A field effect transistor (FET) is an extensively employed component in large-scale integrated circuits to regulate circuit on or off by modulating its gate voltage. The mechanical counterpart of a FET is of great significance for advanced regulations of acoustic or elastic waves. Recently proposed active metamaterials are promising scenarios of FET based on the opening and closing of frequency bandgaps through active elements, but hindered by the structural complexity in practice. In this study, we propose and experimentally realize a nonlinear pendulum metamaterial (NPMM) as a mechanical ultra-low-frequency FET. By employing the perturbation method to investigate its dispersion relationship, we show that the NPMM possess a field effect bandgap. Namely, the bandgap can be opened or closed by controlling the excitation amplitude of the impinging wave, which is largely affected by the relative mass of the inner pendulum unit. As a verification, the transmission behavior of the NPMM is numerically studied and experimentally tested. It shows that, once the amplitude of the impinging wave exceeds a certain threshold, the disappearance of a bandgap will be observed with a sudden amplification of the output wave. The central frequency of the field effect bandgap is as low as 4 Hz, whereas the characteristic size of the unit cell is only 4 cm. We expect that the present work opens a new avenue for low-frequency wave regulation.