Below I've included an excerpt from Andy's website describing his Flexi-50 head,  but really,  you HAVE to plug into this thing to get the story.  When I say the different tones you can achieve with this amp are limitless,  it just sounds like more of my hype,  but really,  when you start plugging in the many different preamp and power amp tubes into this unit you realize that it's true.  This is one VERSATILE unit!!!

    "The Flexi-50 is a precision hand-built 50-watt Class-AB amplifier with foot-switchable overdrive/boost, foot-switchable master volume control and the ability to use almost any preamp and power tubes in any combination, including 6L6, EL34, 6V6, 6CA7, 8417, 6550, KT66, KT77, KT88, KT90, KT100, EL84 (with Yellow Jacket adapter), 6K6, 6F6, 12AX7, 12AY7, 12AU7, 12AT7, 12AZ7, 12DW7, 12BH7, ECC83, ECC82, ECC81 and many more, giving the user a huge pallet of available sounds simply by changing tubes. The amplifier has a fat, clear, full clean sound reminiscent of late 1950s to early 1960s American combos, and overdrive to rival the best British heads. Even at the highest overdrive settings, the amp still demonstrates dramatic touch sensitivity, cleaning up very well when the volume on the guitar is reduced. Additional features include external bias test points and individual bias controls, permitting the user to quickly and accurately set the output tube bias with any digital voltmeter. The dual bias controls permit the user to correctly bias even mismatched tubes, eliminating the need for matched power tubes.

    What does all of this mean for you, the player? It could very well mean that the search for your sound is finally over. While the THD UniValve and BiValve-30 amplifiers offer the same flexibility and choice of tubes, they do so in a Class-A circuit. While Class-A amplifiers do have a strong following due to their smooth, even character, there are many players who need, or even crave the immediacy, clarity and "punch" of a grid-biased Class-AB amplifier.

    How is the Flexi-50 different? First off , it is a single-channel amplifier, but one that can be switched and blended among a number of different sounding and different feeling voices. The changes can come from a number of methods including foot-switching a preset boost level with its own, dedicated tone control that allows you to make your boosted sound darker than the clean sound, brighter, or anywhere in between. 

    The unique combination of our touch-sensitive input circuit and the wonderfully active and reactive tone control section (that we slaved a year to perfect) make for a front-end that really responds to subtle and not-so-subtle changes in the signal being fed into the amplifier either by the guitar or any effect that may be between the guitar and the amplifier. Back off on the volume control and the amp gets much cleaner without losing the fatness of the full-volume sound. How do we do this? You buy the sushi and well,  get as technical as you want, but what really matters is that it works, not so much how it works."

*  50 watt class AB amplifier
*  footswitchable overdrive/boost
*  footswitchable master volume control
*  able to use almost any power tube or preamp tube including 6L6, EL34, 6V6, 6CA7, 8417, 6550, KT66, KT77, KT88, KT90, KT100, EL84 (with Yellow Jacket adaptor), 6K6, 6F6, 12AX7, 12AY7, 12AU7, 12AT7, 12AZ7, 12DW7, 12BH7, ECC83, ECC82, ECC81 and many more
*  external bias test points and 
individual bias controls
*  50 watt / 20 watt front panel switch
*  fully shielded power & output transformers
*  series/parallel switched impedance selector that can take any cabinet from 2 ohms to 16 ohms impedance
 

   "THD Electronics BiValve is a single-ended pure Class A design.  It has two output tubes which are wired in parallel and combined through a special output transformer to deliver up to around 30 watts. You can use almost any output tube 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 tube, this makes the BiValve even more of an amp-tweakers dream than its predecessor. Because there's such a wide range of tone available you your style. 

    Inside there are two ultra-thick PCBs: one for the power supply and one for the audio. The four tube 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 home town 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 BiValve into total meltdown without annoying the neighbours; 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 tubes 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'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 tube combination under the sun, and as a result the tonal range is virtually unlimited. 

   While we liked the dual 6L6 arrangement the amp is supplied with, switching from these Russian tubes to a pair of the new Groove Tubes 6L6GEs (made in America to the old General Electric spec) really brought things to life. The cleaner tones have a rich, sonorous and detailed mid-range that will flatter any guitar, with a bell-like treble that almost sounds like a chorus effect. 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. We lost count of the different valve combinations we tried but a few really stood out. A 6V6 with an EL34 yielded one of the absolute best British blues/rock sounds we've ever heard. The smaller valve works to balance out the EL34's hollow midrange and aggressive distortion with a unique high-end response - somewhere between the best vintage Marshall and tweed Fenders, with a hint of AC30 thrown in. 

   Tough construction, ultra-hip styling and a unique collection of features add up to an amp you'll probably never tire of playing through. The best things in life are rarely cheap, and the BiValve is definitely an amp for the well-heeled purist - no footswitches to change channels means you have to use it the oldfashioned way, driving things from the guitar."

*  single ended Class A design
*  30 watts output
*  able to use almost any power tube or preamp tube including 6L6, EL34, 6V6, 6CA7, 8417, 6550, KT66, KT77, KT88, KT90, KT100, EL84 (with Yellow Jacket adaptor), 6K6, 6F6, 12AX7, 12AY7, 12AU7, 12AT7, 12AZ7, 12DW7, 12BH7, ECC83, ECC82, ECC81 and many more
*  Volume, Treble, Bass & Attitude
rotary controls
*  built in Hot Plate power attenuator
*  front panel Hi/Lo power switch
*  2 speaker outputs with impedance changer
 

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