This bothered me for a long time until I did enough research to understand the current flow through 2 and 4 diode rectifiers. Why does a power transformer with a center tap allow rectification with just two diodes versus four needed with no center tap? Because the center tap provides the path for returning current. If you use a transformer without a center tap then a 4 diode bridge rectifier is needed to provide a current return path from the amp circuit back to the transformer. Bridge Rectifier Current Flow During Top Half of AC Wave All four diodes act as one-way valves that allow current to only flow one direction. The two diodes on the left side are the 'bridge' back to the transformer. They provide a return path for the current from the amp (circuit load) to the transformer. Bridge Rectifier Current Flow During Bottom Half of AC Wave With a Center Tap Two Diodes are Enough Conventional rectifier showing current flow during the top half of the AC wave. The power transformer's center tap provides the current return path from the amp circuit. A tube rectifier works the same way as this two diode rectifier, that's why tube rectifiers are always paired with power transformers with center taps--the center taps are required to provide a current return path from the amp circuit back to the transformer. Conventional Rectifier and No Center Tap With no center tap there's no return path to the transformer so this will not work.