WIRELESS NEWS.

Press, Volume LXII, Issue 18674, 24 April 1926, Page 8

WIRELESS NEWS.

RESISTANCE AMPLIFIERS. NOTES AND COMMENTS. (SPECIALLY WEITTM *o* "THS mSS.") (By "Electra.") As there seems to d number of radio enthusiasts still in doubt as to whether to install transformers or resistance amplifiers, it is advisable to further my notes of a few weeks ago on the subject. As stated before the present day tendency in radio development leans more toward loud speaker reproduction than perhaps to the sensitivity or selectivity of the receiver itself. Both loud speakers and amplifiers have undergone considerable development during the past year or so, and what was an open discussion two years ago is an established fact to-day. Ity this, I inean that resistance amplification is now the accepted method for getting the ltest quality speech and music. This is due, perhaps, more to the development of valves than to the amplifier itself.

Before going into details of the advantages of resistance coupling over otlief methods, it would be advisable to review tho disadvantages of the other two methods, namely, transformer and impedance coupling. The A.F. transformer, while giving enormous amplification, has certain restrictions, inasmuch ns the best transformer in tho world does nob amplify evenly over the entire bond of speech and musical frequencies. In other words, the percentage distortion at various frequencies is not anywhere like constant. I mention percentage distortion, since there is another kind of distortion which is railed wave-form oi modulation distortion. This is caused principally by hysteresis of the iron core Or to residual magfietism, which may be considered as another form of hysteresis.

It has been conclusively proven by eminent physicists in the radio field that the inductance value of the primary winding of an A.F. transformer must, be of the value of at least 50 henries together, with an impedance of eti'Al to at least the internal impedance if tho value of the transformer n to be ret-lly efficient. However, no matter how well the windings are designed, there always exists the trouble due to the iron core, a point which has been previously raised. Impedance coupling, although being infinitely superior to the majority of transformers, possesses many of tho inherent disadvantages /of the transformer, and gives less amplification per stage. With these main disadvantages enumerated, it would be advisable to see what advantage# resistance amplification offers over other methods.

The resistance amplifier itt its present day form is, without dispute, the best method of amplification to be employed in conjunction with a radio set designed for broadcast reception. There are no windings of iron cores to cause wave-form distortion, and the percentage distortion may be disregarded if the amplifier is designed with a suitable Bize of coupling condenser. This is realised by very few manufacturers of resistance amplifiers. This condenser acts as a dlroct current split between the plate and the grid of two successive valves. It must pass 'without hindrance however, the voltage variations produced across the resistor, Which is connected in the plate circuit of each amplifier valve. Since the resistance or reactance of this condenser increases as the frequency decreases, it is obvious that there must be at this point, a condenser of suitable size. Here it might be mentioned that a condenser of .006 mfd is not always suitable. Mention must be paid to this, since the majority of manufacturers employ a condenser of this capacity. The resistors and grid leaks must be of first class manufacture and must conform t.O some really stringent requirements, Which need not be enumerated here.

Until recently, it was found necesBAfy to employ twice as many stages of resistance coupling to obtain the same volume as that given by a good tWA Stage transformer amplifier. Today thefe is upOll the market a Hi-mU tube having an amplification constant of twenty or more. This type of tube, used in conjunction with & resistance amplifies*, Will give results Superior to two stages of (transformer Coupling. The improvement in tonal quality haS to be heard to be believed. Apart from the high amplification constant of these tubes, the other characteristics remain practically tho same as the standard U.V. 201 A. This ,tube is unsuitable for use in a transformer coupled amplifier, SittCO the plate impedance is much higher than the standard tube.

It is a common impression that the voltage to be used on the B. battery side of a resistance amplifier is much greater than with a transformer coupled amplifier. Strictly speaking, this is not so. Excellent results will be obtained using only 90, volts on the plate.' Of Course, 120 to 150 volts will produce more volume. With a resistance amplifier, using approximately 120 VOlts, "B." battery, the current consumption will be only one-third of that consumed by a transformer coupled amplifier. Consequently tho "B" batteries last throe timea as lopg. It will be seen, therefore, that the resistance amplifier is cheaper to run than any other type, as also is its initial cost much lower.

Perhaps a few points as to how to obtain the best results with a resistance amplifier Would be .appreciated by many readers. Where standard tubes of the 20lA type are used, the plate circuit registers may be between 80,000 to 100,000 ohms. The grid leaks should be one megohm, .5 megohm and .25 megohm respectively for a three-stage amplifier. With these values of resistance no blocking should be experienced in the last tube.

Where Hi-mu tubes are used of the Mu-20 type, the plate circuit resister should be 100,000 ohms. The question has been raised as to whether a "C" battery is desirable or not. From experiments conducted, it has been concluded that where three stages of resistance coupling are employed, a "C" battery is desirable in the last stage, and where the plate voltage exceeds 90 volts a "C" battery is essential. In the first two stages a "C" battery need not be used, since the wave*form distortion at this point is negligible, provided there is no distortion T»efore the sound reaches the amplifier. The experimenter must remember that for perfect reproduction only a first-class loud-speaker may be used. With the many types of cone speaker on the Market,, there is no reason why the broadcast listener should not secure results equal to those achieved by the engineer; and provided the programme leaves the station in an undistofted form, there ib no reason why the resistance amplifier should not be termed the super-amplifier of t6-dav.