R-55 Thyratron VCO

The R-55 is essentially one of the VCOs in our legendary S-1000 Wretch Machine, boiled down into a 22U Eurorack module. It produces the notorious “sharkfin” sawtooth waveform of thyratron VCOs, replete with the weird pitch variations and apparent “noisiness” of a thyratron oscillator.

Turn up the overdrive control, and the oscillator output clips the output amplifier pentode tube, for a unique soft-clipped square-wave sound. The simplicity of the R-55 will fool you--its behavior is erratic and weird, it always sounds “broken”, and the waveform is “deviated” in every way. You don't even need to feed it Hz/v CV: just run a square-wave or pulse signal from a conventional VCO into a pitch CV input, and it will “force” the R-55 to track along with the other VCO.

CV response: Hz/v, giving about 1.5 octaves of usable range with a Kenton Pro-Solo MIDI/CV. Span and offset trimpots on front panel, adjustable to make the R-55 track roughly along with the Metasonix R-54 Supermodule, or with other Hz/v VCOs. Maximum pitch range, 60 Hz to more than 3 kHz (approximate), range with CV typ. 2.5 octaves maximum. Octave switch raises pitch range about 2 octaves. Input jacks: two CV for pitch, plus one that optionally controls the gain of the pentode clipping circuit, acting as a VCA. Maximum output signal, 20 v p-p. Power requirement: +5v at 1.8 amps cold, 650 mA hot, plus +-12v dc, 50 mA.

(All specifications are approximate and may vary from sample to sample.) Tubes used: 2D21/5727 thyratron, 6AK5 pentode.

WARNING: the R-55 draws a LOT of power from the +5v line when first powered up. Your cabinet MUST be able to produce 1.8 amps at +5v to warm up the tubes in the R-55. Doepfer and Kilpatrick +5v power adapters cannot supply this much current. Addition of a large +5v supply to your Euro cabinet may be required. Monorocket now offers a special cabinet with +5v power sufficient to run at least three R-55 modules, plus +-12v power sufficient for many other Metasonix R modules plus modules from other manufacturers.