Valve amplifier
[1] The simplest valve (named diode because it had two electrodes) was invented by John Ambrose Fleming while working for the Marconi Company in London in 1904.
In 1906 Lee De Forest added a third electrode and invented the first electronic amplifying device, the triode, which he named the Audion.
The overwhelmingly dominant circuit topology during this period was the single-ended triode gain stage, operating in class A, which gave very good sound (and reasonable measured distortion performance) despite extremely simple circuitry with very few components: important at a time when components were handmade and extremely expensive.
A range of topologies with only minor variations (notably different phase splitter arrangements and the "Ultra-Linear" transformer connection for tetrodes) rapidly became widespread.
This family of designs remains the dominant high power amplifier topology to this day for music application.
Valves remained in certain applications such as high power RF transmitters and the microwave oven, and audio amplification equipment, particularly for the electric guitar, recording studios, and high-end home stereos.
In audio applications, valves continue to be highly desired by most professional users, particularly in recording studios' equipment and guitar amplifiers.
Most audio technicians and scientists theorize that the 'even harmonic distortion' produced by valve tubes sounds more pleasing to the ear than transistors, regardless of style.
In a tube amplifier, the transition from linear amplification to limiting is less abrupt than in a solid state unit, resulting in a less grating form of distortion at the onset of clipping.
Although the cathode resistor can be many kilohms (depending on biasing requirements), the small-signal output impedance is very low (see operational amplifier).
Valves remain in widespread use in guitar and high-end audio amplifiers due to the perceived sound quality they produce.
A specific issue for the telecommunication industry was the technique of multiplexing many (up to a thousand) voice lines onto a single cable, at different frequencies.
The problem is that the amplifiers need to be extremely linear, otherwise "intermodulation distortion" (IMD) will result in "crosstalk" between the multiplexed channels.
Post World War II, the majority of valve power amplifiers are of the Class AB-1 "push pull" ultralinear topology, or lower cost single ended i.e. 6BQ5/EL84 power tubes, but niche products using the DH-SET and even output transformer-less topologies still exist in small numbers.
The vacuum tube voltmeter (VTVM) uses the high input impedance of a valve to buffer the circuit being measured from the load of the ammeter.
An op-amp typically has a differential input stage and a totem pole output, the circuit usually having a minimum of five active devices.
For example, an RF device might be required to operate over the range 144 to 146 MHz (just 1.4%) Today, radio transmitters are overwhelmingly silicon based, even at microwave frequencies.
