Radio Broadcast (May 1929-Apr 1930)

A Tube Making Possible Many Economies ENGINEERING FEATURES OF THE UX-245 By F. H. ENGEL Technical and 7Vsf Department, Radio Victor Corporation The ux-245 is a power amplifier which has been developed to meet the demand for an outi put tube which would ~r fl| permit the manufac r~~MM ture of broadcast receivers having good tone quality and reasonable volume at a price well within the /'. H. Lngel reach of the average purchaser. Heretofore, the medium-priced receiver has been limited to the use of the ux-112a and ux-171a types of output tubes, and, while the fidelity of reproduction was satisfactory, the volume obtainable with these types was, in many instances, not considered sufficient for all purposes. Receivers utilizing ux-210 or ux-250 tubes are inherently above the middle-priced range so that a gap has always existed between the medium-priced, good-fidelity, low-volume receivers and the high-priced, good-fidelity, high-volume receiver. The ux-245 tube is intended to fill this gap. The electrical rating of the ux-245 tube is as follows: Filament volts 2.5 Filament amperes 1.5 Plate volts 250. Some misunderstanding has existed in the past regarding the voltage ratings of tubes so that a few words in explanation will not be amiss here. The filament voltage rating of a Radiotron, for example, as given in the instruction sheet accompanying each tube, is a normal value, i.e., optimum set performance and life will be obtained when the tube is operated at its rated (normal) value. This means 'that receiving sets operating on socket power should have transformers or resistors in the filament circuit which are designed to operate the filament or heater at rated value under average line voltage conditions. A reasonable amount of leeway is incorporated in the tube design so that ordinary line fluctuations downward will not cause undue loss of electron emission and appreciable decrease in set performance. In the normal voltage range the filament of the ux-245 tube operates at a dull red color and, with normal plate voltage and grid bias, the sturdy, coated filament gives exceptionally long fife performance. The plate voltage ratings of tubes are maximum values and are so indicated on the instruction sheets. In the case of the ux-245, the 250-volt rating means just what it implies, i.e., the value beyond which it is unsafe to go from the viewpoint of life performance. Several methods of obtaining this voltage regulation are available but their discussion is beyond the scope of this article. In general, however, the amount of voltage fluctuation caused by line-voltage variation, load variation, and manufacturing variations in the apparatus must be determined or estimated and an average design value should then be decided upon so that under the operating variations to be encountered the voltage ratings of the tube will not be exceeded. Electrical Characteristics Table i gives the electrical characteristics of the ux-245 tube under the two operating conditions which will be used most generally. It should be remembered that the values given are average and that individual tubes may vary somewhat from those stated below. Table I AVERAGE CHARACTERISTICS THE UX-245 TUBE Characteristic Filament volts 2.5 2.5 Plate volts 250 . 180 . Neg. grid volts 50 . 33 . Ampl. Factor 3.5 3.5 Plate Resistance (ohms) 1900 . 1950 . Mutual Conductance (micromhos) 1850 . 1800 . Plate Current (mA.) 32. 26. In Fig. 2 a family of plate current-plate voltage curves for various grid-bias voltages is shown. These curves are useful in determining the undistorted power output of the ux-245 tube. The method employed has been referred to in an article which appeared on page 329, March, 1929, Radio Broadcast ("A High-Power Output Tube— The 250," by K. S. Weaver) and has also been described, among others, by Messrs. J. C. Warner and A. V. Loughren in the I.R.E. Proceedings, December, 1926, so that this discussion will not be repeated. The undistorted output obtainable from ux-245 tube is shown in Table II together with similar data on ux-171a, ux-210, and ux-250 tubes. Table II OUTPUT OF VARIOUS POWER RADIOTRONS Undistorted power output in milliwatts PLATE VOLTAGE UX-171A ux-210 ux-245 ux-250 180 720 780 250 340 1600 900 425 1600 450 4650 It will be noted that the ux-245 lies between the ux-171a and ux-250 with regard to the undistorted power which it is capable of delivering to a loud speaker, and it has the same output as the ux-210 tube. The advantage of the ux-245 over the ux-210 lies in the fact that the ux-245 delivers the same power as the ux-210 but at about one half the plate voltage required by the latter. This feature is of great importance in connection with set design from the cost standpoint and is one reason for the assured popularity of the new ux-245 tube. Operation As stated above, maximum power output from the ux-245 is obtained when it is operated at normal filament potential, 250 volts (maximum) on the plate, and with a negative grid bias of 50 volts. Filament voltage recommendations for the ux-245 have already been discussed. However, a few remarks concerning the characteristics of this filament may be of interest. The filament oi the ux-245 is of the coated-ribbon type and has high thermal inertia which, in addition to the relatively low filament voltage required, insures "humless" operation on alternating current. This type filament is not affected by the presence of small amounts of residual gas and a slight blue glow in the tube is no indication that the tube is defective or that it is improperly operated. The usual midpoint connection to a re Fig. J — These diagrams illustrate four methods which may be used for obtaining all potentials required for the operation of a receiver without exceeding the rating of a standard UX-280 full-wave rectifier tube. Each of these power supply systems is designed to supply a receiver of different voltage and current requirements. • JULY 1 929 • • 167