The conical cavity has been modified for use in electron beam devices. In this new cavity resonator a short radial line section couples the beam hole with the conical region providing a suitable interaction gap. Operated in a higher order TM0p0mode the cavity size is large compared to wavelength making it attractive for millimeter or submillimeter applications. In this mode, both Q0and Rshuntare large. For analysis, the modified conical cavity can be considered as a composite consisting of a circular waveguide at cutoff, a radial waveguide, and a shorted "quasi-wedge" guide. Resonant frequencies are found by impedance matching at a transverse plane in the cavity. Unloaded and Rshuntare found by calculating total energy stored and power loss along with an equivalent voltage across the gap. An experimental cavity was tested. This cavity was scaled for and Ku bands and operated in TM0p0modes from p=5 to p=9. Measured results agreed well with theory. At the p=9 mode, the radial dimension of the cavity was 4.2 λ. Design curves are presented for the modified conical cavity without beam hole.