THD Univalve Rackmounted Guitar Amp Head Purple

The THD UniValve Rackmounted Guitar Amplifier Head is a single-ended class A amplifier head with a single output tube that can be switched at will among many octal-based power tubes, including 6L6, EL34, 6550, KT90, KT88, KT77 and KT66, for different tones without re-biasing the amp. Likewise, the two preamp tubes can be any combination of 12AX7, 12AT7, 12AU7, 12AY7 or 12AZ7. The UniValve delivers tones from smooth and clear to very aggressive overdrive. It is quite small and light, yet easily capable of driving a 4 x 12" cabinet. It has a built-in hot plate power attenuator that allows for full output distortion at almost any volume. And it doesn't cost as much as you might think.What is a Class A? Class A is a term given to an amp that runs its tubes at full current all the time, unlike most tube amps that alternate between running one set of tubes and the other set, each for one half of the wave. The set not in use is turned off by a positive swing of the grid voltage. Single-ended output stages always operate in Class A. Most push-pull amplifiers, including the venerated Vox AC-30 operate in Class AB when overdriven, even if they are in Class A while clean. The upshot is that Class A operation has its own unique tone characteristics that set it apart from other tube amp classes. Class A amps sound great at low volumes, and even better as you turn them up. Thus, with the relatively low wattage of the UniValve you can turn up the amplifier to take full advantage of its stunning output distortion tone without deafening anyone. Why the self-biasing feature? The UniValve's circuitry senses the current needs of the output tube and provides the correct bias volt-age to the cathode of the output tube regardless of tube type or wear. This means that, without any trip to an amp technician, you can simply replace the output tube with one that has different sound characteristics. (Of course, you can also change the preamp tubes at will for even more tonal variations.) For example, a 6550 will stay cleaner at higher volumes, but when overdriven, yields an astoundingly "woody" character to the overdrive. On the other hand, an EL34 will break up earlier than a 6550 and gives a more British sound. The ever-popular 6L6 family of tubes gives a tight overdrive with less power than a 6550 but more detail than an EL34. Even within the same type of output tube there are "harder" and "softer" tubes, so two guitarists playing with the same guitar through the same UniValve, but simply switching the tubes to their preference, can have noticeably different tones.Controls and Features The UniValve has two inputs: one for high gain and one for low gain ranges. The low gain input with 12AX7 preamp tubes in the amp gives a wide range of clean sounds from country clean and crisp to jazz warmth. With the controls pegged and a relatively high-output humbucker, the low-gain input can deliver a very convincing Texas blues type of ov