A 140 GHz high‐power gyrotron has recently been operated in a 14TBitter magnet to characterize emission as a function of magnetic field and beam current. The velocity ratio (or pitch angle) &agr;=⟨v⊥⟩/⟨v∥⟩ of the beam electrons is a critical parameter for high‐efficiency gyrotron operation and was measured using a capacitive probe located in the beam tunnel before the cavity. The observed velocity ratio decreased as the beam current increased while the beam voltage and magnetic fields were held fixed. This decrease in &agr; partially explains the reduced gyrotron efficiency observed at high‐beam currents. The velocity ratio exhibited saturation effects as a function of both the beam current and the control‐anode voltage, at low cathode magnetic field values. Particle code results show a decrease in &agr; as a function of beam current that is consistent in magnitude with the observed values.