Each tuned RF stage consists of an amplifying device, a triode (or in later sets a tetrode) vacuum tube, and a tuned circuit which performs the filtering function. The tuned circuit consisted of an air-core RF coupling transformer which also served to couple the signal from the plate circuit of one tube to the input grid circuit of the next tube. One of the windings of the transformer had a variable capacitor connected across it to make a tuned circuit. A variable capacitor (or sometimes a variable coupling coil called a "variometer") was used, with a knob on the front panel to tune the receiver. The RF stages usually had identical circuits to simplify design.