Change the potentiometer to one of 200k ohm and you may have less minimum output power. What you say about the capacitor does not cause any functional difference in the circuit. That type of regulator works by phase partition and what it does is to control a timing by means of a RC time constant to produce the priming of the thyristor so that the longer it takes to produce the priming in the thyristor gate, the less energy it will receive The welder, therefore, by increasing the value of the potentiometer we get more delay in the dispro of the triac. When the voltage in the condenser reaches the trigger threshold of the diac (plus or minus 30 volts) the current causes the gate to prime by discharging the capacitor. And a new capacitor charge cycle is initiated by the potentiometer and the limiting resistor in series with the capacitor.