Page:March 1916 QST.djvu/3
42 MARCH 1916
It has often been asked, “How is an X grade Audion Bulb made, and what makes it different from the S grade?” Also, “How are the bulbs tested?” These are questions of interest to all users, and rightfully so.
The process of testing Audion Bulbs is one of the most careful and expensive tests in the entire electrical industry. A bulb used in the proper detector may give extremely loud signals from stations twenty or thirty miles away, and yet not be really sensitive to weak signals from great distances, or it may give much weaker response to nearby stations than a crystal detector and still be extremely sensitive to weak signals over long distances.
Theoretical methods of testing are, therefore, of no value. The only practical method is to compare with a standard, under actual working conditions, receiving weak signals. The standard is set by comparison with the best crystal detector. The unknown bulb is connected in circuit on a double detector, and the oscillations are tuned from one bulb to the other, reducing coupling and making the necessary changes in capacity to counterbalance change of mutual inductance until the signals can be heard on one bulb and not on the other. Testing by throwing the circuits out of tune is not satisfactory because the Audion is a potentially operated detector.
If the unknown bulb is equal to or better than the standard, it is passed, but otherwise discarded. If it is sufficiently more sensitive, it is passed as the “X” or extra sensitive grade, There may be one X grade bulb in 100 or there may be twenty—no one can tell. There may be ten S grade bulbs in 100 or there may be fifty. The Audion bulb looks simple, but is one of the most difficult instruments to make ever invented.
The testing is done by expert, commercially licensed operators, of years of experience, and thoroughly familiar with the Audion. The employment of a beginner, or one only experienced in operating, for the work, would be fatal. The tuning apparatus is regular amateur equipment, not the specially designed and necessarily more efficient apparatus made for the Audion.
AUDION TROUBLES AND HOW TO CORRECT THEM(1). The most common cause of trouble is due to exhaustion of one or more of the “B” high voltage dry flashlight batteries. These batteries are at best unreliable, from their very nature. If only one becomes exhausted until it registers 3½ volts or less on a voltmeter, the efficiency of the entire
receiving set is remarkably decreased. The great difficulty is to convince the operator of the necessity of testing every battery often with a voltmeter. As soon as one or more shows 3½ volts or less, it must be replaced, and all connections must be soldered. If not replaced at once, the operator always brings up the intensity of the filament, and, of course, burns it out before long—then says the bulb was defective.
(2) The next most common cause of trouble occurs when the operator replaces one or more cells, and connects them backwards with respect to the others, connecting the carbon to the carbon of the next battery.
Then again, when the operator replaces the entire set of batteries, he often connects the carbon to the filament of the bulb. The detector cannot operate unless the negative or zinc is connected to the filament. And again, the bulb was all right, but now has lost its sensitive qualities!
(3) The third cause of complaint, due entirely to the operator, is the fact that, “The old bulb was very good, and the new one cannot compare with it.” This is because of either of the above causes, or he does not try reversing the connections from the “A” or lighting battery. Some bulbs will not operate at all unless this is done to find which way is best.
(4) Then, some operators use the Audion in a cold place. Consider the surface of glass of the bulb, and realize that the very small quantity of gas in it must necessarily be affected by change of temperature. The Audion works best when the temperature of the room is 60 degrees or higher.
(5) Then comes the complaint from a beginner who says, “The Audion brings in 600 meters and amateur stations, better than any other detector I have ever tried, but Arlington on his 2500 meter wave is much stronger with a crystal detector.” This is easy to explain. The Audion is selective to some extent as to spark frequency, but positively is not selective as to wave length. It is exactly as sensitive at 3000 meters as at 200. If you will consider the proportions of an amateur tuner, you will realize that it is most efficient from 200 to 1500 meters. They are all built in this way. When a loading coil is used in the primary circuit, the efficiency of the primary is decreased, although its period is increased.
The secondary, if one is used, is seldom loaded. A variable condenser will help to some extent, but unless the tuner can receive with high efficiency the long waves, it is impossible to operate the Audion to full