Radio Broadcast (May 1928-Apr 1929)

JUNE, 1928 THE NEWEST POWER TUBE 75 adjusting output transformer to adapt the tube impedance to that of the loud speaker. Although an impedance. adjustment is not necessary when using a 250 type tube, it is necessary to use an output device to protect the windings of the loud speaker from the high plate current of the 250 tube. COMPARISON OF SENSITIVITY UNDER this caption we discuss how the two arrangements compare with regard to power output for a given input voltage. Let us, therefore, calculate the power output, for a given input, for both arrangements. If each arrangement works into a load impedance equal to twice the tube impedance, then, per volt input squared, there will appear in the load the amounts of power, as indicated below. Arrangement Power in Load, Per Volt Squared, on Grid of Tube Single 250 0.8 milliwatts Push-pull 210's 3.0 milliwatts In this table the 250 shows up very poorly, being only about one-quarter as sensitive as 210's in push-pull. However, the turns ratio of the average push-pull input transformer available to-day is lower than an ordinary transformer and if we assume that with a single tube the input transformer has a ratio of 4 and that the push-pull transformer has a ratio of 2 (many push-pull transformers have a lower ratio than this) between the primary and one side of the secondary, we then obtain the following figures: Arrangement Power in Load, Per Volt Squared, Across Transformer Primary Single 250 12.8 milliwatts Push-pull 210's 12.0 milliwatts These figures give a truer picture than those given previously. The difference between the two arrangements — a matter of 0.8 milliwatts per volt squared — is too small to be appreciable to the ear but at least we may be sure that an amplifier using a push-pull arrangement wilth average transformer ratios and with 210 tubes won't give any greater volume, with a given input, than an amplifier using a single 250 tube. This analysis, summarized in the accompanying table, shows the 250 tube to be about equal to a 210 push-pull amplifier in most respects excepting that of cost. The complete characteristics of the 250 tube are given in the table accompanying this article which shows that this tube requires a plate voltage of about 450 volts and takes a plate current of 55 mA. The power supply must provide this amount of current and voltage. The loud speaker must be isolated from this high plate current by an output device. In some cases it will be found possible to substitute a 250 type tube for a 210 tube in an amplifier, but frequently this will not be possible because the 250 takes three times as much plate current as a 210 and the rectifier-filter system may not be able to supply sufficient voltage at this higher current drain. The higher current drain may reduce the voltage output of the power unit from 450 volts, when a 210 type tube is used, to 300 volts or so and it is not worth while to operate a 250 at this voltage. Under such conditions the tube cannot deliver much more power output than a 210 type tube, operated at 450 volts. allow one watt per loud speaker THE 250 type can be used in a properly designed push-pull amplifier and such a use for the tube should prove useful where sufficient power is required for the operation of several loud speakers or for auditorium work. A push -CHARACTERISTICS OF THE TYPE 250 TUBE Plate Voltage Negative Grid Bias Plate Current Plate Resistance (a-c) Mutual Conductance Voltage Amplification Factor Max. Undistorted Output Recommended Maximum Filament Max. Overall Base 250 300 350 400 450 Volts 45 54 63 70 84 Volts 28 35 45 55 55 Milliamp. 2100 2000 1900 1800 1800 Ohms 1800 1900 2000 2100 2100 Micromhos 3.8 3.8 3.8 3.8 3.8 900 1500 2350 3250 4650 Milliwatts 7.5 Volts Height 6i" Large Standard UX 1.25 Amperes Diameter 2}J" Filter Choke Coils 15 henries or more GENERAL CIRCUIT REQUIREMENTS FOR THE 25O TYPE TUBE All values are indicated except the voltage dividing resistances which vary according to the receiver to be supplied Comparing the 250 Type Tube With Other Power Tubes IN THIS article the new type 250 power amplifier tube is compared with other power * tubes now available. A summary of the points developed more fully in the article follows: (a). The maximum power output of a type 250 tube is 4.6 watts, which is about three times as much as can be obtained from a 210 type tube and about seven times as much as can be obtained from a 171 type tube. (fe). The power output of a single 250 is about equal to the power that can be obtained from a push-pull amplifier using 210 tubes. (c) . The power supply equipment necessary for the operation of the 250 type tube is about the same as is required for the operation of a push-pull amplifier with 210 type tubes. (d) . Because ordinary transformers are generally cheaper than push-pull transformers, it is cheaper to construct a power amplifier with a single 250 type tube than it is to construct a push-pull amplifier with 210's. (e) . An amplifier using a 250 will give excellent quality. When the tube is worked into a load impedance equal to twice the tube's plate impedance, rated output will be delivered to the load without creating more than 5 per cent, of second harmonic current. (/). The volume output of a singlestage amplifier using a 250 will be about equal to that from a push-pull amplifier with type 210 tubes, assuming that the input voltage is the same in each case and that the transformer feeding the 250 has a ratio of 4 and the input push-pull transformer has a ratio of 2 (average figures). (g). A single 250 tube will supply all the power, with plenty in reserve, that will be required for the operation of any radio receiver in the home. Where only moderate volume is required, smaller tubes such as a 171, may be used but where reserve power is desired to take care of greater volume, the type 250 may be used.