Radio Broadcast (May 1928-Apr 1929)

-Charge Amplifier By H. R MANLY Thordarson Electric Manufacturing Company Supply A+ +90 INPUT A ? +45 SWITCH VT3 p VT 1 VT 2 HOW THE INSTRUMENTS ARE MOUNTED THE popular quest for greater volume generally leads to a multiplicity of tubes which add to the cost and complication of the receiver while opening the way to various troubles difficult to overcome. An amplifier which adds the effect of more tubes, without the tubes, forms an interesting and worth-while solution of the problem. This unit is of the two-stage type and in its first stage uses one of the new screen-grid tubes as a space-charge amplifier connected to the power tube through a Z-coupler, a device which consists of two high-impedance windings built on two separate cores and placed within one case. As employed in this amplifier, the inner grid of the tube takes no part in handling the signal but is used solely to reduce the effect of the space charge around the filament. This space charge, which in other tubes strongly opposes the flow of electrons between filament and plate, is a negative charge. By maintaining the inner grid, near the filament, at a rather strong positive voltage, the troublesome space charge is reduced and the amplification is greatly increased. The outer grid, which almost completely surrounds the plate, is used as a control grid and receives the voltage changes which represent the signal to be amplified. Such use of the two grids is the reverse of the practice followed when the tube is used as a screen-grid amplifier. With a strong voltage from the detector stage almost any audio amplifier will deliver all the voltage a power tube will handle with only moderate amplification in the first audio stage. But with a weak signal and correspondingly weak voltages from the detector, this moderate amplification will not be sufficient to produce good loud speaker volume. It is under such conditions the space-charge amplifier stage affords real THE new screen-grid tube may be used in two ways, one as a screen-grid amplifier at radio frequencies, the other as a space-charge tube at audio frequencies. So far as we know this is the first description of an amplifier using this tube as a space-charge audio-frequency amplifier. A unit built in the Laboratory and operated from d.c. shows a good low-frequency characteristic, but fell off somewhat above 2000 cycles due to the high input capacity of the screen-grid tube. The ratio of voltage {across 4000 ohms) in the output to the input 0.1 volt at 1000 cycles, was 240, which indicates a voltage step-up in the system from input to the grid of the power tube of 120, compared to 72 which is the step-up in the average two-stage transformer-coupled amplifier. The Thordarson amplifier power-supply illustrated here when tested in the Laboratory had a tendency to motorboat which was corrected by connecting a 25,000-ohm resistance in the 45-volt lead and by passing it to ground as shown in the circuit diagram. Owing to the fact that the entire unit is built in a rather small space, the output is not too free of hum, and the experimenter is advised to arrange his apparatus with a little more room between parts. ■ — The Editor. improvement because it steps up the weak voltage and delivers a comparatively strong impulse to the power tube's grid. FACTORS DETERMINING VOLUME IN OUTPUT AS IS well known, the effective or overall amplification of any tube and its coupling device depends not only on the amplification factor of the tube but also on the amount of impedance in the external circuit of the tube. 163 Using the 222 type tube as a space-charge amplifier it is found to have a plate impedance of about 125,000 ohms and an amplification factor of approximately one hundred. The greater the external impedance, the greater will be the proportion of this theoretical amplification actually realized and impressed as grid voltage on the power tube. The required high impedance for the external plate circuit is secured from one winding of the Z-coupler, connected between the plate of the space-charge tube and the plate current supply for that tube. The other winding is placed between the grid of the power tube and the filament circuit of that tube. The a.c. voltage variations across the plate circuit winding are passed on to the grid of the power tube through a coupling condenser, C3, in Fig. I. With an impedance type of coupling using a resistance grid-leak there is a decided tendency for the power tube's grid to block on strong signals. The second coupler winding, connected between the power tube grid and the filament circuit, provides an escape for the excessive collection of electrons which produce the blocking. It has a relatively high impedance and comparatively low d.c. resistance. Heretofore, the screen-grid tube has been used chiefly in radio-frequency amplifiers, and then with the screen-grid method of connection. The screen-grid connection is especially favorable for such use because it reduces the tube's gridplate capacity and lessens the consequent internal feedback. In audio-frequency amplification, this capacity is of comparatively little importance and using the tube as a space-charge device secures advantages which are especially desired for this kind of work. As a space-charge amplifier, the tube is free from a tendency to howl which,