In this paper, the nature of the primary instability present in the pedestal forming region prior to the transition into H mode is analyzed using a gyrokinetic code on JET-ILW profiles. The linear analysis shows that the primary instability is of resistive nature, and can therefore be stabilized by increased temperature, hence power. Its growth rate reaches a minimum for a temperature corresponding to the magnitude of the experimentally measured temperature at the L-H transition. The minimum of the growth rate is shifted towards lower temperature for lower effective charge Zeff. This dependence is shown to be in qualitative agreement with recent and past experimental observations of reduced Zeff associated with lower L-H power thresholds.