THD Introduces the UniValve Head
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(Click for a close-up) |
January 18, 2001The new THD UniValve 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, 7027, 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 easily
capable of driving a 4 x 12" cabinet, yet quite small and light. 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 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
it 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 voltage 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, has 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-direction warmth. With
the controls pegged and a relatively high-output humbucker, the low-gain
input can deliver a very convincing "Texas Blues" type of overdrive. Plug
into the high-gain input and you are in a whole different world from early
"British Invasion" tones to contemporary Heavy Metal sounds. The most
surprising aspect to the UniValve's high-gain overdrive is that even with
stunning levels of grind, you can still play complex chords and hear every
note of the chord clearly. There is none of the mush and confusion normally
associated with high-gain tones.
The controls are the traditional Volume, Treble and Bass controls along
with
what we call an "Attitude" control. The Attitude control is not a Presence
control, as the amplifier has no negative feedback from the output
transformer back into the driver stage. The Attitude control determines how
the driver stage responds to signal and how it drives the power tube. There
is also a built-in THD Hot Plate power attenuator that lets you have full
output distortion at any volume level. The Hot Plate has its own
hard-bypass
switch on the front panel, which completely removes it from the circuit for
Full-Power playing into a speaker.
The UniValve features a light bulb on the front panel that not only glows
when the amplifier is played, but it also provides very effective
noise-reduction. For occasions when you do not want this feature, there is
an On-Off switch for the light bulb circuit right on the front panel.
The UniValve has a transformer-isolated line out on the
back
panel complete with an adjustable level control and a switch to chose
Line-Level or Instrument-Level signal. The transformer used in the line-out
was chosen for its pickup-like impedance and tonal characteristics. Most
amplifiers' line-outs only work well when used to drive other power
amplifiers, and sound harsh and thin when plugged into the guitar input of
an amplifier. Not so with the THD UniValve. Even into the input of a
Marshall 100-watt head, the UniValve provides a warm, fat, tight and sweet
sound that feels like an amplifier, not like a preamp-amp rig.
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 to allow the silent use of the amp for
direct recording purposes when miking a cabinet is not practical.
Configuration
The UniValve will initially be sold only as a head. It is a metal chassis,
powder coated and enclosed in a perforated steel cage to protect the tubes.
In the coming months, we will be introducing a number of different combo
cabinet and head box configurations. The UniValve can be installed in any
of them simply by screwing it into the box. There will be a huge range of
choices and options in the cabinets including head boxes, 1-12" combos,
2-12" combos, 2-10" combos and others. THD will periodically produce
"Limited Edition" cabinets with special hardwoods or other unique features.
Anyone with a UniValve will be able to use these as swapping cabinets only
takes about five minutes. For more information, visit their web site at www.thdelectronics.com. |
