Like the UniValve, the THD BiValve Rack-mounted Guitar Amp Head is a single-finished pure Class A design. But, it has two productivity valves that are wired in parallel and combined through a special productivity transformer to deliver up to around 30 watts. You can use very nearly any productivity 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 very nearly 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 make 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 very nearly 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 simpler to see.Underneath the perforated steel lid there’s a larger pair of transformers to handle the increased productivity, 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 benefit the aircraft industry.The front panel looks unrepresentatively simple. Working from the left, there’s a pair of input jacks labeled “more” and “less”, followed by a treble cut switch then rotary reins for volume, treble, bass and what THD call “Attitude”. It’s not a fancy name for a presence control – there’