In most electron-tube circuits, the operating level of a tube is determined by the level of bias. When a
negative-bias voltage is applied to the control grid of a tube, with no input signal, the conduction through
the tube is reduced; thus the damage to the tube is minimized. There is one drawback to this. Because the
control grid is already negatively charged by the bias voltage, the negative alternation of a large input
signal will drive the tube into cutoff long before the positive alternation can drive the tube into saturation.
Once the negative alternation reaches a certain level (determined by the bias voltage and tube
characteristics), the tube simply cuts off. For this reason, conventional tubes, which you previously
studied, are called SHARP-CUTOFF TUBES. Because of this sharp cutoff, the range of amplification of
the conventional tube is limited by the bias voltage and tube characteristics. Once this range is exceeded,
the output becomes distorted due to cutoff.
In most applications, the sharp cutoff feature of conventional electron tubes causes no problems.
However, in some applications electron tubes are required to amplify relatively large input signals
without distortion. For this reason, the variable-mu tube was developed. VARIABLE-MU TUBES have
the ability to reduce their mu, or (µ), as the input signal gets larger. As the mu (µ) decreases, the
likelihood that the tube will be driven into cutoff decreases. (For an AMPLIFIER, this may appear to be self-
defeating, but it isn't.) The idea is to amplify large input signals as much as possible without causing the
tube to cutoff or create distortion. Because of their ability to avoid being driven into cutoff, variable-mu
tubes are called REMOTE-CUTOFF TUBES. You should be aware, however, that a variable-mu tube
can be driven into cutoff, but the amplitude of the input signal required to do so is considerably greater
than in conventional sharp-cutoff tubes.
The key to the ability of a variable-mu tube to decrease gain with an increase in the amplitude of the
input lies in its grid construction.
To understand how the unique grid construction of a variable-mu tube works, we will first examine
the grid operation of a conventional tube during cutoff. Look at figure 2-7. Here, you see a diagram of a
conventional sharp-cutoff triode with zero volts applied to the control grid. In view A, the majority of the
electrostatic lines of force leave the positive plate (+) and travel unhindered between the evenly spaced
grid wires to the negative CATHODE (-). Electrons emitted by the CATHODE travel along these lines from the
cathode, through the grid spacings, to the plate.