Audio synthesis via vacuum tubes

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The 6ME8 beam-deflection tube.
A vacuum tube synth.

Audio synthesis via vacuum tubes is a selection of synthesizer module circuits which can be used in a manner similar to the modules of Moog and Buchla. Yet, the signal paths of these modules is implemented using only vacuum and gas tubes. No semiconductors are needed (except in control circuitry) to create a viable music system with the most extreme flexibility; patchable voltage control, a concept not previously seen with tube design. The result is an entirely new and little-known world of tonal variations, due to the unique linearities and nonlinearities of tubes.[1]

The vacuum tube

In electronics a vacuum tube or (thermionic) valve (outside the USA) is a device generally used to amplify, modify or control a signal. Mostly the vacuum tube has been replaced by much smaller, more reliable and less expensive solid state technology. Tubes are still used in several specialized applications such as audio systems and high power RF transmitters.[2]

Simply put vacuum tubes are arrangements of electrodes in a vacuum, (except for some special-purpose tubes which use a gas in the envelope) within an insulating, temperature-resistant envelope, (most often of glass). The electrodes are attached to leads which pass through the envelope via an air tight seal which are designed to plug into a tube socket for easy replacement. A filament (the cathode) is heated so that it releases electrons into the vacuum (thermionic emission). These are drawn to a metal plate (the anode) inside the envelope. Another electrode (the grid) is placed between the filament and plate. A voltage applied to the grid will control the plate plate current.[2]

In the history of synthesizer

Some people insist that science and technology advance by continuous and predictable movement. Yet many of the important advances of history have not occurred as a result of planning and R&D expenditure, but as blind accidents. Often, the perceptual weaknesses and limited artistic vision of real human beings tend to impede rather than assist the process of invention.[1]

The history of electronic music in the 20th century has followed such a path. Large research institutions, such as Bell Telephone Laboratories or RCA's research division, have made contributions of one kind or another. Yet the modern equipment of music bears very little resemblance to their contributions. Most of what we call a 'synthesizer' today came about as a result of the work of a few lone inventors with very little financial backing, and then selling their products to avant-garde composers who were willing to take a chance in order to explore a new musical world. The rest of the world's musicians tend to exhibit great inertia in the face of change, until the change becomes too great to ignore.[1]

Think of most of the major electronic keyboards and music devices before the early 1960s, when the Moog and Buchla synthesizers appeared. These 'presynths' tended to be exotic and unusual by prevailing standards. Yet they had little impact, even though most received great attention in their time.[1]

And in spite of their new design philosophies, these instruments usually followed a basic plan. The vast majority used a keyboard with top-octave division or a single LC oscillator with time constants switched in with the keys. This encompasses the Hammond Novachord (1939) and Solovox (1940), the Clavioline (1941), the Ondioline (1941) and the Univox (1946), its derivatives such as the Multimonika (1949), and virtually all electronic organs of the 1950s. Modulation was very primitive, usually involving selection of tremolo and key-down percussion effects; and waveform filtering was available only with fixed passive 'formant' filtering.[1]

The few devices outside this scheme usually had some kind of linear or position-controlled pitch selection rather than a keyboard. This includes the Theremin (1927), the Hellertion (1928), the Ondes Martenot (1928), the Trautonium (1928) and the Mixtur-Trautonium (1932). Except for the Theremin, these all used a linear strip controller or a moving indicator on a linear strip to control pitch. Waveform modulation was as simple as on the keyboard devices; infinitely-variable pitch control was considered radical enough already. The Theremin has proven to be the most popular of these instruments, although the Ondes and Trautonium have had some success with composers over the years. And automated devices, such as the RCA Synthesizer (1955) and the Kent Music Box (1951), were highly automated yet still limited in tonal effects and real-time modulation.[1]

All such instruments above were based on vacuum tubes or gas-filled tubes (in the case of the Trautoniums). It seems only an accident of history that generalized modulation systems and tone control only arrived with the Synket and the Moog and Buchla synthesizers, in the early '60s. All these were solid-state. Before 1961, tubes did not have the concepts of voltage control applied to them except in very primitive ways, such as pitch or tremolo. And tubes essentially disappeared from the scene until recently, as the electronics industry embraced the transistor and discarded what came before.[1]

Ken Stone's tube synth. Above: VCA, VCF, beam modulator and VCAs, beam modulator, Hellfire clone. Below: 2 x VCO, 2 x HFM, beam modulator, VCF
From the rear: The top rack contains the Hellfire clone, and the two 6BN6 type modulators, one with two tube voltage controlled amps. The bottom rack contains the two 1 volt per octave VCOs, Modulators, VCF, Beam modulator and a preamp.
The bottom rack from another angle.

The circuits

Listed in order of complexity/difficulty of construction.


This circuitry is intended for the more advanced builder. Because high voltages are used, a shock hazard exists. We do not recommend that the novice DIY musician try to construct this synthesizer. Some experience with tube electronics is highly recommended.

All these projects and designs should be considered dangerous if not lethal if not used safely. When working on projects based on these designs, use extreme care to ensure that you do not come into contact with mains AC voltages or high voltage DC. If you are not confident about working with mains voltages, or high voltages, or you are not legally allowed to work with mains voltages, or high voltages, you are advised not to attempt work on them.


Readers are permitted to construct these circuits for their own personal use only. Eric Barbour retains all rights to his work. Ken Stone retains all rights to his work.

See also


  1. ^ a b c d e f g Audio Synthesis via Vacuum Tubes (archived) by Eric Barbour, 1997, with permission of the author
  2. ^ a b "Vacuum tube – Basic electronics tutorial". Archived from the original on 2019-06-07. Retrieved 2019-06-07. 

External links