Vacuum tubes, amplifier tubes, or thermionic valves, are the heart of guitar amplification. Even solid state amps are usually designed to emulate the sound of tube amps. But the vacuum tube was actually critical for all electronics until the transistor replaced it for most uses in the 1960’s.
The earliest, simplest tubes were diodes. John Ambrose Fleming invented the Fleming Diode in 1904. Diodes have a heated cathode and an anode and allow current to flow in only one direction – from the heated cathode to the plate, or anode. We use diode tubes as rectifiers in tube amps because they convert AC to DC. But the amplification with tubes was not possible until the invention of the triode.
The triode differs from the diode in that a control grid is placed between the anode and cathode. The control grid is so named; because as the voltage on the grid is changed it controls the current flowing from cathode to anode. The triode was invented in 1907 by Lee DeForest, but it was not until 1912 that its use in amplification was discovered. The triode was a game changer, revolutionizing telephone and radio and creating the field of electronics. The 12AX7 dual triode found in modern tube amps was invented in 1947. Of course, lots of other tube types have been used in preamps, from the EF86 to the 6SJ7, 6SC7, and others.
The next development in tubes was to add a second grid, known as the screen grid. This screen was located between the control grid and the plate (anode), and was designed to counter some problems with capacitance between those two electrodes. The tetrode was more powerful and had a higher frequency capacity than the triode, and was common by 1926. Unfortunately, in certain situations electrons striking the anode could cause other electrons to be ejected from it (think of a ball hitting water and causing a splash), and those electrons could be captured by the screen grid. This is known as the “tetrode kink”. We can think of the tetrode as in interesting stop along the way to the power tubes we use in amplification – the pentode and the beam pentode.
The pentode (classic examples are the EL84 and the EL34) solved the secondary electron emission problem by adding a “supressor grid” between the anode and the screen grid. The first pentodes were invented in 1926. While we think of these as power tubes, the EF86 is a pentode that was used in early VOX amplifiers as a preamp tube.
Closely related to the pentode is the beam tetrode. The 6V6 and 6L6 are examples of beam tetrodes. The beam tetrode was patented in 1932 and solved the tetrode kink by focusing electrons on certain parts of the anode and repelling secondary electrons back to the anode.
Developments in tube technology were important for radio, telephone, television, computers, and virtually every other area of electronics. When most tube applications in the US were replaced by transistors, the worlds of hi-fi audio, guitar amplifiers, and tube based studio equipment such as compressors and preamps did not provide enough demand for continued manufacturing here. As a result, we now get them overseas or from the diminishing supply of NOS US made tubes.
Alex’s Bio: Alex Anest has been performing, recording and teaching music in the Southeast Michigan area since 1996. He was a founding member of the Jericho Guitar Trio, Never Nebula, and Delta 88. With Delta 88 Alex performed across the Midwest and played at the Ann Arbor Folk Festival in 2004. Since then he has toured Switzerland, Germany, Austria and Italy with songwriter Kevin Meisel. Alex currently performs with Ryan Racine and Gas for Less and the electric anti-jazz ensemble Giraffe. Giraffe is a chance for Alex to bring his many musical influences together – a very enjoyable, though sometimes difficult task for a musician who finds inspiration from artists as varied as Paco de Lucia, George Harrison, Thelonious Monk, and Jimi Hendrix. The common thread among these giants (and the goal to which Alex aspires) is the ability to transcend stylistic boundaries while keeping their own unique musical voice intact.