Basics
Plate: attracts electrons. Grid: controls their flow. Cathode: emits electrons when heated. Heater: warms the cathode.
Thermionic emission
A metal cathode heated to a high temperature releases electrons from its surface. In an ordinary lamp those electrons are mostly unimportant. In a vacuum tube they become the working current.
Early tubes often used the filament itself as the cathode. Later tubes commonly used an indirectly heated cathode: a separate heater warms a coated metal sleeve that emits electrons but remains electrically separate from the heater circuit.
Plate or anode
A positively charged plate attracts emitted electrons. Conventional current is drawn in the opposite direction to electron motion, so tube schematics normally describe plate current as flowing from the positive power supply through the plate circuit toward the cathode.
The control grid
A triode places a wire grid between cathode and plate. A small change in grid voltage strongly changes the number of electrons that can reach the plate. This is the basic amplifying action.
A grid made more negative relative to the cathode repels electrons and reduces plate current. Making it less negative allows more plate current.
Bias
An amplifier normally establishes a steady operating point before the audio or radio signal is added. This steady grid-to-cathode voltage is the bias.
Several bias methods are common:
- Cathode bias: cathode current develops voltage across a cathode resistor.
- Fixed bias: a separate negative supply sets grid voltage.
- Grid-leak or contact bias: used in selected low-level circuits.
Voltage gain
In a resistance-loaded triode stage, a small increase in grid voltage can increase plate current. The greater current causes a larger voltage drop across the plate resistor, so plate voltage falls. The output is therefore amplified and inverted.
High voltage
Many vacuum-tube circuits operate with hundreds of volts on the plates. Power amplifiers may use still higher voltages.
Heaters
A tube heater may operate at 6.3 V, 12.6 V, or another specified voltage. Heaters draw substantial current and can introduce hum if their wiring, grounding, or supply arrangement is poor.
Tube sockets
Unlike most modern transistors, tubes are usually plugged into sockets. Octal, noval, miniature 7-pin, and other socket families allow a worn tube to be replaced without soldering.
Warm-up time
A cold cathode emits poorly. Tube equipment therefore takes a few seconds to reach normal operation. Large transmitting tubes may require carefully timed warm-up and power sequencing.