May a primary source be biased?

The short answer is that anything produced by a human from the human perspective is inexorably biased. All of us, no matter how much we may delude ourselves into believing we are objective, are locked into a single perspective, our own. This being the case, we are trapped within our own point of view that is filtered through our own experiences. It is painted by our own perceptions and abilities to perceive. Some may be less biased, some more; however, this is no way negates the fact that every perspective is biased, primary, secondary, tertiary, or beyond.