Explanation: When an RF amplifier's plate circuit is tuned to resonance, it presents its maximum impedance to the tube. This maximizes the RF voltage swing developed across the tank circuit and, consequently, maximizes the power delivered to the load. This peak in output voltage and power can subtly affect the grid circuit. Even with fixed bias, if the amplifier is driven into the region where grid current flows (i.e., the grid is driven positive relative to the cathode), the increased RF voltage swing at the plate can feed back a small amount of energy to the grid via the internal grid-plate capacitance (Cgp). This feedback causes the effective RF voltage on the grid to be slightly higher, driving it marginally more positive during its peak and resulting in a slight increase in grid current. It's a common indicator that the amplifier is optimally tuned for maximum output. Therefore, the grid current will rise slightly at resonance.