1.1 Set up the following: Vacuum Tube display; transistor display; Welch vacuum tube demo; broken UV199 tube; cut-open metal octal tube; large transmitting tubes;

1.2 Write the following on the whiteboard: semiconductor; intrinsic; P-N junction; P-N-P junction; doping; cathode; anode; (draw symbol for diode); (draw symbol for transistor, with E, B, C labeled).

(See 1.2 above).

Are there any similarities between the vacuum tube and the transistor?[think of each of them as control devices, with input, control and output] What is different about the two? [main difference is that the vacuum tube requires a heated cathode to release electrons --other differences are fragility, lack of reliability, much wasted power, much higher voltage for tube; more reliability for transistor]

Where was the transistor invented ? [Bell Telephone Laboratories, Murray Hill NJ]. About how many years ago? [December 1947]

Why was the transistor put into a hermetically sealed [what does that mean?…sealed so that air, water, and any contaminants cannot get inside] can?

Even more important than hermetic sealing, what was done to make the transistor more immune [means?] to outside influences? [

Early Vacuum Tube History:

1887 Edison introduced an anode into the light bulb and observed current flow.

1904 Fleming, using the Edison effect and knowing that the electron existed, made the first diode. He used it as a ??? [detector for radio]

1906 DeForest invented the "audion", a triode tube. How did this tube differ from the Fleming valve? [DeForest added a wiggly wire that was used as an electrostatic control on electron flow].

1912 Western Electric, a Bell System company, developed a tube amplifier from a greatly improved "audion".

1914 Using the tube amplifier, the Bell System made a telephone circuit from New York City to Los Angeles. For comparison, the longest telephone line before amplifiers were used was from New York to Denver (which in itself a fantastic achievemen!).

1915 Western Electric made the first commercial triode tube

1916-1918 (during World War I)Western Electric and General Electric made tubes for the US military.

1916 the tetrode, a space charge tube, was invented by Langmuir (GE Co) and Schottky (Siemens Germany).

3.2 Vacuum tube operation: how does it work? Let's recall the transistor: it has an input, an output, and a control element. The output controls the current flow coming from a separate source [draw an NPN XSTR]. The vacuum tube is similar. [draw schematic with a filament, anode, and grid]. There is a current flow through the tube. Remember that current flow is in the opposite direction to electron flow. Where do the electrons come from? [by heating the filament red-hot or hotter] After a while, we figured that this is the hard way to get electrons --so we coated the filament with another material that gave off electrons at a lower temperature. We didn't have to put so much power into the filament now. A few years later, about 1926, we figured out how to put a "heater" in for just one reason: to heat the cathode and liberate the electrons. Just about all small receiver tubes have been made that way ever since.

3.3 Why aren't vacuum tubes used to day? [They are power hogs; space-wasters; fragile; unreliable; short-lived ..To quote Jack Morton of Bell Labs:

"Using a vacuum tube is like sending a freight train for a pound of butter!"

But there are some tubes that have run for many years. Can you name a few of the places where they are used? [microwave oven; big radar sets; TV picture tube; all kinds of big transmitter tubes for FM and AM radio; the electronics in Russian fighter planes] However, there is one usage over which you find a lot of controversy. This is for audio hi-fi (high fidelity) amplifiers. The argument is primarily about how the system acts when it is distorted. That happens if the volume is turned up too high. The vacuum tube purists feel that a vacuum tube goes into "graceful" distortion, which is not as objectionable as the hard crash that occurs with transistors.

3.4 Vacuum tube construction: the simplest tube is a diode, which has only a cathode and an anode. [draw the picture, circular plate, leave room for the grid]. This is enclosed by a glass bottle, and then a vacuum pump is connected to pull a vacuum. After the pump has done all it can, the tube is sealed, or tipped off. [draw picture of exhaust stem, fires playing on it]. A base is put on the tube, and then it goes to a flash function: voltage is applied to the tube and the getter is flashed. The getter is just what you might think -- it "gets" the air that's left inside after the vacuum pump work. The results of the getter coat the inside of the tube. Notice that with the old tubes, when the vacuum pumps weren't very good, the getter covers the inside of the tube. With the newer tubes, there's hardly any sign of a getter. The all-metal tubes include little insulated "feedthrough" points where each of the wires (never more than eight) passes through the shell of the tube.

3.5 Metal cap on the top of the tube: There are two reasons for the cap, and they are not related. In older tubes, the cap is a connection to the control element, the control grid. Who invented the control grid? [DeForest] We could not bring the control grid too close to the output or we had feedback. It is similar to that of a public address system when the volume is turned up, and the signal from the speaker comes back to the microphone and goes around that same loop. And so, the control grid came out of the top for isolation, away from the rest of the tube. Now, the reason for having a cap on top of other tubes has to do with the voltage at which the tube output -- the plate or anode--operates. Up to 500-600 volts, the connection to the plate might still be made at the base. Above this, one has the likelihood of seeing an arc from the plate pin to any other pin. For this reason, the plate is brought out the top (or sometimes the side) of the tube.

3.6 Extra Power required: The vacuum tube has one power requirement that the transistor does not have; where does that power go? Hint: the transistor gets its electrons for flow from impurities added to the semiconductor in a process called doping. [filament or heater, to release electrons]. This voltage ranges from 3.0 volts at .06 amperes, about the same as a small flashlight power, up to tens or hundreds of amperes, but still at a fairly low voltage.

3.7 Anode voltage: Small radio tubes operate with anode voltage from 30 volts to 400 volts. Just before transistors took over completely, tubes for auto radio were developed that could work with only 12 volts on the anode. This voltage came from the car battery. In just a few years however, the auto radio was completely transistorized.

4.1 Using the EP (Experimenter's Playground kit), hook up the attached circuit for a two-stage transistor amplifier. This amplifier can be used for a microphone, or for a crystal radio set, to give a speaker output.

4.2 As an extra project, the student may work on any of the other experiments available in the EP.

5.1 Give out Graduation Certificates for all students

5.2 Give out the Final Test to each student. Emphasize "answers on back, no peeking until through."

Page last updated on January 28, 2002