According to some embodiments, NMR (nuclear magnetic resonance) circuits include one or more variable capacitors each having two concentric cylindrical conductors. The inner conductor is connected to an upper external contact disposed closer to the NMR sample coil than the outer conductor. The inner conductor and upper contact are generally exposed to higher voltages than the outer conductor, and the upper contact is shorter than the outer conductor. Capacitance adjustment is performed by moving the inner conductor longitudinally within the outer conductor. Using a relatively short contact for the higher-voltage capacitor node allows reducing the surface area most conducive to arcing, and allows reducing lead lengths and stray capacitances. Shorting the capacitor may be performed by moving the inner conductor to contact a capacitor base situated opposite the upper external contact.