CHAPTER 14
MORE AMPLIFIER TUBES
THE TETRODE

The place to cure a pain-such as interelectrode capacitance-is at its source. Since the source of interelectrode capacitance is in the tube, the tube is the place to work the cure.

This is done by putting a SECOND grid between the first grid and the plate. Both grids are alike in design. To prevent confusion, call the original grid the CONTROL GRID, and this new grid the SCREEN GRID. The new tube is called a TETRODE. The SCREEN GRID is connected to a positive potential that is usually lower than the plate potential.

SWAPPING TROUBLES

The tetrode cures the trouble you had with interelectrode capacitance in the triode. BUT you run into a new trouble with the tetrode-since the screen grid is

But what is more serious-the electrons that do arrive at the plate will be traveling at a velocity high enough to KNOCK OTHER ELECTRONS LOOSE FROM THE METAL OF THE PLATE. The process of "knocking" other electrons off the plate is called SECONDARY EMISSION. These electrons will form another space charge between the plate and screen. Now, if the screen is at a potential equal to or greater than the plate potential, the electrons of the space charge will flow to the screen, further reducing the NUMBER that reach the plate.

WHAT IS WRONG WITH THE TETRODE?

The output of a tetrode has a tendency to make the music sound CORNY. The voice of your favorite blues singer would sound like a bull-frog with a cold. This is DISTORTION. You want amplifiers to put out exactly the SAME SOUNDS that are put in. You want the signal amplified, but you don't want any hash added to it.

The tetrode was used extensively. If you watched your operating potentials, the distortion could be kept to a minimum. Today, this tube has been largely replaced by a much improved type known as the PENTODE.

THE PENTODE

The secondary emission of the tetrode was responsible for the development of this new tube. Since the effect of the secondary emission is largely that of robbing the plate of its electrons, the search naturally turned to finding a way of either preventing or reducing this effect. The outcome is a new tube, with ANOTHER GRID placed between the screen grid and the plate.

The new element is the SUPPRESSOR GRID, or just SUPPRESSOR. Observe in figure 107 that it is connected directly to the cathode. Thus, whatever potential is placed upon the cathode is ALSO PLACED ON THE SUPPRESSOR. In some pentodes, the connection to the cathode is made internally-that is, the cathode and suppressor are

HOW THE PENTODE REDUCES THE EFFECT OF SECONDARY EMISSION

The way the suppressor grid reduces the effect of secondary emission from the plate can be understood by examining figure 108.

Notice, in figure 108, that before any electrons of the secondary emission from the plate could get to the screen grid, they'd first have to pass the NEGATIVE SUPPRESSOR. But that's impossible-the NEGATIVE CHARGE on the suppressor drives all of these electrons BACK TO THE PLATE. In this way, the suppressor grid prevents the electrons of secondary emission from reaching the screen.

AMPLIFICATION OF TRIODES AND PENTODES

The amplification factor for this pentode is about 60 times as great as it is for the triode. For example-if one volt of a.c. were placed on the grid of each tube, the triode would produce an a.c. voltage change in the plate circuit of only 20 volts. But with the same grid voltage the pentode would produce a plate voltage change of 1,200 volts. In practice these values are not easily attained, but in theory it is possible. Here's the important thing-with a pentode, you can START WITH LESS AND FINISH WITH MORE.

INTERELECTRODE CAPACITANCE

TRIODES VS PENTODES

You already know that the interelectrode capacitance of a pentode is much less than that of a triode. Usually the plate-grid capacitance of a pentode is about 1/1000 that of a similar triode. For example, compare this capacitance for the 6J5 and 6J7 tubes-

Triode-6J5--4.000μμf.
Pentode-6J7--0.005μμf.

These examples are representative of most triodes and pentodes.

BEAM POWER TUBE

The BEAM POWER TUBE is a new design that combines many of the advantages of the triode and pentode. Whether this tube is a tetrode or a pentode depends entirely upon your definition of what constitutes a tetrode and a pentode. Actually, it is neither a tetrode nor a pentode-better call it a BEAM POWER TUBE.

In figure 109 you'll see that this tube has a cathode, a control grid, a screen grid, and a plate. In addition to these parts, it also has TWO BEAM-FORMING PLATES. Each beam-forming plate extends about one-fourth the distance around the grids of the tube. The remaining distance is open The plates are connected to the cathode, and therefore they operate at the cathode potential.

The openings in the grids are arranged in such a manner as to cause the electrons to form into LAYERS or SHEETS as they pass between the windings on their way to the plates. After passing the screen grid, the sheets of electrons combine to form a BEAM.

