Projection engineering (Sept 1929-Nov 1930)

Page 18 Projection Engineering, June, 1930 Trouble Shooting In Amplifiers By James R. Cameron. Operating Characteristics of Vacuum Tubes Are Such That Certain Conditions Must Be Fulfilled in Order That Amplification Be Free From Distortion. AUDIO amplifiers in use today employ three distinct methods L of coupling: (1.) Transformer, (2.) Resistance, (3.) Impedance, with variations of 2 and 3, where combinations of resistance and impedance and resistance and auto transformer coupling are involved. However, no matter what the arrangement the function of the system is always the same. This does not mean that the performance is the same. Our purpose is not to outline the operating characteristics of various types of audio amplifiers. Instead to concern ourselves with troubles in audio amplifiers. We take for granted and we are justified in so doing, that the audio amplifier incorporated into any receiving system is best suited for that system. Furthermore that the design of that audio amplifier is as good as can be made under existing conditions. Operating on this premise we eliminate the subject of amplitude distortion in audio amplifiers, wherein is involved the frequency characteristic of the amplifier in question. The vacuum tube employed in an audio system plays a very important role, because it manifests a great influence upon the performance of the amplifier and the attainment of satisfactory results. 1. High mu tubes or tubes designed to possess high values of amplification constant are suitable for use in two types of audio amplifiers, resistance coupled and impedance coupled and are not suited for use in transformer coupled systems. 2. The conventional type of low mu tube represented by the 226 and 227 is suitable for use in all types of audio amplifiers but is limited for use in all stages except the output stage. This does not mean that a power tube cannot be employed in an intermediate stage in the audio system, but it is mentioned so as to define the limits of the regular tube. 3. D-C. tubes such as the 239A are not suited for operation with raw a-c. filament potential, but may be employed in series filament connection when the filament supply is d-c. secured from the a-c. power line or batteries. The same applies to the 112. 4. The screen-grid tube utilized as a space charge tube is unsuited for use with transformer coupling. This applies to the a-c. and d-c. tubes of this type. This tube is suited for use with resistance and impedance coupling when the resistances and the chokes are specially designed for the tube. 5. The 226 and the 227 are suitable for use in audio amplifier systems operating with raw a-c. filament po tential, but greater satisfaction is secured with the latter. 6. All types of tubes may be employed in push-pull arrangement providing that the system is designed for such work. 7. The utility of power tubes such as the 210, 245, and the 250 is governed by the required output power and the amount of voltage available for application to the grid of the output power tube. "~8. .Output power tubes should be interchanged unless the required operating potentials, grid, filament and plate, are available. The reason for this is that the output power is a function of the operating potentials. 9. It is necessary to remember that the operating characteristics .of the output tube differs from the other tubes in the amplifier. This difference is found in the function of the output tube. Whereas the other tubes in the amplifier perform as voltage amplifiers the output tube is a power amplifier and the power output varies as the square of the input. In other words if a tube is rated at 4 watts output with a certain value of plate potential and 40 volts applied to the grid, the application of 20 volts to the grid reduces the output power to one-fourth of the maximum output or 1 watt and not to one-half the maximum power or 2 watts. 10. The difference between the output power tubes and the other tubes in the audio amplifier is one of physical structure which permits the application of higher values of plate potential and grid bias so as to enable the application of high signal voltages to the grid of the power tube without causing overloading and distortion. Furthermore, the design of the power tube is such that it affords best energy transfer between the tube and the speaker over the normal audio frequency range, therefore affording best tone quality. 11. All tubes are suited for series or parallel filament connection, although it is not customary to employ the a-c. tubes in series connection. 12. The design of all vacuum tubes is such that operating voltage specifications must be fulfilled and adhered to. Excessive operating potentials may cause irreparable injury to the tube. This is particularly true of the filament. 13. Insufficient filament, plate or grid potentials will impair the operation of all vacuum tubes. The advent of the screen-grid tube makes necessary mention of the screen-grid voltage in addition to the other voltages quoted. 14. Amplification in a system should not be secured by increasing the oper ating potentials unless the value in use before the change is made is less than the maximum. 15. Volume control should not be in the form of reduction of operating potentials, such as the filament or plate voltages. Reducing the grid voltage is out of the question. (The above applies particularly to the audio amplifier, since plate voltage volume control is used in radio frequency amplifiers.) 16. Vacuum tubes of all types have definite operating lives, determined by the operating potentials applied to the tube. The most valuable part of the tube is the filament and operating life is limited by the deactivation of the filament or decrease in electronic emission. The 227 type of tube involves the cathode as the source of electrons. 17. Operating characteristics of vacuum tubes are such that certain conditions must be fulfilled in order that the amplification be free from distortion. The first is the elimination of grid current. D-C. current must not be permitted in the grid-filament circuit since it introduces distortion and reduces amplification by varying the characteristic of the tube and the coupling device. This applies equally to all forms of audio amplification. The second is that circuit arrangement must be such that the plate current variations in the plate circuit should be faithfully amplified reproductions of the signal voltage applied to the grid circuit. This means that every increase and decrease in signal voltage should be faithfully followed by plate current variations in the plate circuit. Since these plate current variations are a-c. the d-c. plate current must be steady, hence fluctuations of d-c. plate currents should not be permitted during operation. They indicate lack of balance between the plate and grid voltages. Increase in plate current during passage of the signal indicates tube overloading, distortion or insufficient grid bias. 18. Where more than one tube is employed in a single stage, as in a parallel or a push-pull stage, it is best if the tubes are of like design and characteristics. This condition affords more stable operation. With respect to the plate current drain in push-pull stages, best results are secured if the total current drain is divided equally between the various tubes in the stage. This division of plate current is also advantageous with respect to the effect of the direct current upon the core of the coupling unit (if it employs a core) in the plate circuit. 19. Vacuum tubes may be connected in parallel and in push-pull arrangements when additional output is re