We　report　a　picosecond　ytterbium-doped　double-clad　fiber　amplifier　with　a　semiconductor　saturable　absorber　mirror　(SESAM)　mode　locking　large　mode　area　photonic　crystal　fiber　(PCF)　oscillator　as　the　seed　source.　In　the　seed　source,　without　any　dispersion　compensator　or　tunable　elements　in　the　cavity,　we　obtain　self-starting　mode　locking　of　a　tunable　ytterbium-doped　fiber　oscillator.　The　1037-1054　nm　tuning　range　is　achieved　by　proper　adjustment　of　the　half-wave　plate　and　SESAM　in　the　laser　cavity.　More　than　1　W　output　power　at　the　different　wavelengths　of　operation　has　been　achieved　in　experiments.　For　the　fiber　amplifier　seeded　with　an　SESAM　mode　locking　PCF　fiber　oscillator,　an　average　output　power　of　21　W　has　been　demonstrated　at　a　repetition　rate　of　45.9　MHz　with　a　pulse　duration　of　8.8　ps,　corresponding　to　a　pulse　energy　of　458　nJ　and　a　peak　power　of　52　kW.　The　nonlinear　effect　of　self-phase-modulation　(SPM),　which　results　in　spectral　broadening,　has　been　observed　during　the　amplified　experiments.