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Cathode

The cathode is the electron emitting element in a vacume tube. In most tubes it is heated by a filament similar to that in a light bulb, but only to a dull red, as electrons are more easily released from hot conductors. Certain elements such as thorium are better emitters than others and are plated on or added to the filament. The thorium boils off slowly and when it is used up, emission drops off and the tube must be replaced. See below for the two common types of cathodes used in radio tubes.

Carrier:

The carrier is the radio frequency signal which is radiated by the antenna. To make it useful, it is modulated by an audio or video signal. Morse code is sent simply by turning the carrier on and off to form the dots and dashes.

Detector:

The circuit in a receiver used to extract the audio or video signal from the radio wave. Early detectors were simple selenium, gallium or quartz crystals. These were later replaced by vacuum tubes and eventually modern transistors.

Directly vs Indirectly Heated Cathode

In a tube, electrons are emitted by a hot cathode, similar to a filament in a light bulb. Two methods are used. In a directly heated cathode, the filament itself is coated with the element thorium (a good emitting material) and serves as the cathode. In an indirectly heated cathode, a standard tungsten filament is place inside a thorium coated metallic cylinder and electrically separated from it by a layer of glass or porcelain. In this case the filament serves only as a heater. There are many advantages to doing this, one is that it allows AC current to be used to heat the cathode without introducing AC hum and noise into a sensitive circuit.

Heterodyne or Super Heterodyne:

A radio circuit which utilizes a local oscillator to generate a radio frequency. This frequency is then mixed with the signal from the antenna producing a third frequency, which depending on the design, is the mathematical sum or difference of the two mixed signals. This third frequency, known as the intermediate frequency, is then amplified and filtered prior to extracting the audio content. This circuit was patented by RCA in the early 1920s but due to licensing issues and the resulting litigation, was not put into production until the latter part of the decade. The heterodyne circuit is still used in modern TV and radio receivers.

Modulation:

Modulation is the audio, video, data, or other intelligence which is added to a radio frequency signal for transmission over the air. The receiver then extracts or "detects" the intelligence from the radio signal.

Oscillator:

A circuit used to create an audio or radio frequency. Oscillators may be variable, or fixed on one frequency.

Pentode:

  Pentode Vacuum Tube

A tube having 5 elements, a Cathode to emit electrons, a Grid to control the flow of the electrons, a second grid referred to as a screen grid, for additional control, isolation, or for injecting a second signal, a third grid known as a suppressor grid, and a Plate to receive the electrons. The filament or heater, is assumed and not counted when describing a tube.

Regenerative Receiver:

An early radio circuit which mixed a small amount of signal from its output back into its input. This "controlled feedback" resulted in tremendous signal gain from early tubes which otherwise exhibited marginal gain at best. The circuit was used in some of the earliest receivers from 1918 to the mid twenties. It was somewhat unstable and would often break into a howling oscillation. It was replaced by the TRF circuit as the performance of tubes began to improve. The circuit experianced a brief comeback in the 1930s with the growing popularity of short wave reception but this was short lived. It remained popular with hobbyists and ham radio operators into the 1950s as it required a minimum number of parts to build.

Selectivity:

The ability to separate nearby and adjacent stations.

Sensitivity:

The ability to receive weak stations.

TETRODE:

  Tetrode Vacuum Tube

A tube having 4 elements, a Cathode to emit electrons, a Grid to control the flow of the electrons, a second grid referred to as a screen grid, for additional control, isolation, or for injecting a second signal, and a Plate to receive the electrons. The filament or heater, is assumed and not counted when describing a tube.

TRF:

  Tuned Radio Frequency

An early radio circuit whereby the composite radio signal was amplified by several vacuum tube stages prior to extracting the audio content. Most sets used three stages of amplification but a few high end sets used as may as seven. The TRF circuit was common in radios manufactured throughout the 1920s. The TRF receiver was eventualy replaced by the heterodyne circuit.

TRIODE:

  Triode Vacuum Tube

A tube having 3 elements, a Cathode to emit electrons, a Grid to control the flow of the electrons, and a Plate to receive the electrons. The filament or heater, is assumed and not counted when describing a tube.

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