THD Electronics UniValve User Manual
THD UniValve
In s t r u ctio n M a n u a l
Thank you for your purchase of the THD UniValve amplifier! The 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, EL-34, 6550, 6CA7, 6V6,
6K6, KT-90, KT-88, KT-77 and KT-66, 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 4x12" cabinet, yet quite small and light.
Finally, it has a built-in Hot Plate® Power Attenuator that allows for full output
distortion at almost any volume. 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.
THD UNIVALVE INSTRUCTION MANUAL 1
TA B L E O F C O NT E NT S
Instruction Manual...................................................................................................1
What Does “Class-A” Mean?........................................................................................2
UniValve Controls.........................................................................................................3
Front Panel..................................................................................................................3
Back Panel ..................................................................................................................5
Switching Tubes in the UniValve..................................................................................7
Power Tubes................................................................................................................7
Preamp Tubes..............................................................................................................7
THD Electronics Contact Information.........................................................................8
What Does “Class A” Mean?
At the beginning of this manual, we explained that the UniValve is a “Single-Ended
Class-A” amplifier—but what does that mean? “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 AC30 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. There is no point at which you will be waiting for the tube to
receive its full current for the best sound—it always will. 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.
THD UNIVALVE INSTRUCTION MANUAL 2
UniValve Controls
Front Panel
First, let’s go through the various control options on the front panel:
1. ROCK Input: This jack fits a standard 1/4" mono shielded instrument cable. If you
use a stereo cable, the sleeve signal will be ignored. Do not use speaker cable into this
input. The ROCK input is the high gain input, meaning that both preamp gain stages will
be engaged in order to pass the hottest possible signal to the power section.
2. ROLL Input: This jack fits a standard 1/4" mono shielded instrument cable. If you
use a stereo cable, the sleeve signal will be ignored. Do not use speaker cable into this
input. The ROLL input is the low gain input, meaning that the first preamp gain stage
will be bypassed.
NOTE: You cannot plug into both inputs simultaneously or otherwise A/B between
inputs. Plugging into the Roll input will disable the Rock input.
3. Volume: This knob controls the loudness of the amplifier. The higher the volume
knob setting, the more the tubes are engaged.
4. Treble: This knob can be used to boost or cut the high frequency response of the
UniValve.
5. Bass: This knob can be used to boost or cut the bass frequency response of the
UniValve.
6. Attitude: The attitude knob determines how the driver tube responds to the signal, and
how it drives the power tube. If the Attitude knob is turned up, the UniValve will sound
edgier and more aggressive. With the Attitude knob turned down, the UniValve sounds
sweeter and more controlled.
THD UNIVALVE INSTRUCTION MANUAL 3
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