Hi-Fi - Tubes: Terms & Types

Analog playback has had a resurgence, here is some information about Tubes:

Tube Terms and Types

Because tube-based electronics are also part of analog's comeback, you may need to be familiar with the terms used to describe tube amplifiers and their performance. Here is a summary of some popular tube types and term from Cary Audio:

12AU7: a miniature nine-pin medium-gain dual triode vacuum tube. The 12AU7 is popular in hi-fi vacuum tube audio as a low noise line amplifier, driver and phase-inverter used in vacuum tube push-pull amplifier circuits.

12AX7: a miniature dual triode vacuum tube with high voltage gain. The 12AX7 was originally intended as replacement for the 6SL7 family of dual-triode amplifier tubes for audio applications. The tube is praised for its distinctive sound, and its wide use in guitar amplifiers has caused it to be one of the very few small-signal vacuum tubes to continue in production since it was introduced. The 12AX7 vacuum tube is used extensively in preamplifier circuits.

EL84: Sometimes called the 6BQ5, the EL84 is a vacuum tube of the power pentode type. It has a nine-pin miniature base and is found mainly in the output stages of amplification circuits, most commonly in guitar and stereo amplifiers.

EL34: The EL34 (6CA7) is a vacuum tube of the power pentode type. It has an octal base and it is found mainly in the final output stages of an amplification circuit. The EL34 was widely used in higher-powered audio amplifiers of the 1960s and 1970s, and it continues to be very popular in vacuum tube hi-fi products today, including many Marshall guitar amplifiers because of its greater distortion (considered desirable in this application) at lower power than other octal tubes such as 6L6, KT88 or 6550.

6L6: This tube is the designator for a vacuum tube introduced by RCA in 1936. At the time Philips had already developed and patented power pentode designs, which were fast replacing power triodes due to their greater efficiency. The beam tetrode design of the 6L6 allowed RCA to circumvent Philips' pentode patent. Further testimony for this vacuum tube's success would be even simpler: the 6L6GC version is still being manufactured and is used in guitar amplifiers, which makes the 6L6 one of the oldest tubes in the category.

6V6: The 6V6 is the designator for a vacuum tube introduced by RCA in late 1937. The 6V6 is a beam-power tetrode that is similar to its predecessor the 6L6. In the world of consumer electronics applications, the 6L6 was not suitable for consumer amplification because it required a lot of input power, which means it needs a robust power supply. With the introduction of the lower-powered 6V6, which required only half the power of the 6L6, the beam-power tetrode became a usable technology for consumer electronics companies.

300B: The 300B is a directly heated power triode using a four-pin base. The 300B was introduced in the late 1930s to amplify telephone signals. In the 1980s the use of 300Bs by audiophiles had grown. The 300B has high linearity, low noise and good reliability. It is often used in single-ended triode (SET) audio amplifiers. These SET amplifiers output is approximately eight watts, while a push-pull pair of 300Bs can output about 20 watts. The 300B is a musical tube that produces a natural harmonic structure with low noise, and a realistic sound.

KT88: The KT88 fits a standard eight-pin octal socket and has similar pin-out and applications to 6L6 and EL34 tubes. Technically, the KT88 has similar ratings to the 6550 which was designed for use as a servo amplifier. It is one of the largest tubes in its class and can handle significantly higher plate voltages than similar tubes; up to 800 volts. A KT88 push-pull pair in class AB1 fixed bias is capable of 100 watts of output with about 2.5 percent total harmonic distortion or up to about 50 watts at low distortion in home stereo applications. It has a clean and tight, punchy sound.

2A3: To true tube fans, the single-ended triode (SET) power amplifier and the 2A3 triode tube are the equivalent of Megan Fox. Many believe it has the best balance; a combination of audio charm and power compared to other power triodes. While power amps that use the 2A3 were rare just a few years ago, they are now plentiful.

Single-Ended Triode: A single-ended triode uses a single triode to produce an output. It is generally designed to operate in Class A, and many consider SET products to be classic because of their. Simplistic circuit designs.

Push-Pull: In a push-pull amplifier, engineers connect the power supply to the center-tap of the transformer, and a vacuum tube is connected to both the upper and lower end of the center-tapped primary circuit. This configuration allows the tubes to run on alternate cycles of the input waveform. A push-pull design requires at least two tubes to operate and most designs are run in Class AB for power and efficiency.

Triode: An electronic amplification device that contains three active electrodes. The term is usually applied to a tube with three elements: the filament or cathode, the grid, and the plate or anode.

Pentode: An electronic product that uses five active electrodes. The term most commonly used to describe a three-grid vacuum tube. One of the most popular pentode vacuum tubes is the EL34.

Ultra-Linear: A type of electronic circuit that is used in tandem with a four-element tube (tetrode) or pentode tube to a speaker. Audiophiles like ultra-linear operation because of its lower output impedance and distortion levels. Engineers can utilize an ultra-linear design with push-pull or single-ended amplifier concepts.

NOS: New old stock (NOS) is used to describe tubes that were manufactured decades ago, but never used. These vacuum tubes are no longer manufactured by companies under brands like RCA, GE, Sylvania, but they are coveted by tube enthusiasts.

Tube Rolling: When a person decides to change the sound of a tube amplifier by switching vacuum tubes to a different type/version. The idea behind the concept is that each different tube type has a different electrical characteristic and that by testing various tube types, you will find the Holy Grail of audio. Tube enthusiasts find the practice of tube rolling fun because of the dramatic and not so dramatic changes it makes to the tone of an amplifier.

Solid State vs. Vacuum Tube Rectifier: A rectifier converts alternating current (AC) to direct current. A vacuum tube rectifier has resistance, and the more current that travels through a tube rectifier, the more the voltage drops. When the voltage drops, the power of the amplifier also drops, which in turn makes a tube rectified amp sound "spongy" in the bottom end. A solid state rectifier has no internal resistance. It has a fixed voltage drop, and when an amplifier needs power at low frequencies, there's no limitation to its power. A solid-state amplifier has more headroom and audio enthusiasts describe their sound as punchy and articulate.