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HOW DOES A VACUUM TUBE WORK?

Old technology? Yes.

Does it have application now? Yes.

Where?

Vacuum tubes or as the English call them, valves, are still used in many of our linear amplifiers. There is some audio applications also.

A linear amplifier is a relatively simple piece of equipment. That is if you have a simple one. Many that have been manufactured are within the scope of most Amateur operators to repair it.

The amplifier is designed around one or more tubes. Some of the older one used six.

What does the tube do?

Use a triode as an example. It has three parts to the tube. A cathode, control grid and plate. Sometimes the cathode is also the heater and in others it may have a separate heater. This is the part that glows in the dark.

The Cathode. This is the negative side of the tube. In some circuits it is at ground potential. In other applications it is slightly above ground potential. This means if you put a volt meter on it to ground, you will see a low positive voltage.

The Control Grid. This is normally a fine wire or mesh physically close to the cathode. This is the control part of the tube. Normally with the grid at 0 volts in respect to the cathode, the tube will be in full conduction. This will vary with tubes. Putting a little negative voltage in respect to the cathode, the tube will start increasing in resistance until it completely turns of.

The Plate. This is the positive side of the tube. In linear amps the + voltage is quite high. Anywhere from 1K to 5K volts. Needless to say EXTREME caution is needed when working with this part of the tube.

The tube is a voltage amplifying device. That means it operates at a very high voltage and relatively low current. When the tube is turned on by lowering the negative voltage (going toward +) on the grid, the resistance of the tube drops sufficiently to make a large change in a decrease in voltage on the plate.

Keep in mind the resistance of the tube from the cathode to the plate is quite high even when in full conduction. That is a reason for the device using such high voltages. The tube will produce big swings in voltage resulting in an amplified output.

There are a variety of configurations on setting the tube up in a circuit. However, I am at the end of the paper so it will be for another time.

73,
Ralph WD0EJA

11-17