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Space-Charge-Grid Amplifier

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Space-Charge-Grid Amplifier Space-Charge-Grid Amplifier MELVIN C. SPRINKLE* A new low-power amplifier using the space-charge-grid tubes first publicized as audio output tubes almost four years ago. By the designer of the Musician’s Amplifier. rom time immemorial, it seeing the audio enthusiast has been searching F lor wider frequency range and lower distortion in his amplifiers. These desirable features are expensive, and cost is an important consideration to the average audio fan. So. the eltort has gone on to get the most for the least. The amplifier to be described represents what is believed to be the highest quality yet attained for the required financial imestment. The remarkable pertorm- ancc is made possible by two important components: a, new and radically differ ent tube type and a high-qualffj low- cost output transformer. In spite of its well known limitations which increase cost, the triodc tube has for years been the standby of the audio crowd. The beam-power tube overcame a number of the limitations of the triode, but many builders never accepted the beam power tube as the equal of the Fig. 2. Schematic of the space-charge-grid amplifier chassis. triode. Thus the argument has raged for some years on the beam tube vs. the triode, with good points on both sides. benefit of those who do not have access monic distortion is reduced. It has been Now, the National Union Radio Corp. to the references, its characteristics are mentioned that the plate current family has developded for commercial use a briefly summarized. The space charge resembles triodc curves. To this must new tube type which combines the best tube is a tetrode, with a cathode, two be added the fact that the plate resist­ features of the triode and the beam tube, grids, and a plate. The grid nearest the ance is low—2.500 ohms—the m- of 16 and which opens a new era in high- cathode is operated at a positive d.c. is medium, and the transconductance is qualitv amplifiers. This tube is known voltage with respect to the cathode, the 6.500 micromhos. The uniformity and as the NU 2160 and is a space charge effect being to counteract the negative regularity of the plate family indicate tube. Its plate family of curves resembles space charge and produce a larger cloud that these parameters are fairly constant those of triodes, but its efficiency and of electrons at the plane of the first grid. and therefore the distortion is low. The drive requirements are like a beam This cloud acts as a virtual cathode. The tube is operated with a bias of about 18 power tube. second grid is then the control electrode, volts on the control (No. 2) grid so that The space charge tube has been de­ being operated with a negath e bias in operation out of a resistance-coupled scribed in the literature1,2 but for the the conventional manner. The operation stage or phase inverter is very prac of the space charge tube is similar to a tical. * Peerless Division, Altec Lansing Cor[> triodc with a large effective cathode. It Through the courtesy of National 1Electronics, Vol. 20, No. 8, August should be pointed out that it is not pos­ Union, the writer was provided with a 1947. p. 121. sible to connect an ordinary tube as a pair of NU 2160’s for experimental use. 2 A udio E ngineering, Vol. 31, No. 9, space charge tube because so doing As of the present, National Union is October 1947, p. 20. would cause excessive current in the planning to market the NU 2160 first grid. In the NU 2160 the current through radio parts distributors, but in the first grid has been lowered by material uncertainties make the general mounting a pair of side rods between availability of the tube a question mark. the space charge grid and the cathode, It is also not known whether other tube and connecting these rods to the con­ manufacturers will make space charge trol grid. The side rods thus shield the tubes for sale through their distribution space charge (No. 1) grid from the channels. cathode except in the vicinity of the lateral wires of the grid. They also have Design of Amplifier an effect in reducing harmonic distor­ tion. Examination of the data sheets pro­ The positive voltage on the space- vided with the tubes, showed that the charge grid is supplied from the plate tube was ideal for a low-cost amplifier, supply through a 10,000-ohm resistor. the power output as calculated being of The space charge grid is not bypassed, the order of 6 watts for two tubes in and the omission of the bypass capacitor push-pull. The Peerless S-508-A output produces three beneficial effects: the transformer, originally designed for type power output is increased slightly for a 6V6 tubes, has ratings as to plate cur­ Fig. 1. Separate chassis for the amplifier and given average cathode current, the out­ rent and primary impedance which suit power supply simplify mounting in many ap­ put is less affected by changes in ex­ it to the 2160. Its design, based on the plications. ternal load impedance, and the odd har­ 6V6 plate resistance, means that it will perform even better at low frequencies ohm resistor, which is heavdy bypassed. this ha , been accomplished, as vvdl be with the low-platc-resistance space- The tubes are operated more Class ABX seen from the performance data. The charge tube. Its physical size is small, than recommended by the manufacturer, NU 2160 was not intended as a high- and its price—scarcely higher than a &s tests with intermodulation equipment power tube, and hence the power output large replacement type transformer— showed that reducing the bias increased is limited when compared v\ ith 6L6 puts it whhin the reach of the most mod­ the distortion slightly at the higher out­ tubes. The useful power output of the est budget. This transformer has low put levels. amplifier in a load resistor connected insertion loss, wide frequency response, Inverse feedback around the amplifier to output transformer secondary is about and ability to deliver power at frequency is sufficient to reduce hum, noise, and 5.5 watts, based upon the point where extremes. The performance data on the distortion almost to the vanishing point the intermodulation begins to climb rap­ complete amplifier to be given later at­ at the usual levels. The source imped idly. This power rating compares very tests to this. ance is lowered to 2.5 ohms on the 16- favorably with the data sheet which The amplifier itself is simple and with ohm tap. Connections are available on gives the power in a resistive load con- the possible exception of the space- the output transformer to match loads lected from plate to plate as 6 watts for charge stage is conventional. As will of from 4 to 16 ohms. Values for the 2 per cent harmonic distortion, as shown he seen from Fig. 2 it consists of a feedback resistor for various load im­ in Fig. 4. The single-frequency sine- pentode amplifier stage using a type pedances arig given in Fig. 2. wave output power before the waveform 6SJ7, direct coupled to a 6J5 split-load begins to be distorted (about 5 per cent) phase inverter which drives the output Construction is 7.5 watts. The power output as a func­ tubes. The circuitry of the pentode stage The amplifier is constructed on an tion of frequency is given in Fig. 5. is conventional, the only precaution be­ It is the power output at frequency ex­ ing to use an adequate bypass capacitor aluminum chassis 5x7x3 in. There is plenty of room for all parts, most of tremes that makes an amplifier sound on the screen. The split load phase ir good and which is reflected in the low verter is also well known and conven­ which are mounted on two terminal boards. One board is used for the pen­ intermodulation distortion. Power out­ tional. The writer has found it to he put at frequency extremes is a function about as good a circuit as is available, tode and plffce-in verter components, having none of the drawbacks somq- while the other holds the coupling ca­ times ascribed to it. One of the big ad­ pacitors and resistors for the space vantages is that it lends itself so well charge tubes. \ ground bus is used with to direct coupling from a preceding connection to the chassis at the input so stage. Use of direct coupling extends as to avoid chassi* currents and hum the frequency response of the stage to The controls are simple—a volume con­ d.c. and eliminates coupling networks trol. a pilot lamp and an a.c. line switch. which can cause phase shift and low- The power supply, Fig. 3, is simple frequency attenuation. There are only and conventional. The power trans­ two coupling capacitors in the amplifier, former is rated at 120 ma, and the am­ from the phase inverter plate and cath­ plifier draws just about that current. ode to the power amplifier grids. The After 12 hours continuous use in 75° F. low-frequency response in the entire am­ ambient, the transformer is just warm. plifier is very good, much better than A 6-wire cable is used to connect the the transformer curves alone might lead power supply to the amplifier. The a.c. one to believe. It should also be pointed power line is run to the amplifier so out that the direct coupling system used that there is no possibility of the power Fig.
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