THD Univalve. The THD Univalve is an innovative tube head designed by Andy Marshall that allows you to easily swap a wide range of preamp and power tubes creating an unlimited variety of tonal characteristics from the same head. Lightweight and compact, yet powerful enough to power any cab and fill a room with sound. This amp is in great condition with very little wear and functions perfectly. Also features a built-in THD Hotplate attenuator for getting authentic output tube saturation at any volume.