Dangerous Curves
As discussed in the previous lesson, a Characteristic Curve is found by
applying several different voltage levels, and measuring plate voltages vs.
plate current. We note that in a diode, if we go below a certain plate
voltage, ( in this case 0 volts ) no plate current flows. The minimum point at
which the tube no longer operates is called the
CUTOFF POINT
. Above a certain plate voltage, additional plate voltage has very little
effect in increasing the plate current. The maximum point where raising the
plate voltage no longer increases current is called the
SATURATION POINT
.
In reality, there are two different factors involved in the control of the amplitude ( or level ) of the plate current that flows in a diode. These are the filament voltage ( sometimes called the heater voltage ), and the plate voltage. Remember that the cathode must be HEATED into thermonic emission. The temperature of the cathode must be high enough to "boil" the electrons from it's surface. It stands to reason, that the higher the temperature we heat the cathode to, the more electrons will be "boiled off". Much like raising the temperature of a pan of water causes it to boil away into steam faster. There does come a point though, where we can boil the electrons off no faster. As we raise the voltage on the heater, it will actually begin to slow down the movement of electrons toward the plate, and begin drawing them toward the heater itself. Therefore, when setting up a tube for operation, you want to make sure that you don't set the filament voltage of the tube so high that it will cause this effect, as it will reduce the efficiency, as well as the life expectancy of the tube. Most tubes are operated at standard values of 6 or 12 Volts. Often, if a tube gets weak, you can boost, say a 6 Volt filament to run as high as 7.5 or 8 Volts. This may extend the life of the tube for a while, but you must watch the plate current. If plate current begins to drop off, then you have the filament voltage set too high. It is usually considered standard practice to run a tube at the lowest filament voltage that will make the current necessary for proper operation of the device. This maximizes the life expectancy of the tube. 
Assuming that the filament voltage of the tube is set, we can increase the
plate current by increasing the plate voltage. Normally when plotting ( or
drawing ) an operational curve for a tube, we assume that the filament is held
constant, and the plate voltage is raised. As the plate voltage rises, so does
the plate current. We do note, however, that there is a minimum and maximum
point, at which the curve is no longer linear ( in a straight line ). We call
the minimum point the "lower knee" and the maximum point the "upper knee" of
the curve. The saturation point occurs at the beginning of the upper knee,
while the cut-off point occurs at the beginning of the lower knee. Under
normal conditions, we usually operate the tube within the linear portion of the
curve.
Later, we will discuss the characteristic curves of various other components. Each type of component has a slightly different curve, which dictates how the component will operate under different conditions. Understanding these curves will give you a more thorough knowledge of how the component works, and insight as to what it will do when it fails. |
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And for what it's worth... this page was last updated HexDate 01-11--7D1
