An Experimental High-Transconductance Tube Using Space-Charge Deflection of the Electron Beam

In the past, electron tubes have been built using either current density control or deflection control; the tube to be described uses a new principle combining both of these methods of operation, with control by a conventional grid. This grid controls the space charge in the usual manner, and the space charge produces a displacement of the electron beam from its space-charge-free path. By means of a fixed intercepting edge, this in turn can be converted into additional current variation at an anode. An analysis of this new principle has been made and demonstrates the possibility of an increase in both transconductance and the transconductance-to-plate-current ratio of the tube. Experimental tubes of the orbital-beam type, utilizing this principle, have been built in nine-pin miniature envelopes. In the output structure, that portion of the beam which passes an intercepting edge enters a single-stage secondary-emission dynode and collector system. Transconductances of 25,000 micromhos have been obtained with only 3 ma output current. These values, particularly the transconductance-to-current ratio, are many times better than those obtained from an ordinary grid-controlled tube, and no increase in capacitances is involved. The measured equivalent noise resistance is about 900 ohms, and the gain-bandwidth product is 320 mc. Because of its mode of operation, the tube requires stable voltages and good mechanical alignment. However, neither of these requirements appears to be severe, and the high transconductance with low current and small capacitance should make this type of tube quite valuable for broad-band amplifier applications.