A 166.6-MHz 50-kW power transmitter is required to drive the dressed superconducting radio-frequency (RF) cavity during horizontal tests and to condition the fundamental power couplers as well as higher-order-mode ferrite absorbers. A highly modular yet compact amplifier adopting solid-state technologies was therefore developed in the framework of High Energy Photon Source - Test Facility. All power amplifier and power supply modules are pluggable from the front panel of the amplifier system. A 3-stage power combining topology was realized by using 32 power amplifier modules, 4 eight-way stripline power combiners, 2 two-way 35-kW coaxial power combiners, and 1 two-way 65-kW coaxial power combiner. Each power amplifier module is equipped with individual circulator and load utilizing an optimized cooling circuit design to ensure the entire amplifier system to be operational in both full-transmission and full-reflection scenarios. No final-stage high-power circulator or load is needed. The amplifier system is tunable in pulsed mode for any duty factors and fully functional in the continuous-wave mode as required. Excellent noise suppression and high wall-plug-to-RF efficiency were achieved at nominal power. Comfortable redundancies for both power amplifier and power supply modules were reserved to compensate for any hardware degradations during operations. Temperature, current, voltage, and RF signal of each module as well as cooling were monitored and managed by using EPICS in the control system of the amplifier. An interlock system was also integrated as well as an event logging system for post-mortem analysis. The system has accumulated over 2000 hours of operation. The design and performance tests of the first 166.6-MHz 50-kW modular solid-state power amplifier are presented.