Variable MU Tubes

Special Purpose Variable MU Tubes

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 variable mu tubes 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 variable mu tubes 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 variable mu tubes work, we will first examine the grid operation of a conventional tube during cutoff. Look at the illustration below. 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.

Cutoff in a conventional tube.

In view B, a bias voltage of -6 volts is applied to the grid. As you can see, some of the electrostatic lines of force are attracted to the negatively charged grid wires while the rest pass through the grid spacings. Because there are fewer lines of force reaching the cathode, there are fewer paths for electrons to use to reach the plate. As a result, conduction through the tube is decreased.

In view C, the negative potential of the grid has been raised to -20 volts, which drives the tube into cutoff. All of the electrostatic lines of force terminate at the negatively charged grid, instead of continuing on to the cathode. The electrons emitted by the cathode will not feel the electrostatic attractive force of the positively charged plate. Under these conditions, current cannot flow through the tube.

Now look at the picture below. Here you see a diagram of a variable-mu, or remote-cutoff, tube. The only difference between the remote-cutoff tube depicted and the sharp-cutoff tube is in the grid wire spacing. In the conventional sharp-cutoff tube, the grid wires are evenly spaced, while in the remote-cutoff tube the grid wires in the middle of the grid are placed relatively far apart. This is shown in view A.

Grid operation in a remote-cutoff tube.

In view B, the control grid is at zero potential (0 volts). Just as in the sharp-cutoff tube, electrons leave the cathode and travel along the lines of electrostatic attraction, through the spaces between the grid wires to the plate. In view C, a bias voltage of -6 volts is applied to the grid. Because of the close spacing of the grid wires at the ends of the grid, electrostatic lines of force at the ends are effectively terminated. The lines of force can only pass between the widely spaced grid wires closer to the center of the grid.

In view D, the same negative potential -20 volts) is applied to the grid that caused the conventional sharp-cutoff tube discussed earlier to go into cutoff. This voltage is high enough to terminate most of the electrostatic lines of force on the grid wire. But, because of the wide spacing between the center grid wires, some electrostatic lines of force are still able to pass between the center grid wires and reach the cathode.

Conduction will still occur in the tube, but at a reduced level. If the grid is driven even more negative, lines of force will be blocked from reaching the cathode, except at the very center of the grid. As you can see, the remote-cutoff tube, by its ability to reduce gain (conduction), handles large signals without going into cutoff. Variable mu tubes such as a 6SK7 with -3 volts applied to the grid will have a transconductance of about 2000 (µ) mhos. If the grid is driven to -35 volts, the transconductance of the tube will decrease to 10 (µ) mhos. This same increase in negative-grid voltage would have driven a conventional tube into cutoff long before the grid reached -35 volts.

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