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www.americanradiohistory.com 444 Radio News for November, 1928 Four commercial transformers sold for radio use. A, B and C are filament -lighting transformers, which step down the 110 -volt, 60 -cycle alternating house current to values between 1 and 15 volts. D is a combination unit which furnishes "B" voltage as well as filament current. Transformers are also available, which supply both fibcment and plate current, with one core. tion, or 450 volts maximum. This transformer is used with the filament type or gaseous- content type rectifying tubes to supply the plate current to the 171- and 112 -types, as well as to the other tubes in the set. The other common type supplies current at about 500 volts and is of either the half- or full -wave type. This transformer is Used with the filament type of rectifier tubes to supply plate current to the 210 and 250 power tubes. The current required from the secondaries of these transformers depends upon the type of rectifier tube employed, and the amount of current necessary for the receiver. It usually varies between 60 and 150 milliamperes. This is a rather small current, since a milliampere is one- thousandth of an ampere. Besicles these two general types of transformers and the variations of each type, there are several combination transformers now being sold. These transformers have both large and small secondary windings, so that the filament and plate supply can be obtained from a single unit. A number of these transformers are wound with high - voltage, full -wave windings and either center -tapped or untapped windings to supply filament current. AUDIO -FREQUENCY TRANSFORMERS The problem of designing good audio - frequency amplifying transformers is very different from that of designing power transformers. In the latter, currents at only one frequency have to be considered, and the windings and core can easily be arranged to give the greatest efficiency at this figure. However, in amplifying transformers, a very wide band of frequencies must be covered with uniform efficiency, so that the signals and music will not be distorted. By referring to the chart of frequencies covered by common musical instruments that will be found on the preceding page, it will be seen that an average broadcast transmission covers a band between 30 and 5,000 cycles. An ideal transformer should transfer currents of any frequency in this band equally well. Transformers f o r audio-frequency amplifying circuits can be divided into four types; the ordinary step -up, push -pull, auto- transformer and output. The problems involved in each of these types are approximately the same and a general discussion of these problems will be worth while. The purpose of the transformer used as a coupling device between two vacuum tubes in an amplifier is to receive the current changes from the preceding tube and Above: A, standard step -up transformer amplifier circuit. B, push -pull hook-up. C, auto -. transformer arrangement. Below: An amplifying transformer (left) compared in size with a "B" supply transformer. deliver them to the following tube with an increase in voltage. However, the compa r- ative voltage changes on the different signals must all be the same, so that natural reproduction will result. If the primary of the transformer is too small (if it has too low an impedance), the lower frequencies will pass through without affecting the secondary. The low impedance does not allow the current to magnetize the core or transfer the energy to the secondary, and the low frequencies are by- passed through the primary winding. It has been found that the primary impedance should be two or three times the tube output resistance in order to fully amplify the lower notes. OBTAINING CORRECT IMPEDANCE In order to obtain the correct primary impedance, it is better to use a large core rather than increase the number of turns in the primary. If a small core is used, the primary 'must contain a great number of turns and naturally this also means an unusually large secondary coil in order to get the step -up ratio between the two coils. 