The THD Univalve... 

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, 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 a Class A?

Class A operation means that a tube is conducting for the entire 360 degrees of the signal cycle.  By contrast, Class AB amps have their power tubes alternately cutting off for a portion of the signal, while the opposite tube (or set) picks up the slack.  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. Class A amps sound great at low volumes, and even better as you turn them up. 

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. 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 and give 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.  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 low gain input with 12AX7 preamp tubes in the amp gives a wide range of clean sounds from "Country" clean and crisp to a "Texas Blues" type of overdrive. Plug into the high-gain input and you go from early "British Invasion" tones to contemporary Metal sounds. 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, along with an "Attitude" control. This is not a Presence control, as the amplifier has no negative feedback around the power stage. Rather, 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 provides very effective noise-reduction. 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 and Line/Instrument level selector. 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.