The effect of filament voltage upon vacuum tube characteristics

One means of increasing vacuum tube life and at the same time decrease filament power supply requirements, heat dissipation problems, and d-c drift, is to operate the vacuum tube with reduced filament supply voltage. If the tube is operating well below temperature saturation, which is the case for the majority of circuits other than power amplifiers, the effect of reduced filament power will be to decrease the initial velocity of electrons emitted from the cathode. This reduces the magnitude of the charge at the virtual cathode, and increases the charge density at the grid, which decreases the velocity of electron flow through the grid. By creating an imaginary cathode, called an image cathode, which is located behind the real cathode, and defined as the plane from which the electrons appear to be emitted, the principles of electrostatics can be applied to determine the effect of lowered cathode temperature upon amplification factor. As the cathode temperature decreases, the charge of the virtual cathode decreases, and the distance from the real cathode to the image cathode decreases. The apparent grid to cathode distance has thus been decreased and amplification factor goes up. This is experimentally verified by tests on a 6C4 triode and a 6AG5 pentode. The effects of decreasing filament voltage upon μ, rp, and Gm are plotted. It is shown that μ increases, rp increases, and Gm decreases. The actual gain that may be achieved in a voltage amplifier increases. Increases in gain of 1.5 times have been measured when the filament voltage was decreased from 6.3 volts to 3.0 volts.