The combined effect of the grounded-grid to cathode C (9pF) and the combined L of the gid-cage, the grid-header, and the extenal pins (320nH). I have no idea what the Q is, but the sharp dip suggests that Q is not low. >What is the origin of this grid resonance, and approx. what is its Q? > >Eric K8LV > - R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K, www.vcnet.com/measures. end