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THD UniValve Amplifier
The 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 pre-amp 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.
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 out-put 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, 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 GREAT-sounding transformer-isolated line out on the
back panel complete with an adjustable level control and a switch to
choose 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 is initially sold only as a head. It is a metal chassis,
powder coated and enclosed in a perforated steel cage to protect the
tubes. THD plans to introduce 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 also plans to 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! |
