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!