'l'he use of a very large secondary will also have a bad effect, since it has a tendency to increase the capacity between the turns of wire in the secondary. This value is known as the distributed capacity, and when it is increased, the higher frequencies are by- passed by it and are not properly amplified. It is generally considered that, the larger the core of a transformer, the more uniformly it will tend to amplify both the high and low frequencies. The core must be made of special magnetic material with a high magnetic value, or permeability. '/'here are two currents flowing through an audio- frequency transformer. The first is the alternating current which constitutes the signal and the other -is the direct current of the "B" supply. In a transformer with a small core, these two currents together may be sufficient to saturate the iron. In other words, the core is not large enough to handle all of the magnetic field produced by the primary winding. This condition may cause the production of harmonics of frequencies which (Continued on. page 486)
www.americanradiohistory.com Radio News for November, 1928 445 SHOULD BE GOOD 'SERVICE MAN (after listening to local in- / terference in re- , ceiver): "I am afraid, madam, that it is your location which is to blame. The set is working all right." OWNER (justly indignant) : "I'll have you know, young man, I have as good a location as anyone in this town. I pay $80 a month rent here !"-S. O. Taylor. LOOKED LIKE PHONES It was so cold that day that the traffic cop stationed at the / 1 school corner had to + - wear muffs over his ears. But he was slightly surprised when one little tad stopped to look at him carefully and then came up close and inquired, confidentially: "Say, mister, what station are you getting now ? " -Mollie Zacharias. THE LOST ART A local (Providence) station ended a morning talk for housewives with the promise of a free cook -book to those listeners who would call at certain chain stores. One young housewife who had purchased baked beans, salmon, condensed milk, canned tomatoes, etc., inquired as an afterthought for the cook -book offered by radio. "Lady," said the clerk, thoughtfully, "you don't want a cook -book; just a can - opener."- Theodore .4. Monahan. NEED ANY ASSISTANTS? W I F E (awakening her husband) : "T h e early morning radio setting -up exercises are on! You left the radio turned on when you went to bed! TIRED DX FAN (after a liard night): "Please be quiet. I'm doing my daily dozin' ."-A. H. Rodiek. HIS DX WAS NB (NEAR -BY) FIRST RADIO FAN: "What sort of a set has Joe got ?" SECOND Dirro: "Well, you don't need a radio log with his receiver. All you need is a splinter. "-A. H. Rodiek. page is devoted to humor of purely THIS radio interest; and our readers are invited to contribute pointed and snappy jokes -no long -winded compositions -of an original nature. For each one of this nature accepted and printed, $1.00 will be paid. Each must deal with radio in some of its phases. Actual humorous occurrences, preferably in broadcasting, will be preferred. Address Broadcastatics, care RADIO NEWS, 230 Fifth Avenue, New York City. PUSHING THE SCOTCH TOO FAR What is the difference between a storage battery and a native of Aberdeen? The storage battery can be overcharged! -From The Saveloy (house organ of the British Broadcasting Co.) SHADOWED TO HIS DOOM FAN No. 1: "Say, why are you so interested in getting a television set working ?" FAN No. 2: "I want to see what Old Man Static looks like, so I'll know him. Then 1'11 get him and get him good, too!" -George Jess. COUNT 'EM AND SEE CITY BOARDER (watching F a r ni e r Timothy milk the cow): "Oh, yes, that set has four tubes, hasn't it RUNNING DOWN HIS BATTERIES JUNIOR FAN (entering hastily from outdoors at night) : "O papa! The cat's tubes are lit! I saw his panel lights !"-Philip Tracy. EDISON'S GREAT RIVAL CASEY: "My static eliminator was invented by an Irishman!" JONES: "What was his name ?" CASEY: "Pat Pending." -Billy R. Meredith. NO KEYHOLE WORK RADIO SALES ?IAN: "Good morning, madam. With this fine six -tube radio you can listen in on what all the world is doing." COLORED PROSPECT: "No, sah, ah believes in minding mall own business, sah r' -Wilfred Anderson (Bermuda). CHEAP ENOUGH A resident of Melbourne recently had a radio set installed,' and when his bill was presented this aston fishing item was at the .end of the account: "For hanging aerial aid myself-22 shillings."-Arthur Russell (.Australia). RADIO RHYMES No. 13 THERE'S JUST ONE THING- I'D LIKE TO KNOW THAT'S QUEER TO ME IN RADIO - I WONDER wHV EACH TUNEFUL 51-12 AIN LIKE ANESTHETIC , DRUGS THE HRAIN I. -. THeN O'ER 'mu 'DROWSY NO DOUBT THE SCiENT15tS FEELWGS CREEP. WILL CLAIM AND LULL YOU TO PRO- 1-I -105E ETHER WAVES FOUNDEST SLEEP. MUST BE To BLAME!
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