Tube rectifier

A tube rectifier converts the power transformer's high-voltage AC into the DC that feeds the B+ rail using a vacuum diode — a 5Y3, a 5U4, a GZ34. Unlike silicon, a hot vacuum diode has real internal resistance, so when the amp draws a heavy current transient the rail voltage briefly drops. That drop is sag.

The corpus keeps tube rectification because that sag is musical, not a defect. The rail gives a little under a hard-picked chord and springs back, so the amp compresses and blooms in a way players feel as touch response. Different bottles sag by different amounts — a 5Y3 is soft, a GZ34 comparatively firm — which is why the specific rectifier is worth reading off each circuit; the table below names it for every amp.

The costs are a voltage drop you design around, a warm-up delay, and a tube that eventually wears — which is exactly why the stiffer, later amps moved to solid-state rectification. For how the rectifier sits in the supply, see the power section in reading a schematic.