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RK7 Thyratron VCO


  • $ 66000


The RK7 is an 8HP Eurorack version of the 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 then our other Eurorack modules.  Like the R-055, 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 of military equipment.


The RK7 is profoundly weird and erratic.  Don't expect it to act perfect - it's far from that.  Metasonix has been making products with thyratron tubes for over 20 years and still cant figure out the whole situation. Apart from the early Trautonium in the 1930s, no one ever used thyratrons as music VCO's before.


The Pitch and VCA CV Inputs accept -10 to +10 with minimum 50k ohm impedances. The Pitch CV Input is limited to +/-10v range, the usable pitch range control CV is roughly 0-4 volts, depending on the setting of the SPAN trimpot. The VCA CV input uses bipolar CVs (such as LFO's) or audible signals, and cannot be damaged by over-voltage.


OCTAVE lowers the pitch of the VCO roughly two octaves when pushed up.

SIN/SAW switches between crude variances of a sine and a sawtooth waveform.

DRIVE enables waveform clipping and gain level from the pentode. when set above 10 o'clock, the waveform will begin clipping.

TUNE enables master tuning of the entire pitch range. A SPAN trimpot is accessible thru the front of the panel (below the tubes) for re-tuning. Accessing this trimpot may require some care, or removing the front panel.  BEWARE OF THE HIGH VOLTAGES AROUND THE TUBES, you jackoff.


Unlike the R-55, the RK7 can self-oscillate without the need of an external VCO applied to the Pitch CV Input. But it's advised that you apply an external square/pulse wave VCO to this input,  to force the RK7 to sync to the external VCO, if you prefer a more "stable and conventional" pitch range. This oscillator is always unstable and its waveforms are "uncharacteristic" of any other VCO on the market, like it's predecessors.  It drifts for no apparent reason, temperature has little effect.  Have fun keeping it from drifting.

Note that these controls have very large ranges, and their effective range is much smaller than their full rotation. This is normal, it was done to allow for variance from the vacuum tubes and other components.


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