Radio Broadcast (May 1929-Apr 1930)

RADIO BROADCAST No. 298 Radio Broadcast Laboratory Information Sheet Sept., 1929 Circuits for the 245-type Tube "V/Tost of the new radio receivers are using audio power ampli6ers which employ either one or two 245-type tubes, the tubes being arranged in push pull when two are used. Because of the wide use of this tube we give on this sheet and sheet No. 299 some data on -the various circuit arrangements generally used with the 245. Circuit A Two 245-type tubes are indicated in circuit A on "Laboratory Sheet" No. 299. The plate voltage required is 300 volts, 250 of which is impressed on the plate of the tube and the remaining 50 supplying the necessary C bias. The filament is shunted by the 10-ohm resistance. The C-bias resistance is calculated by dividing the required C bias, 50 volts, by the plate current, 32 milliamperes, and dividing by two since there are two tubes. This calculation shows that the C-bias resistance should have a valup of 750 ohms as indicated. Circuit B This circuit shows a single 245-type tube with transformer output. The required plate voltage is 300 volts and the C-bias resistance is 1500 ohms. Since the a.c. current in the plate circuit must flow through the C-bias resistor to get to the filament, it is essential that the resistor be bypassed with a condenser of 1 or 2 mfd. The output transformer may be an ordinary one with a ratio of about 1 :1, or it may be the transformer which couples the tube to the moving-coil system of a dynamic loud speaker, in which case, of course, it should have a step-down ratio. Circuit C This arrangement is similar to that indicated at B except that a choke condenser output is used. With this arrangement d.c. current is kept out of the primary of the loud speaker or coupling transformer if one is required. Since one of the loud speaker terminals returns directly to the center-tapped resistor connected asross the filament, if is not essential that any condenser be connected across this resistance. No. 299 Radio Broadcast Laboratory Information Sheet Sept., 1929 No. 300 Radio Broadcast Laboratory Information Sheet Sept., 1929 Center-Tapped Filament Resistors Oeveral readers have written us request^ ing information on what determines the value of the center-tapped resistance connected across the filament of an a.c. tube. In some cases it is apparently felt that this resistance must have a definite value in order to produce a definite load on the transformer secondary supplying the filament. This is seldon, if ever, the case, however. No hard and fast rules can be given for the value of the resistance used across the filament. In fact a wide range of resistances can bo used with equally good results. The importan* point to consider is the resistance of the filament across which the center-tapped resistor is to bconnected and to make sure that the latter's resistance ii fairly high in comparison with that of the tube. For example, if a tube filament has a resistance of 1 ohm then the total resistance of the center-tapped resistor should be at least 10 ohms and might well be higher. Never use a center-tapped resistor of an ohmage comparable (o that of the tube across which it is to be connected. One other factor is of some importance, especially in connection with the resistors placed across the filaments of power tubes. C bias for these tubes in a.c. sets is obtained by means of an additional resistor whose value is equal to the C-bias voltage required divided by the plate current of the tube. Actually, however, the center-tapped resistor also supplies some of the bias for the current in returning to the filament must flow through both halves of this resistance. In effect, therefore, the plate current flows through a resistor equal to one half the total value of the center-tapped resistance, since both halves of it are in parallel from the standpo'nt of the plate current. In calculating required values of C-bias resistance, it is wise, therefore, to substract from the calculated results one half the value of the center-tapped resistance. For example, two 171a tubes in Eush pull draw 40 milliamperes and require a ias of 40 volts. The value of the C-bias resistor should, therefore, be 1000 ohms. If, however, a 200-ohm center-tapped resistor (values as high as this are frequently used) is placed across the filament of the tube, one half of this value (100 ohms) should be subtracted from the required 1000 ohms leaving 900 ohms. This value should then be used for the C-bias resistor. THAT TRADE GOSSIP (Continued from page 259) other, and have worked together. Every difhculty as to harmonious action has been the breeder of some new form of distribution, the mail order house, the department store, the chain store, the manufacturer's own retailing system — and yet the independent dealer, in the aggregate, remains the principal retail factor. He and the manufacturer understand each other a little better than they did once — but not well enough yet, by far. Welcome the wholesaler's salesman, make him your friend, make him work for you, tell you the news of how other dealers are making a success, but do not exchange cheap and unreliable gossip with him. You can give him much and he can give you much that is real and important. To your customer you must always appear substantial and reliable. There is money in your pocket in having him respect you. In your business, your customer looks up to you as an expert in a mysterious business. "The customer is always right?" Not always. He is not right when he wants to take your profit away from you and you should have a nice way of telling him so. There is a nice way to do this, also a nasty way. The customer is not right when he wants to trade-in a bum set for many times what it will bring as junk. He generally is not as up to date about radio as he likes to appear. No he is not always right but he is interested, and that is the thing for you to build on. He needs education, both in your merchandise and in the common rights of a merchant. Remember this about customers. They have been catered to, flattered, made love to by advertising to such an extent that they are all puffed up with the importance of their own little dollars. They do not know how to treat a dealer fairly. They do not realize that if he gives up his profits he cannot continue to serve them. But they are human. Most of them are trying to make a living for themselves. They are quite capable of being educated into being profitable customers. News vs Gossip The distinction we have made between "news" and "gossip" is often a hair-line distinction. How can we get the "news," which is wholesome, and kill the bad effects of "gossip"? The first step is to see the seriousness of what it means and act accordingly. The next is to fix the imagination on being a successful dealer and act accordingly, that will keep any man busy. Simple and self-evident as all these things are, we are very apt to ignore them. We are very prone to help foolish gossip along, then wonder why" the good will in our industry is no better than it is. Everybody has a stake in this. "Among the many things we talk about, what is dangerous gossip, and what is real and valuable news?" The only answer I can give to that is — You generally know that difference yourself if you will stop to think. Broadly speaking, any display of real enthusiasm for radio and radio merchandise is promotive and valuable, while knocking of radio, knocks at people in radio, indications of fear for radio's future are taking money out of your own till and throwing it away. [Another article by Mr. Dickinson will appear in the October Radio Broadcast. In this article the advertising problems of a retail radio store will be discussed from all angles — Editor.] 3 0 2 • • SEPTEMBER 1929 •