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Metasonix TS-23 Dual Thyratron VCO

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The TS-23 uses two 2D21/5727 thyratron tubes to make musical pitches. These are not "vacuum tubes" proper, but gas-filled switching tubes. They act like unstable switching devices, allowing a capacitor to charge, then discharging it at a certain voltage level. By applying a varying voltage to the tube-s grid, you can change the switching point; thus changing the length of the charging cycle; thus, the pitch of the resulting sawtooth wave. The TS-23 uses a solid-state op-amp IC to drive the tubes; it is in a plastic-potted module inside the cabinet, and operates from its own DC power derived from the 6.3v AC heater supply. (We could have used a low-impedance tube, such as a 6AS7, to drive the thyratrons, but this would have increased the size and cost of the unit greatly, with no real sonic difference.)

Because the thyratron is not a very "good" switch, its sawtooth output is not a mathematically-perfect sawtooth. Instead, the discharge cycle is noisy and rounded-off. And the charging follows an exponential curve, not a straight line. The resulting sound is distinctive to such tubes, and unlike any solid-state VCO or pitch source. And as the control voltage (CV) is increased, the amplitude of the waveform decreases, as well as its discharge shape. This was previously seen only in pioneering electronic musical instruments, most notably the "Mixturtrautonium" used by Oskar Sala in a number of movie soundtracks in the 1940s and 1950s. So, though the TS-23 won't be confused with a high-performance laboratory oscillator, its unique sound makes it a worthy adjunct to the analog synthesis studio.

The TS-23 contains two thyratron VCOs, both driven from the same CV source (no, they can't be modulated separately). Switching on VCO 2 allows the player to tune the two VCOs together, or detune them, for a variety of "beat note" sound effects. VCO 2 is mixed into VCO 1's output at a lower level, to differentiate them. The SOFT SYNC switch allows VCO 1 to "force" VCO 2 to follow it over a narrow range of detuning (usually less than one whole interval, approximately).

Sawtooth waveforms are not the only ones available from the TS-23. It contains a special waveform clipper, based on the obscure 6BN6, a beam modulator tube intended for use in FM radio receivers. The 6BN6 is very, very nonlinear--but it doesn't clip like a diode or other conventional circuit. It provides more of a waveform "squishing" effect, which may be varied by changing the tube's screen voltage. The 6BN6 is unique, and does not behave like a typical triode, pentode or other vacuum tube.

Technical Specifications
Type: Analog
Synthesis: Oscillator(s)
Oscillators: 2
Waveforms: Saw Down
Osc Modulation: Input, Knob
Polyphony & Tuning
Polyphony: 1
Timbrality: 1
Tuning: Micro
Modes: Mono
Case: Desktop
Controls: Knobs
Audio Output Connections: 1/4" Phone Jack, Mono Out
CV Ports: CV IN
Year Released: 2001
Manuals & Documents
MSRP List Price: $1,400 - convert
Retail Street Price: $1,099 - convert

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