Tough construction, ultra-hip styling and a unique collection of features add up to an amp you'll probably never tire of playing through.

Like the UniValve, the THD BiValve Rack-mounted Guitar Amp Head is a single-ended pure Class A design. However, it has two output valves that are wired in parallel and combined through a special output transformer to deliver up to around 30 watts. You can use almost any output valve you like without touching any kind of bias adjustment, and get this: in the BiValve you can use them in any combination as well. Combined with the capability to take almost any preamp valve, this makes the BiValve even more of an amp-tweaker's dream than its predecessor. Because there's such a wide range of tone available, you create your own style.

The BiValve Rack mount amplifier head sits in the same compact pressed steel chassis as the UniValve and, except for the extra valve and control plate, it looks almost identical. Glasgow artist Charles Rennie Mackintosh inspired the UniValve's acid-etched control plate; this time round THD have gone for a marine theme, with an intricate diamond scale design. The control markings are screen printed, which makes them easier to see.

Underneath the perforated steel lid there's a larger pair of transformers to handle the increased output, and inside there are two ultra-thick PCBs: one for the power supply and one for the audio. The four valve bases are bolted to the chassis and secured by locking nuts, with hi-fi style internal baffle plates to cut down on radiated hum. The standard of construction is absolutely top class. THD are based in Seattle, which is also hometown to Boeing, and most of the BiValve's heavy fabrication work is contracted out to local specialist companies who service the aircraft industry.

The front panel looks deceptively simple. Working from the left, there's a pair of input jacks labeled "more" and "less", followed by a treble cut switch then rotary controls for volume, treble, bass and what THD call "Attitude". It's not a fancy name for a presence control - there's no negative feedback loop - instead Attitude works on the driver valve to change its response, and does more or less what the name suggests: either smoothing things out or making them more aggressive.

In the center, part of a clever noise reduction circuit, is a light bulb that glows as the amp distorts. Depending on your point of view it either looks very hip or very distracting, hence a small switch underneath to turn it off. Next to this is the level control for the Hot Plate - a built-in output attenuator that lets you run the THD BiValve into total meltdown without annoying the neighbors; there's also a defeat switch for this function, which adds a little extra volume for live work.

The last three rocker switches are for mains, standby and power selection. The hi/lo power switch is like having a built-in Variac; switching to low voltage adds a squashy dynamic feel and reduces clean headroom, and it's essential for valves like the 6V6, which can't handle high plate voltages.

The BiValve's back panel is also similar to its smaller cousin. There's a pair of speaker outlets with an impedance changer, and the excellent transformer-isolated line out - using a 6mm stereo jack socket - is now balanced. Fuse protection is more comprehensive, and the BiValve also benefits from a pair of warning LEDs to let you know if a power valve is faulty.

When it comes to describing the BiValve amp's sounds, it's difficult to know where to start, or when to stop, for that matter. You can use almost any power and preamp valve combination under the sun, and as a result the tonal range is virtually unlimited. Low-end response through a ported cab containing a pair of Celestion Vintage 30 speakers is full without becoming too tubby.

Using the volume control in conjunction with the two inputs you can cover the whole gain spectrum, from squeaky clean to absolute brain-frying power-amp distortion with almost infinite sustain, and all the time the BiValve stays totally musical. Even at full-tilt you can still pick out each string within a chord, and the tone controls seem to have just the right range, whatever valves are in use. The Hot Plate feature lets you play any distortion tone at any volume level. You can even disconnect the speakers for recording, as the amp has a built-in dummy load.

As you'd expect, the BiValve is much louder than the UniValve. There's more than enough headroom for live use, and notes seem to jump out of the loudspeakers a lot quicker. It's a different effect to the UniValve's three-dimensional warmth, but it is equally rewarding. The BiValve delivers totally on the UniValve's promise of more to come, and the result is one of the best playing experiences any guitarist could hope for.

THD BiValve Rack-mounted Guitar Amp Head Features:

• 2 output valves of nearly any type in any combination allows incredibly diverse tone possibilities
• No bias adjustment necessary
• Rackmountable
• Top class construction
• 2 input jacks
• Controls: volume, treble, bass, attitude
• Switchable noise reduction light bulb
• Built-in THD Hot Plate

This may be the last amp you ever need! Call or click today to order.

THD BiValve Rack-mounted Guitar Amp Head Specifications:

• Output up to approx. 30 watts