THD UNIVALVE INSTRUCTION MANUAL
2
TABLE OF CONTENTS
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 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. 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.