Beam control pulse modulation (CPM) in an RF transmitter tube (101) is provided by a solid state switch on (S1) connected between the cathode power supply (103) and the cathode (102) of the transmitter tube. The beam controlling element (104) is returned to the normal cathode power source such that closing the switch (S1) between the two cathodes and the cathode power source brings the beam controlling element voltage to cathode potential causing beam current to flow, while opening the switch (S1) provides a very high equivalent cathode resistance, which self-biases the tube in the cutoff region, eliminating the need for a separate "off" voltage power supply. Off-edge performance may be further improved by providing an off switch (S2) connected between the cathode (102) and a collector tap on the cathode power supply (103), and closing the switch (S2) when the on switch (S1) is opened to accelerate return of the cathode potential to the cutoff voltage.