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SKU: MET_.RK7 Maker: Types: ,


Thyratron VCO

The RK7 is an 8HP Eurorack version of our legendary TM-3/S-1000/R-55 VCO circuit. The entire circuit is based on two tubes: a 5696 thyratron and a 5702 subminiature pentode, both of which are military surplus tubes used in avionics. The 5696s are still available but getting scarce, and they require a higher plate voltage than our other Euro modules. Like the R-55, the RK7 has a pentode clipping stage which provides voltage control over the gain and clipping level. 5702 subminiature pentodes have always been rare and little used outside military equipment.

The RK7 is profoundly weird and erratic. Don’t expect it to be perfect – it’s far from that. We’ve been making products with thyratrons for 20 years and still can’t figure out the whole situation. Apart from the early Trautonium in the 1930s, no one ever used thyratrons as music VCOs before. Everything about the RK7 is WRONG and CONTRARY to “standard practice in the sainted world of electronic music” blah blah blah. It is a little piece of raw and brutal 1958 reality for your spiffy and bland 21st-century modular synthesizer. Looking for “an authentic experience”? It doesn’t get more authentic than this.

Only two waveforms are available: a crude sawtooth and a distorted sine wave, with some soft clipping available for a “squaring” effect ha ha ha. The normal CV control range is from about C2 to C4, roughly 2 octaves. The OCTAVE switch lowers it by 2-3 more octaves and yes, if you’re insane, you can use it as an LFO.

The Pitch and VCA CV Inputs accept -10v to +10v with minimum 50k ohm input impedances. The Pitch CV Input is limited to +/-10v range and is Hz/Volt only. (If you want it to track v/octave, use a CV keyboard with Hz/v or a Hz/v converter module.) The usable pitch control CV is roughly 0-4 volts, depending on the setting of the SPAN trimpot, while the VCO itself only produces slightly more than 2 octaves from CV control. The VCA CV input uses bipolar CVs (such as LFOs) or audio signals, and cannot be damaged by over-voltage.

All specifications are approximate and may vary from sample to sample. The RK7 uses only about 200 mA (500 mA briefly during cold power-up) from +12v and 5 mA from the -12v supply rail. We deliberately run it colder than usual for maximum lifetime. Input impedance (CV) is 50k ohms and output impedance is 1k ohms. (Do NOT attempt to drive a 600-ohm load.) There is an 80-volt power supply built into the RK7, so do not poke around inside it like a fool.

The Metasonix RK-series of vacuum-tube modules is intended for use in any “Eurorack” standard modular synthesizer system. All RK-series modules are 8HP (40.7mm) wide and fit in any Doepfer(tm), Analogue Systems(tm), Monorocket ™, or other compatible cabinet. They are powered entirely from the cabinet’s internal +-12v dc power supplies, using Doepfer compatible 16-pin power connectors. Their circuits protrude behind the panel less than 31mm (1.2 inch). All vacuum tubes are NOS (new old stock) types from classic American and European manufacturers that are run very conservatively for long life, and are readily available from distributors. The tubes in the RK7 protrudes from the panel for visibility and cooling, less than 20mm (.79 inches), and the RK7 unquestionably fits inside a Doepfer A-100 suitcase cabinet lid or into a Doepfer mini case or Monorocket case cover. All audio inputs and outputs and CV inputs/outputs are 100% compatible with other synthesizer modules. Each RK-module is fully protected against reverse supply voltage, overvoltage, undervoltage, or incorrect hookup to external equipment. So don’t worry! As with all our products, the RK series is totally handcrafted in Northern California, USA.


  • Handcrafted in Northern California, USA
  • +/-12V Doepfer-style power connector
  • power: 500 mA +12V, 5 mA -12V
  • width: 8HP