This is a USED item and has been fully tested.
The RK4 an 8hp module with a single vacuum tube.
It uses a triode-pentode in a unique Wien-bridge circuit, to provide clean sine-wave oscillation. Using a Vactrol, the pitch of the RK4 is sweepable from less than 20 Hz to more than 2 kHz, continuously, via front-panel control or a CV input. By adjusting the waveshape/resonance control, the sinewave signal produced by the RK4 can be trimmed to less than 10% THD (depending on operating pitch)---with NONE of the problems inherent in solid-state oscillators and sine waveshapers. Because the Wien bridge contains no piecewise sine shaper or amplitude-control circuit or component. The tube itself performs that job. So the distortion products are entirely low-order, unlike the "sine waves" that solid-state VCOs or samplers produce.
Turn up the RESONANCE, and the oscillator output gives a unique soft-clipped square-wave sound. The simplicity of the RK4 will fool you--its behavior is odd and radical, and years will be required to fully explore all of its quirks.
Now for the special trick. Plug an audio signal into the audio input. Instantly, the RK4 transforms from a VCO into a bandpass resonant voltage-controlled filter, with the same range as in VCO operation -- 20 Hz to >2 kHz. Its behavior and sound are unique. Again, you will be kept busy for years exploring all of its strangeness.
CV response: Hz/v. Use a suitable MIDI/CV interface, or use our R-60 interface to obtain equal-tempered tuning. Maximum output signal, 12 v p-p. Power requirement: +12v dc, approx 400 mA, and -12v at 10 mA. As with all our modules, it draws about 2.5 times the operating current during "cold" power-up, and you must insure your power supply can handle the inrush current. See our support section for more information and power supply suggestions.