Radio Broadcast (May 1929-Apr 1930)

-z. RADIO BROADCAST considered satisfactory, for it indicates poor reproduction of the bass notes ; a note of 50 cycles would be reproduced at only one-tenth of the amount with a 2000-ohm triode. This is amply proved in practice, for bass notes are almost inaudible. Curve d, however, when a 4-1 transformer is used is very similar to those for 1000and 2000-ohm triodes, and as would be expected there is no audible difference between a 2000-ohm triode and a pentode with a 4-1 transformer, provided that the volume is small. That is where the trouble comes in, for, from what has just been said, it can be seen that for use with the best reed-drive type loud speakers, the pentode is useless unless a suitable output transformer is used to make the load impedance high. But, if the load impedance is high, the undistorted power output is greatly reduced, although the stage gain is increased. In the particular case mentioned, when the 4-1 transformer is used with the Amplion loud speaker, the load varies between 13,000 and 200,000 ohms, and the maximum undistorted power output is consequently only about 85 milliwatts; which can hardly be considered sufficient for good quality if reasonable volume is desired. The control grid input for this output is but 3 volts peak. When a loud speaker of the moving-coil or dynamic-cone type is used the results are different, for this type of loud speaker requires constant current at all frequencies; so that it would seem to be ideal with a pentode. The usual impedance variation is between 1000 and 4000 ohms over a range of 50 to 5000 cycles, being greatest at high and low frequencies and least at a frequency somewhere about 500 cycles. [The author is speaking of English speakers —Editor]. In practice, with a low-resistance triode, this causes a falling off in the current at both high and low frequencies; the plate resistance of the pentode is so high, however, compared with the speaker impedance that the variation makes a negligible difference to the total circuit impedance; and it is this total circuit impedance which alone limits the current. The result is that the current remains constant over the whole frequency range, which, with this type of speaker, is what is wanted. It should be carefully noted that the current will only remain constant provided that the speaker impedance is small compared with the tube's plate resistance, for only then is any variation in its impedance swamped. This means that if a transformer ratio is used to get a large stage gain, the tone of the reproduction will be neither better nor worse than with a triode, but that the power output will be small due to the effective speaker impedance being high compared with the plate resistance. On the other hand, if the effective impedance is adjusted to be about 6000 ohms, the power output will also be at its maximum (640 milliwatts) and the variations of the speaker impedance will be swamped by the relatively high tube resistance, with the result that the current will be constant at all frequencies and a fair output will be obtained. Theoretically, the moving-coil loud speaker would work much better with a pentode than with a triode as regards tone, but in practice so much depends upon the whole design of the loud speaker. With some speakers the results are improved, with others they are the same, while with still others they are distinctly worse. Many speakers of this type have a pronounced resonance at some high frequency, and while this is not serious with a triode, and is even sometimes advantageous owing to the falling current at high frequencies, with a pentode it often makes reproduction sound shrill. A Look at the Future It will be seen from the foregoing that it is useless to expect good quality from a pentode with a reed-drive loud speaker unless only a small output is required, when a step-down output transformer can be used. With many moving-coil loud speakers, however, the results are at least as good as with a triode, while the power output is not much different from that of a triode on the same plate voltage, the input voltage to the control grid to produce this output will be only about onethird to one-quarter of that necessary with a triode. The chief use for the pentode seems to lie in inexpensive receivers which make no pretense of giving the best quality, and where compactness and low cost are of prime importance. It is worthy of note that no English firm is at present using it in their best receivers; it is confined to their cheaper models and to portable sets, where weight is a great consideration. In this connection the large gain which can be obtained from the pentode with a high-impedance load often enables the audio-frequency tube between the detector and the power stage to be omitted. Also, most of the cheap cone loud speakers, such as are used with these sets, have a fairly high impedance, usually about 20,000 ohms and upward, so that no output transformer is necessary for tone correction. Any output transformer or choke which is to be used in the plate circuit of a pentode must