Mode-locked (ML) operation of a simple Yb:KGW oscillator is described, providing 12 W at 1038 nm with 290 fs pulse width. A polarization-coupled scheme is used for efficient longitudinal pumping by a pair of reshaped laser diode bars. With changes in cavity dispersion, the pulse width is adjustable from 134 to 433 fs, in a high quality circular mode. A Saturable Absorber Mirror (SAM) provides self-starting operation, and the cavity is stabilized by the Kerr-lens effect.