The ROG Eighteen is an overdrive/distortion circuit based on the classic 18watt Marshall amp of the late 60s. It uses JFETs in place of tubes, like many other ROG amp-based drive effects. J201s are specified, but socket and try some other JFETs (2N5457, MPF102, etc.) The Volume control will probably act more like a gain control, though there is no resistor limiting the minimum amount of signal sent to ground when it's at zero. For the schematic and more info on the circuit see the ROG article.