Summary form only given. Under two SBIRs from the Department of Energy, DTI is currently building a full scale hard switch modulator for ILC class klystrons. Theses klystrons are expected to be similar to the Tesla klystrons, with the final design in the range of 110-150 kV, 120-166 A, and 1.5 ms pulses of plusmn 0.5% flatness. The relatively long (1.5 ms) pulse width which requires 25 kJ delivered every pulse, without significant cathode voltage droop. DTI's design uses a solid state hard switch for the main pulse control, and a passive bouncer to provide the required pulse flatness with a huge capacitor bank. This modulator will be delivered to SLAC in the 2007. This paper will describe the design, construction, and preliminary test results obtained at DTI and SLAC. This system has four main components: a 150 kV DC power supply, a ~35 kV DC power supply to charge the bouncer, the modulator tank, and the capacitor tank. The modulator and capacitor tanks were split in this design to allow easier transportation and handling. Controls for the modulator are located in 19" drawers above the modulator tank. This design allow s the equipment to be operated at low voltage outside of the oil tank for assembly and testing, without the need to disconnect any wiring. The hard switch represents, in many ways, the simplest modulator design, consisting of only a capacitor and a full-voltage switch. The hard sw itch itself is built using Powerex QIS4506001 IGBTs, which were jointly developed by DTI and Powerex specifically for pulse power applications in a previous DOE SBIR. Meeting the droop requirements with a conventional storage capacitor would requires an extremely large capacitor -approximately 180 uF at 150 kV. This would be very large, and would not only be expensive, but would place a large facility burden on the ILC. A resonant bouncer provides the desired pulse flattop to meet the ILC specifications, at significantly reduced size and capacitor requirements. The main capacitor and bouncer provide 25 kJ over the ILC's 1.5 millisecond pulse at very tight voltage regulation. In this circuit, the bouncer capacitor produces an increasing voltage that compensates for the droop on the main capacitor. The ILC pulse requirements of constant pulsewidth, load perveance, and duty cycle are ideally suited to this type of resonant circuit. DTI estimates that a hard switch with bouncer will be 30 -50% less expensive in production than the published figures for the baseline FermiLab / DESY design. In part, this is due to the elimination of the pulse transformer, but it is also attributable to the smaller, simplified designs made possible using high voltage, solid-state switching. The total floor space required by the hard switch modulator is estimated to lie only 25% of that required in the baseline FermiLab/DESY design.