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.

Electrical Characteristics

The most important electrical characteristics are peak inverse voltage and peak current per plate. This determines how high of a voltage the tube can safely handle, and how much current in can provide. Never exceed the datasheet ratings for either of these parameters, the results can be catastrophic! Additionally, the heater voltage and heater current determine the heater characteristics. Any substitutions must use the same voltage and not exceed the current available from the power transformer heater winding. This is an area where a lot of people get into trouble, using a rectifier tube that draws too much heater current, and then overheating or damaging the power transformer.

Another important characteristic of a tube rectifier is the internal resistance, which determines the much-discussed voltage drop that makes tube rectifiers an important component in the sound of an amplifier. Voltage drop is the amount the voltage is reduced as it passes through the rectifier tube, due to the internal resistance. Due to this internal resistance, the voltage output drops as more current is drawn. This creates a natural compression effect, or "sag" due to the voltage lowering as the output power increases. If you were designing an amplifier with solid-state diodes and wanted to simulate the sound of a tube rectifier, just add an appropriate resistor in series with the diodes, which will behave very similar to a tube rectifier. Some amplifiers have a tube rectifier simulation or "sag" switch which does just that. There are also some plug-in rectifier replacements that work based on this principle. Most rectifier tubes used in guitar amplifiers are directly heated. This means that the cathode and heater share the same connection, and a separate heater winding on the transformer is required due to the DC voltage being present on the transformer windings. Indirectly heated rectifiers have a completely separate heater, which allows for a lot more flexibility in the design of the circuit and selection of transformers.

Common Rectifier Tubes for Musical Applications

5AR4 - First introduced in 1956, the 5AR4 was widely used by many amplifier makers in the 1950s and 1960s. It has been used in the Vox AC30

5U4 - First available in 1935 the 5U4 is an extremely common and relatively inexpensive tube nowadays, but was not used in as many guitar amplifier designs. It is used in the Mesa Double and Triple Rectifier amps.

5Y3 - Having a lower current output than the 5U4 and 5AR4, the 5Y3 is found in low wattage amplifiers like the Fender Champ.

6CA4 - Also known as the EZ81, the 6CA4 is found in many lower power designs like the Marshall Plexi, Vox AC15, and some Hammond Organs as well.