Specified voltage AND current AC/DC - Any filament or heater, unless otherwise stated by the tube manufacturer behaves like a resistive element and then must be operated at the voltage and the current is given in its documentation and generally given in the Vf and If fields. In some cases, as for tungsten filaments of old power tubes, either voltage or current might be not specified, the exact operating value being marked somewhere on the bulb. This option must be applied to filaments/heaters of direct and half-indirect rectifiers and of other tubes intended to operate with cathode floating or pulsed at high negative voltage with respect to ground, as many CRTs and radar transmitting tubes.

The connecting method, parallel or series, to the filament or heater of other tubes is left to the user, who has to follow the recommendations of the tube manufacturer, complying at the same time with the operating limits of each tube.

Examples: CK1, CF3 with e.g. ABC1, CL4 and CY2 (C-series for AC/DC or for car "batteries" 12.6 V).