In this tube, the SCREEN POTENTIAL is made GREATER than the plate potential so that the electrons arriving at the plate will be moving from a HIGHER to a LOWER potential. Their speed will be reduced in a manner similar to that of an automobile coasting up hill.

Because the electrons that reach the plate are traveling at a lower speed, FEWER ELECTRONS WILL BE KNOCKED OFF THE PLATE. Those secondary electrons that do bounce off the plate are caught in the BEAM and swept back to the plate.

The beam-forming plates prevent stray electrons from returning to the screen grid, and the plates also SHAPE the electron beam. The combined action of the plates and grids reduces the screen current to a low value.

The beam power tube has two major advantages-it is as sensitive as most pentodes, and it can handle large currents. These two advantages allow you to CONTROL LARGE UNITS OF POWER WITH A SMALL AMOUNT OF GRID ENERGY

TRANSMITTING TUBES

You have seen pictures of TRANSMITTING TUBES that are as tall as a man, and still others of queer shapes and designs. In spite of their size and shape, they are only diodes, triodes, tetrodes, and pentodes, basically like those you have studied.

Transmitting tubes have to be large since many of them must be able to deliver SEVERAL THOUSAND WATTS of power. In comparison, most receiving tubes cannot carry more than two or three watts. Just as the airplane engine of several thousand horsepower is larger than the automobile engine of 100 horsepower, the transmitting tube must be LARGER AND STRONGER than the receiving tubes.

First of all, the transmitting tubes must be able to dissipate or get rid of large amounts of heat. To do this, the plate, grids, and cathode are made LARGER AND HEAVIER, and the tubes are provided with a cooling system. Some transmitting tubes are cooled by circulating water, but most of them have a forced draft of air circulating around them. Few water-cooled tubes will be found in Navy equipment.

One of the most common causes of failure in transmitting tubes is overheating due either to faulty cooling or to overload. A tube that operates at a dull cherry red is near the danger point. Any slight additional heat may damage the tube. Check the fans and other cooling systems to see that they are operating properly.

As strange as it may seem, most metals have a small amount of some inert gas-nitrogen or argon-dissolved in the metal of the plate. If the plate becomes red hot, this gas will escape from the metal and float around inside the vacuum tube. This gas is objectionable because it prevents the grid from exercising the proper control of the electrons going to the plate. If this

First check to be sure that the gassy tube you are replacing is not one that is SUPPOSED to have this blue glow. Remember, mercury vapor tubes are designed with gas in them and give off a similar bluish glow.

MORE INFORMATION, PLEASE?

So far, you have met only a representative few of the many dozen different vacuum tubes in everyday use. A great number of tubes are similar to these few, and others are identical both in structure and in electrical characteristics. You'll be working with many different tubes, but you are not expected to remember all their different characteristics. The information can be found quickly in any STANDARD TUBE MANUAL, which the chief will have on his book-shelf.

WHAT'S IN A TUBE MANUAL

In addition to dope on AMPLIFICATION FACTORS, PLATE RESISTANCE, TRANSCONDUCTANCE, and INTERELECTRODE CAPACITANCE, you will also find the following information

MAXIMUM PLATE VOLTAGE and MAXIMUM PLATE current-as the names indicate, these are the MAXIMUM permissible values of current and voltage that can be used without damage to the tube.

TYPICAL OPERATING CHARACTERISTICS-A vacuum tube is seldom operated at its maximum rated values. The TYPICAL values are those you will be likely to use in an ordinary circuit to obtain best results and longer tube life.

USES FOR THE TUBE-This section tells you that the manufacturer recommends the tube for use in one or more of these spots-

FILAMENT VOLTAGE and FILAMENT CURRENT-this information is important. If the tube has insufficient filament voltage, it will not operate at top efficiency. But a voltage that is too high will burn it out. When you are installing or replacing a tube, be sure that the filament voltage is correct. The numerals in front of the letter in the tube number give you the APPROXIMATE filament voltage.

POWER RATING-Tubes designed as POWER TUBES will have their MAXIMUM SAFE POWER-HANDLING capacity in WATTS.

SOCKET CONNECTION-tube bases are fitted with KEYS and PINS so that you can put a tube in the socket in only the RIGHT position. Tube manuals give you the socket connections for each tube. Most tubes fit into an EIGHT-PIN SOCKET. The pins are numbered CLOCKWISE as you view the connections from the BOTTOM, the numbering starting at the KEY SLOT.