A carbon nanotube-based high current density electron field emission source is under development at Jet Propulsion Laboratory (JPL) for submillimeter-wave power generation (300 GHz to 3 THz). This source is the basis for a novel vacuum microtube component: the nanoklystron. The nanoklystron is a monolithically fabricated reflex klystron with dimensions in the micrometer range. The goal is to operate this device at much lower voltages than would be required with hot-electron sources and at much higher frequencies than have ever been demonstrated. Both single-walled (SWNTs) as well as multi-walled nanotubes (MWNTs) are being tested as potential field-emission sources. This paper presents initial results and observations of these field emission tests. SWNTs and MWNTs were fabricated using standard CVD techniques. The tube density was higher in the case of MWNT samples. As previously reported, high-density samples suffered from enhanced screening effect thus decreasing their total electron emission. The highest emission currents were measured from disordered, less dense MWNTs and were found to be ~0.63 mA @ 3.6 V/µm (sample 1) and ~3.55 mA @ 6.25 V/µm (sample 2). The high density vertically aligned MWNTs showed low field emission as predicted: 0.31 mA @ 4.7 V/µm.