A new method of controlling the amplified spontaneous emission (ASE) from Yb3+ ions in Er3+/Yb3+ co-doped fiber amplifiers is presented. The 1μm ASE is suppressed by stimulating a laser emission at 1064nm in a fiber amplifier, due to a positive feedback for the 1μm signal. The results are compared to a conventional amplifier setup without any ASE control. We have shown, that applying a feedback loop in an Er3+/Yb3+ co-doped fiber amplifier allows higher power scaling and provides operation without unwanted parasitic lasing effects, increasing the stability and robustness of the amplifier.