I'm nearing completion on a pair of 813's for 20 meters, grounded grid. Plate Pi-net is tuning properly as verify with my trusty Millen GDO. Decided to check for other resonances and found a plate circuit resonance about 125 Mhz. This is in addition to the desired 20M resonance. In troubleshooting, I have removed the tubes and disconnected the plate choke (and parasitic supressors). So now only have the plate coil, tuning variable, and loading variable. Still have resonance at about 125Mhz. Frequency slightly changes with changes in either variable. GDO coil is fairly closely coupled to the plate coil (closer than I need for the primary 20M resonance). Plate coil is #8 copper wire, 1 3/8 dia, 14 turns in 2 1/2". Tuning variable is 23pf wide-spaced, loading is 206pf plus 150pf doorknob in parallel. I have tried without the doorknob and still about 125 Mhz. I connected wiper of both variables directly together, no change. If I hadn't checked, I would be dumb and happy, but now I worry as to why I have this resonance, and what will happen when I fire up the amp.