Rectifier tubes are a critical part of the sound of tube amplifiers. The purpose of a rectifier tube is rectification -- converting an AC (alternating current) to a DC (direct current) voltage which is usable for powering the rest of the amplifier circuits.
Common rectifier tube types which are still in production include the 5AR4, 5U4, 5Y3 and 6CA4. Many other less popular types are great for amp building, many of which are listed below. As amplifier power output increased, many manufacturers switched to using solid state diodes in place of tube rectifiers. An example of this is the Fender Twin. The early tweed Twins used tube rectifiers, and when the blonde version came out in 1961 it featured a solid state rectifier to support the higher power output. However, lower powered amplifiers like the Fender Deluxe Reverb continued to use a tube rectifier.
The most important electrical characteristics are peak inverse voltage and peak current