For many applications it is desirable to develop electron beam driven efficient sources of electromagnetic radiation producing the required power at beam voltages as low as possible. This trend is limited by space charge effects. In the present study, the limiting currents of intense electron beams are analyzed for the case of beam propagation in the tubes with gaps. A general treatment is illustrated by an example evaluating the limiting current in a high-power, 1–10 MHz inductive output tube (IOT) which is currently under development for ionospheric modification. Results of the analytical theory are compared to numerical simulations performed by using the code Michelle.