Radial potential control by use of end-plate biasing in the GAMMA10 tandem mirror is very effective to suppress low-frequency fluctuations and to achieve a reactive plasma with hot ion temperature of up to 10 keV. In order to clarify effects of both radial electric field and its shear on low-frequency fluctuations, basic experiments have been carried out a small linear device, QT-U of Tohoku University, in which systematic control and precise measurements of radial potential profiles can be done. Flute-mode and drift-mode fluctuations appear in the radial region with steep density gradient. The observed flute-mode is identified as Kelvin-Helmholtz instability driven by strong E × B drift shear. The drift-mode fluctuations depend complicatedly on both radial electric field and its shear. The drift-mode is destabilized in the region of weakly negative electric field. In the strong Eτ region, the mode is suppressed, irrespective of its sign. This behavior agrees qualitatively with that observed in the ECH mode of GAMMA10. The drift-mode in the QT-U is clearly stabilized by the increase in net ion drift shear which is defined as the sum of E × B drift shear and ion diamagnetic drift shear.