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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. 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.) 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 THD calls 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.
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