of time to tweak the settings and get it dialed in, but it is well worth it. The THD Univalve breaks up beautifully when pegged to ten. One of the great things about this amp is it is capable of powering any cabinet. I had it hooked up to the factory cabinet as well as a 4 x 12 Marshall 1960 cab. Both sounded awesome. The switching capability of the power tubes is a great touch letting you dial in just about any tone. Country clean to “Grip it and rip it Metal.” Definitely a great amp.

Pros: Limitless tonal capabilities, Sturdy construction, Bass reflex cabinet.

Cons: N/A

MSRP – $1,678.00

Till next week, thanks for reading and keep on shredding.

Nick Arrietta is a staff writer at SMG. Nick is a professional Music teacher from California and has been playing the Guitar for 24 years. Along with teaching music he is a touring guitarist, studio musician and songwriter. Email:

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