High power, high efficiency RF sources are required at frequencies below 700 MHz for ion and proton accelerators. These sources are also used for high energy accelerators, such as the Advanced Photon Source, which is seeking 350 MHz RF sources producing more than 200 kW of continuous power. The program is developing multiple beam triodes to provide this RF power at efficiencies exceeding 80%. The sources would be extremely compact and low cost. Both the triodes and the associated RF cavities are being developed.