The NOS premium, mil-spec, subminiature, USA-made Phillips-Sylvania 6021 dual triodes are really great tubes. They have extremely long life and work well in extreme conditions. Most SFX-03 owners will never have to change their tubes. But in the unlikely event that your 6021s become microphonic over time, you will be able to return your SFX-03 to us for a tube replacement.