Radio Broadcast (May 1928-Apr 1929)

THE COMPLETED POWER AMPLIFIER AHP A DYNAMIC TYPE LOUD SPEAKER This high-quality power amplifier, built around the type 250 tube, must be used with the better types of loud speakers if the builder is to obtain from the unit the quality it is capable of producing. This photograph shows the amplifier in conjunction with the Jensen dynamic loud speaker A Good Amplifier-Power Unit for the 250 Tube By HOWARD BARCLAY WE PROPOSE to describe here a singlestage power amplifier and B supply designed for use with the new type 250 tube. In the following paragraphs we review the general characteristics of this power amplifier and call attention to the interesting features that have been incorporated in it. Sufficient constructional details are given so that the amplifier may be readily constructed by those interested readers to whom this unit appeals. This power amplifier and B supply is designed for use as a last-stage amplifier. The receiver proper must contain at least two stages of resistance-coupled amplification or one stage of transformer-coupled amplification in order that the overall gain of the system shall be sufficient to operate the power tube. A combination amplifier and B-supply unit of this type can also be used to considerable advantage to replace the output stage in a radio receiver not using power tubes or in cases where more volume is desired than can be obtained from the smaller types of power amplifier ordinarily used in radio receivers. Service men and professional set builders may find the construction of an amplifier of this type advisable for customers desiring the utmost in quality reproduction and who desire comparatively large amounts of volume such as is necessary in reproducing radio programs for dances in clubs, and in other large public rooms. The output of this single type 250 tube is about equal to the power output of an amplifier using type 210 tubes in push-pull. More specifically, the rectifier-filter system used in this power unit is such that the plate of the 250 tube receives about 400 volts at which voltage the tube can deliver 3.25 watts. This amount of power is in excess of what is ordinarily required for home reproduction but will not be excessive when the radio installation is called upon to supply more volume — sufficient, for example, for dancing. The construction of a power amplifier that can deliver more power than is ordinarily required falls into the same category as the purchase of an automobile with an 80 horsepower engine. The maximum output of the device may seldom be required, but is available when necessary. [The reader should refer to the article "The Newest Power Tube," by Howard E. Rhodes on page 74, Radio Broadcast, June, 1928. Here the potentialities of the 250 type tube were most thoroughly analyzed and compared with other power tubes that are available to the radio fan and professional set builder. — Editor.} THE CIRCUIT IS INTERESTING THE circuit diagram of this power amplifier and B supply is given in Fig. 3. The circuit contains two innovations. In the first place a small flashlight bulb, indicated as B in the diagram, is connected in series with the output of the rectifier tubes, to protect the rectifier tubes from damage in case the output of the filter system is accidentally short-circuited or in case the type 250 tube is placed in the socket and operated at less than normal bias so that the plate current is excessive. In such cases the small flashlight bulb will glow very brightly or will burn out and thereby warn the user of the device that there is some defect in the circuit. The filter system used in this device is unusual in that the condenser connected across the input to the filter system is eliminated and in this case is connected instead to the center point of the two filter choke coils, Li. There is no condenser connected across the input to the filter system. This revised circuit is used in accordance with recommendations to be found in the Cunningham Tube Data Book. [Available from E. T. Cunningham, Inc. at $2.50. — Editor.] Curves are given in this book indicating that with the familiar type of rectifier and filter circuit, Fig. iA, the load on the rectifier tube isquite heavy, reaching during each cycle, current values as high as 300 milliamperes although the average current drawn from the filter system was only 125 milliamperes. Under such conditions the rectifier tubes are therefore called upon to supply a peak current about two and one half times as great as the load current (300 divided by 125) and thus the fila iy?a» — — nra^ THIS article describes a very satisfactory power supply and amplifier unit using the new type 250 amplifier tube. Technically, the device has two especially interesting and unusual features. In series with the rectifier output a small flashlight bulb is included. This operates to protect the rectifier tubes from damage in case of an accidental shortcircuit. A new type of filter circuit is used, which gives longer rectifier tube life than that usually obtained in the usual type of filter system. This unit tested out very well in the Laboratory. The quality was very good and the a.c. hum was practically inaudible in the dynamic loud speaker which was used. — The Editor. 141 ment of the rectifier must be made heavy and long enough to supply this large amount of current. A very great improvement in these load conditions can be obtained by removing the condenser, Q, across the input to the filter system, as indicated in Fig. 1 B. Under such conditions the peak value of current which the tube must supply is reduced to only 140 milliamperes in comparison with the value of 300 milliamperes which the tube is called upon to deliver when the condenser was included in the circuit. This reduced current drain on the tube results in much longer filament life. The omission of the first condenser from the filter system would normally cause a reduction in output voltage but this can be compensated by slightly increasing the transformer voltages. The power unit described and illustrated in this article makes use of this new filter system so that the user can obtain from the rectifiers as long a life as possible. The list of parts on page 142 are those used in constructing this amplifier. assembly and operation THE wiring of the amplifier is not difficult, using the information given in Figs. 2 and 3. The various transformers, chokes and condensers are supplied with insulated leads sufficiently long so that most of the wiring may be done with them. The leads are colored in accordance with the markings on the diagram. If it is necessary to use any additional hook-up wire it should be of the heavy insulated rubber type not smaller than size No. 18. [The unit described here was made with parts from the Dongan Company and it performs very satisfactorily indeed. Obviously, if the builder prefers, he can assemble a similar amplifier and power supply employing the necessary units made by other reliable manufacturers. We recommend that only 1000-volt test condensers be used in the filter. The "New Apparatus" department, page 85 June issue, and that department in this issue, as well as our advertising pages, give information on the products which might be used. — Editor.] As indicated in the picture diagram, Fig. 2, all of the cases of the transformers, filter chokes, and condenser blocks should be wired together and connected to the negative B. The filament leads of the rectifier tubes should be twisted together as should the filament leads to the type 250 power amplifier tubes to minimize any a.c. hum. It is useless to construct a quality amplifier