All amplifiers require a steady DC power supply for their circuits to operate correctly. A rectifier is the device that converts the Alternating Current - AC coming from your wall plug and transformer, into Direct Current - DC.

Tube amps make use of either a Solid State Rectifier, or a Tube Rectifier.  Up until about 1960, almost all guitar amps used tube rectifiers. However, the introduction of silicon diodes provided manufacturers of high-powered amps with an excellent alternative to tubes. A solid-state rectifier is typically comprised of two, or four diodes in a “bridge” configuration. By switching to silicon rectifiers, amp designers could deliver higher voltages to the output stage and squeeze more power from the tubes.

Many modern and boutique tube amps however still use a rectifier tube to convert power into DC. When used, there is typically one rectifier tube installed, and this is a larger tube, often similar in size to the power tubes.  There are exceptions and some guitar amplifiers feature more than one rectifier tube.

The rectifier tube is not directly in the amp's signal chain, but can have a significant influence on tone. Voltage drop or "sag" due to high power demands can effect the dynamics of any amplifier, and this is often a key contributor to an amp's signature tone.  Sag occurs when the amp is working hard and requires additional current, and the rectifier tube cannot immediately respond to this demand.  As a result the rectified B+ voltage drops momentarily and this can be heard as slight compression, or sponginess of the sound until the supply voltage stabilises again. 

Tube rectifiers are dual diode 4 pin devices that fit into the industry standard eight-pin "octal" socket.  Modern guitar amp builder generaly only use a few different types of rectifier tube - 5U4, GZ34, 5Y3, and EZ81. Each has different power and voltage handling capabilities but all operate in the same way.   Only two types are used in significant numbers, namely the low power 5Y3, and the higher rated GZ34. (5AR4)

Always refer to your original equipment manufacturers specifications to confirm the rectifier tube requirements for your specific amplifier.

Here is a specification table for the most common rectifier tube types. The voltage drop is estimated from a supply voltage of 425V.