The application of dc magnetic field to a 2G high temperature superconducting (HTS) tape can reduce the critical current Ic of the tape, without driving the material normal. This property can be used in the development of HTS switching elements through so-called Jc(B) switching. The design of a dc magnetic field switch is reported, in both single and bifilar configurations. Dc magnetic field is applied by an iron-core field coil with copper windings, providing an applied field of up to 1.4 T. This allows for effective suppression of Ic. Here, it is also reported that the self-field Ic of the single tape is suppressed from 357 A to 195 A. This is due to the presence of the iron core amplifying and distorting the self-field of the tape and is confirmed by experiment and finite-element modelling. This effect can be eliminated using a bifilar configuration, which recovered close to the no-core self-field Ic of 357 A. Active dc switching by applied field was still achieved in the bifilar configuration. Such distortion and suppression may prove useful for the development of novel HTS circuit components.