Tube Types
Diode
The vacuum diode contains a cathode and plate. Electrons emitted by the hot cathode can travel to a positive plate, producing current. A negative plate repels electrons, so current is strongly one-directional.
Vacuum diodes were used as radio detectors and power-supply rectifiers. This is the direct historical bridge into the LearnTronics Diodes section.
Triode
A triode adds a control grid. A small grid-voltage change controls a much larger change in plate current, providing amplification. Triodes remain common in audio preamplifiers and phase inverters.
Tetrode
A second grid, called the screen grid, is placed between the control grid and plate. It reduces the electrical coupling between plate and control grid and permits higher gain and better high-frequency performance.
The simple tetrode introduces another problem: secondary electrons knocked from the plate can be attracted to the screen, producing an undesirable kink in the plate-current curve.
Pentode
The pentode adds a third grid, the suppressor grid, between screen and plate. It discourages secondary electrons from reaching the screen and sends them back toward the plate.
Pentodes can provide high voltage gain and strong output-power capability. The EL34 is a well-known power pentode.
Beam-power tube
Beam-power tubes use aligned grids and beam-forming structures to reduce secondary emission without a conventional suppressor grid wound in the same manner as a pentode. The 6L6, 6550, KT66, and KT88 families are important examples.
Dual and multiple-section tubes
One glass envelope can contain more than one electronic system. The 12AX7, for example, contains two triodes. Other tubes combine triodes with diodes, pentodes, or other sections to save sockets and chassis space.
Rectifier tubes
Rectifier tubes such as the 5Y3, 5U4, and GZ34/5AR4 convert transformer AC into DC for the amplifier's high-voltage supply. Their internal voltage drop and current capability influence the supply voltage and its response to changing load current.
Specialized tubes
Tube technology also includes gas-filled regulators, thyratrons, cathode-ray tubes, photomultiplier tubes, klystrons, magnetrons, traveling-wave tubes, and many other devices. They share electron or ion motion in evacuated or controlled-gas structures but serve functions far beyond ordinary audio amplification.