In the most general sense, a cathode is a terminal or electrode through which electrons enter an electrical device, such as a battery, vacuum tube, or diode. This process is generally associated with the flow of electric current.
To better understand this, let’s start by examining the use of cathodes in vacuum tubes, a technology that is pivotal in the creation of vintage-style sounds in music production.
Cathodes in Vacuum Tubes
A vacuum tube, also known as a valve, contains electrodes, including a cathode, enclosed in a vacuum within a glass envelope. These were the predecessors to modern solid-state electronic devices like transistors.
When a vacuum tube is functioning, the cathode is heated either directly or indirectly to emit electrons. This emission of electrons, known as thermionic emission, results in the formation of an electron cloud. This cloud is then attracted to a more positively charged electrode, known as the anode, creating a current of electrons or a cathode-ray.
Vacuum tubes are fundamental to the iconic sounds of guitar amplifiers, with companies like Fender, Marshall, and Vox all utilizing this technology. The way these tubes distort when overdriven gives the warm, smooth character that musicians often prefer over the hard clipping of solid-state amplifiers.
Cathode-ray Tubes (CRT)
While not directly related to music technology, CRTs provide a good illustration of how cathodes work in electronic devices. In a CRT, electrons emitted from a cathode strike a phosphorescent screen to create images.
Although we’ve largely moved on to digital display technology in the music studio, CRT technology was once standard in visual display units, allowing engineers to visualize waveforms, equalizer settings, and other technical aspects of a mix.
Cathodes in Diodes
A diode is a semiconductor device that allows current to flow in only one direction, and it has two terminals, an anode and a cathode. When a positive voltage is applied to the anode and a negative voltage to the cathode, the diode becomes forward-biased, allowing current to flow.
Diodes are used in many audio applications, including rectifiers in power supplies of audio equipment, clippers and limiters in guitar distortion pedals, and overvoltage protection circuits.
Cathodes in Batteries
In a battery, the cathode is the electrode where the reduction reaction occurs, and it is also the positive terminal. It’s worth noting that this seems to contradict the definition we started with – in a battery, electrons are received at the cathode, not emitted. This is because of the different nature of chemical reactions inside a battery compared to the physical process of electron emission in a vacuum tube or diode.