This Sophia Electric delay action board was designed for retrofitting into tube amps built without pre-heating the tube filament. Research shows pre-heating tubes will have 3-4x more life span than without it. Thus you buy this board will save you a lot of money otherwise spend on replacing expensive output tubes(300B/2A3/845/KT88/Kt66/El34/6L6) often.
Any tube amplifier with solid state rectifier must use this delay action board to warm up filament first to avoid cold start by pre-heat filament before applying B+ to the tube plate. Must have!