tube <77> vco

Description: The VCO described here is a direct derivative of the original oscillator invented by Friedrich Trautwein in 1932. It illustrates the challenges faced by the music world: the Mixtur-Trautonium oscillator was a true voltage-controlled oscillator (VCO), created at a time when the Theremin was still a novel concept. This situation highlights the shortcomings of both the academic and popular music scenes. Had Trautwein received more support, he and his collaborator Oskar Sala might have advanced their work and potentially created the voltage-controlled synthesizer long before Bob Moog and Don Buchla entered the field. The history of electronic music could have been different, as the term "synthesizer" might have emerged three decades earlier. Instead, only a few instruments were constructed by Telefunken, Sala worked on some film soundtracks, and a handful of composers wrote pieces for the Mixtur-Trautonium, which largely went unnoticed outside of Germany.

Today, despite a resurgence of interest in vacuum tubes within recording studios, electric guitars, and audiophile homes, electronic keyboardists often remain firmly attached to the latest digital technologies. This rigid mindset mirrors the attitudes of 1932, albeit in a distorted manner. Digital signal processing (DSP) chips have supplanted traditional instruments like flutes and trumpets. The VCO in question functions as a relaxation oscillator centered around a thyratron tube, which, despite not being a vacuum tube, contains a mixture of xenon and other gases. This device operates in an on-off manner, characterized by two states. The term "thyratron," derived from Greek meaning "door," signifies its binary operation. Initially developed in the early days of tube technology, thyratrons were primarily utilized for basic control functions. The specific thyratron referenced, the 2D21/5727, was originally designed for switching small relays. It is highly sensitive, contributing to the foundation of many electronic control systems that exist today.

The 2D21 bears a rough resemblance to standard vacuum amplifier tubes, though its construction has been modified to ensure gas ionization occurs at the appropriate times. The grids are engineered to shield the anode from the heated cathode until the correct voltage conditions are met. The cathode utilizes materials distinct from those in vacuum tubes to prevent damage from gas ions. Additionally, various wiring connections are coated with high-temperature cement to ensure that only the anode is exposed to ionization current. Different types of thyratrons exist, such as triode, tetrode, and pentode versions, with the extra grids in tetrode and pentode types providing additional shielding for the anode, enhancing the tube's sensitivity.

To achieve oscillation with a thyratron, it is necessary to connect its cathode and control grid(s) to ground, apply a positive voltage exceeding 70 volts to the anode through a resistor, and connect a capacitor between the anode and cathode. This configuration results in a relaxation oscillator that generates a sawtooth waveform. The oscillation frequency is determined by the RC time constant of the capacitor and resistor, as well as the ionization voltage of the thyratron, which is approximately 70 volts. Variations in frequency can be achieved by adjusting the resistor, capacitor, or the voltages applied to the grid or anode. The most straightforward method to create a voltage-controlled oscillator (VCO) involves altering the sensitivity of the circuit components.The VCO presented here is a direct derivative of the original oscillator invented by Friedrich Trautwein in 1932. And it perfectly demonstrates why I`m so cynical about the music world: the Mixtur-Trautonium oscillator was a true VCO, invented at a time when even the Theremin was a new and radical concept.

All this shows the academic and popular musical scene at its worst, for if Trautwein had received some encouragement, he and his collaborator Oskar Sala might have persisted with development. And they might have invented the voltage-controlled synthesizer, at a time when Bob Moog and Don Buchla were still sucking on their pacifiers!

History might have been very different, if only the average musician were not so stuck in relentlessly rigid thinking. The term "synthesizer" might have appeared, 30 years before it actually did. (Instead, a few instruments were built by Telefunken; Sala did some movie soundtracks; a few composers wrote a few pieces for the Mixtur-Trautonium; and it was basically ignored outside Germany.

) Are things so very different today Even though vacuum tubes are enjoying a considerable surge of popularity in the recording studio, with the electric guitar, and in the home of the audiophile. the electronic keyboardist continues to march grimly forward, hopelessly wedded to whatever the latest digital technology is.

This closed mindset seems amazingly like 1932 to me, only distorted in a funhouse mirror. DSP chips have replaced flutes and trumpets. The VCO is simply a relaxation oscillator built around a "thyratron" tube. This device is not a vacuum tube, but contains a tenuous mixture of xenon and other gases. It`s an on-off device, having only two operating states. "Thyros" is Greek for "door", so it`s a "door tube"; either open or closed. The thyratron dates from the early days of tube technology, and was usually used for simple control functions. The small thyratron used here is the 2D21/5727, originally intended for use in switching small relays on.

It is very sensitive, and much of the electronic world we know today originated from the electronic control systems made possible by thyratrons. (Indeed, today`s SCR or triac is a "thyristor", simply a semiconductor copy of the thyratron. Larger thyratrons were made for controlling AC motors and other loads. ) The 2D21 is very roughly similar in construction to any vacuum amplifier tube. Some details have been changed, since you want the gas to ionize only at the right time. The grids are designed to shield the anode from the hot cathode, until the right set of voltages is seen.

The cathode uses different materials from vacuum tubes, since the gas ions can damage a regular oxide-coated cathode. And various wiring connections are coated with high-temperature cement, so that only the anode sees ionization current.

There are triode thyratrons and tetrode thyratrons (and even a few pentode thyratrons). The extra grids in the tetrode and pentode serve to further shield the anode from the grid potential. So, the "shield grid" further increases the sensitivity of the tube. To make a thyratron oscillate, simply connect its cathode and control grid(s) to ground; apply a positive voltage greater than 70v to the anode via a resistor; and connect a capacitor from anode to cathode.

Presto, a relaxation oscillator, producing a sawtooth-shaped waveform. The frequency is dependent on the RC time constant of the capacitor and resistor, and on the ionization voltage of the thyratron (about 70 volts). It varies from tube to tube, due to manufacturing tolerances. To control the frequency means either changing the R, the C, or varying the grid or anode voltages. It`s easiest for us to make a VCO by varying the sensit