Radio Broadcast (May 1929-Apr 1930)

STRAYS from THE LABORATORY """"" """ 1 The curves in Fig. 1 Data on and Fig. 2 are published Foreign-Made through the courtesy of Pentode Tubes M. E. Bond, engineering »"" ■ i »>""""»" department, American Bosch Magneto Company. They show the power output from a Mullard Pentode, and the manner in which it varies with various load resistances. It is interesting to note that with an input of 5 volts on the grid, a power output of 800 milliwatts can be obtained. In the Laboratory it was possible to get this same power from the Philips (Holland) Pentode with a grid voltage of 16, and so the Mullard valve is more sensitive. Experiments in the Laboratory with Ediswan Pentodes indicate that such a tube with 150 volts on the plate and drawing about 12milliamperes will deliver power equal to that from a push-pull 171 amplifier with about one third the input voltage. Listening tests, comparing the Ediswan Pentode working into standard loud speakers through an Ingranic output transformer designed to couple these high-resistance tubes to low-resistance loud speakers, indicate that there is still a lack of lowfrequency response compared to trie reproduction from the 171 push-pull amplifier using an Amertran transformer. This lack of low frequencies, however, is not as bad as may be noted from some of the newly designed loud speakers sold in this country, and touted as being the last word in "perfect reproduction." """ """"""»"» JN RADIO LANGUAGE Symbols Used there are many symbols, in Technical short-hand expressions, Radio Writing that make it rather pic i minimi iiiiiimi .. turesque and rather unintelligible for the layman. Some of these symbols have international acceptance, some are used only in this country, and some have not been agreed upon generally even in this country. For example the Greek letter "omega" (u) is sometimes used to indicate resistance in ohms, and sometimes megohms. Similarly, the large Greek letter "omega" (O) is used indiscriminately for ohms and millions of ohms. The Greek letter "m«" ((jl) is used generally in this country for the amplification factor of a vac 140 uum tube. It is also an abbreviation for "millionths"; thus, microhenry is abbreviated to iih. The small "omega" (<.>) is used §iqo o for the "expression 2x f or 6.28 X f where 1 is the frequency in cycles. It enters into many elec ^ trical calculations. Thus, the re <« 6° actance of an inductance is <oL or o 6.28 X L X f. Strictly speaking, 5 40 a) is an abbreviation or symbol for the "angular velocity of the vector representing a sine wave of current or voltage, and is expressed in radians." A radian is a measure of the arc through which the end of the vector travels, 6.28 radians constituting an entire circle of 360 degrees. The Greek letter " lambda" (X) is used for the wavelength in meters. Thus X = 300 would be read "wavelength equals 300 meters." The small or capital letter "r" and "R" is used almost universally for resistance. 0 22 020 0 18 0 16 014 "0 12 0 10 0 08 006 004 0 02 1 J — — Epr=2 mstant) HAB ULLJ M T! Rl TICS OF ROPH 24 TUBE 1 i— ( r—T :6< Iff i T 1 v -*-£ R+ Conditions lor de!e o«r, me optimum load F ] OV V 1 0 2 4 6 8 10 12 14 16 18 20 22 " 10' LOAD RESISTANCE OHMS Fig. 1 Thus, RP indicates the plate resistance of a tube. The letter p is called a subscript and states that "R" in this case is a particular resistance, that connected with a tube. In a similar manner, subscripts are used on the letters "E" and "I", denoting voltages and currents to form EP, Eg, Er for the plate, grid, and filament voltage of a tube, and IP, Ig, and If to indicate the plate, grid, and filament currents. Generally speaking, large letters denote d.c. values, small letters indicate a.c. values, but here the usage is not universal and some authors use one convention, some another. Thus, IP may refer to the steady B-battery current in the plate circuit of a tube. This flows whether or not there is any a.c. grid voltage, eg. When there is an a.c. grid voltage there is an a.c. plate current, iP, as well as the d.c. plate current. The letters "db" are used as an abbreviation for the " decibel, " a unit of transmission loss or gain. It has the same value as the tu which it replaces in technical language. The letter "G" is a symbol for conductance. A conductance is an expression telling how well a certain circuit conducts currents, just as resistance tells how much it impedes the progress of a current. Conductances are measured in mhos (ohm spelled backwards). Thus, the mutual conductance of a tube is its Gm and is usually expressed as micromhos. In England this expression becomes "S," the steepness of the grid voltage-plate current curve. The Greek letter "tau" (t) is sometimes used for the coefficient of coupling between two coils; sometimes the letter "K" is used for this same expression. Fluorescence in Radio Vacuum Tubes 4 5 6 7 VOLTS ON GRID Fig. 2 We now have the last word on the strange flickering of light within a vacuum tube when """""" 11111 signals are put on its grid. We had laid it to fluorescence, and, following the dictionary, stated that this fluorescence was due to organic material within the tube. This theory was exploded by a reader who wonders how any organic material could survive the heat which the tube undergoes in the manufacturing process. He is correct; no organic matter could. And here is the correct explanation, coming from the engineering department of the General Electric Research Laboratory. "Fluorescence under electron bombardment is a property possessed by a number of different chemical salts. For X-ray work zinc sulphide is one of the ingredients most commonly used. It is also well known that certain salts of barium and calcium fluoresce when bombarded by electrons under certain conditions. "It, therefore, seems very probable that the outside of the anode of the tube accumulates a small amount of some salt which shows fluorescence under electron bombardment. "There may be several sources for such material; notably, from the active filament coating or the getter used. The gas free specially cleaned anode surface is particularly sensitive to receiving such an active material. During operation of the tube there are many stray electrons that have escaped through the end of the grid and plate and these strike the outside of the plate. Variations in grid voltage at an audio rate change the path of the electrons and, therefore, cause different portions of the plate to fluoresce. "Fluorescence is not associated with organic material, as is well proven by the ordinary X-ray fluorescent screen. Phosphorescence, however, is usually associated with an organic material." 154 • • JULY • 1929 •