A 120-V, 100-W lamp has R = 144 ohms at full brightness. Most tunable pi networks can match that. A cold lamp filament has about 1/10 the resistance of a white-hot one; with resistance steadily decreasing as the lamp is run at less than full power. So if you use a 150-W lamp on a 100-W output rig the impedance will be quite close to 52 ohms, purely resistive. Dan, K8JWR