have a much higher inductance than is necessary with a triode, owing to the higher plate resistance. An inductance of between 40 and 80 henries is usually considered satisfactory. As the present tendency in radio is toward better and better quality, it seems probable that the present popularity of the pentode will decline; particularly as, even now, triodes are available giving larger power outputs with a fairly large stage gain. In this connection a recent addition to power tubes in England is the Marconi P. 625, which is rated to withstand 250 volts on the plate with a current of 24 nxA., and a negative C bias of 24 volts. It has an plate resistance of 2400 ohms and an amplification factor of 6, giving a mutual conductance of 2.5 mA./v; the undistorted power output is approximately 1000 milliwatts. The filament is of the coated type and requires a current of 0.25 ampere at 6 volts. Now this tube will give a stage gain of about one-fifth of that which can be obtained with a pentode with a load of 20,000 ohms, but it will give an undistorted output of about 12 times that with the pentode. It is true that the plate voltage required is nearly double, but this is of little moment nowadays when socket power units are commonly used. If the pentode is used with the best load for power output the stage gain is about three times that of the P. 625 tube, but the latter tube still gives a greater output, about 1.5 times. It is the writer's opinion, therefore, that there is little advantage to be gained by using a pentode in place of a really good triode power tube, the only thing in which it scores is in stage gain; and its superiority here is not sufficient to make its use worth while, except in receivers where quality is only a secondary consideration, such as lightweight portable sets. This latter type of set is very popular in England, and the pentode is becoming very common in them, due solely to its high stage gain, for the high plate current does not make for economical operation from small dry batteries, which have nearly always to be used in these sets. DESIGN OF THE COLONIAL MODEL 32 A.C. (Continued from page 359) picked up at several different points in the band. It results from poor selectivity in the first r.f. tube on signals of sufficient intensity to overload this tube. When the first tuned circuit is not sufficiently selective to prevent two different carriers from simultaneously reaching the grid of the first r.f. tube, a series of beat frequencies will be set up in the plate circuit of the first tube as well as the interference caused by the modulation of either carrier, by the audio component of the other carrier. Their amplitude will vary with the combined amplitudes of the two carriers. When the receiver is located in the vicinity of several broadcasting stations, these beat frequencies become very important in the creation of cross-talk. To prevent the cross-talk which occurrs when the voltages impressed on the grid of the first tube are sufficiently high to cause it to draw current, a 750,000-ohm resistance shunted by an r.f. by-pass condenser is placed in series with the r.f. inductance in the d.c. path of the grid current. This reduces the d.c. voltage on the grid without reducing the r.f. signal. At the same time any slight grid current flowing through it increases the negative voltage on the grid due to the IB drop through the resistor. In this manner the selectivity of the first tuned circuit is preserved and cross-talk eliminated. Volume is controlled by variation of the cathode potential with respect to the other elements of the tube, at the same time varying the antenna input by means of a 10,000-ohm potentiometer on the same shaft as the bias unit. The volume control is a 75,000-ohm variable resistance which acts as a grid-biasing resistor or more properly as a cathode bias and resistor. A screen-grid tube is used in this receiver as a detector. By shunt feeding through a 400,000-ohm resistor, 280 volts are applied to its plate. Plate rectification is used and the graph of Fig. 8 shows the gain of this tube as a detector as well as the high audio-frequency output available without overload. The characteristics of the loud speaker were taken by a series of sound pressure measurements. The procedure employed was as follows: A radio-frequency carrier of constant amplitude, modulated at audio frequencies by a variable beat-frequency oscillator, was introduced into the input of the receiver. A correction for the pressure increase due to the reflection from the microphone was made on the basis of an equivalent sphere. A typically representative curve of the Model 32 receiver is shown in Fig. 9. This measurement is actually a complete overall fidelity measurement of the entire receiver. It tells the complete story of the fidelity of reproduction and takes every factor into account, including side-band distortion, audio amplifier characteristics and speaker characteristics. The characteristics of the speaker itself may be determined by comparing the sound pressure curve with the fidelity curve previously 362 • • OCTOBER 1929 •