WIRELESS

Wanganui Chronicle, Volume LXXXIII, Issue 19511, 13 February 1926, Page 1 (Supplement)

WIRELESS

By

The Grid

The Audio-Amp lifter (Continued)

In last week’s notes we referred to amplification by the methods known cs impedance-capacity and resistance-capa-city, and gave some figures showing the voltage amplification obtained in amplifiers of these types in comparison with the more usual transformer coupled type. We also referred to the two main kinds of distortion, known respectively as 1 ‘amplitude’* distortion and ‘‘wave form distortion.” experienced in audio amplifiers. This week we desire to discuss the impedance* capacity and resistance-capacity methods as regards the apparatus used and the difficulties encountered. The first question is the type of valve to be used, and readers of last week’s notes will remember that we referred to a valve known as the Daven highmu, which has an amplification factor of 20, compared with 6 to 8 in ordinary valves. For either impedance-capa-city or resistance capacity work it is desirable that a high factor valve be used. Other manufacturers besides the Daven Corporation are now making these high factor valves and it should not be long before they are available to amateurs in New Zealand. Of course ordinary amplifying valves will work in these circuits but the voltage amplification obtained will be low. in fact three stages will not be quite equal to two stages transformer coupled. In these circumstances the most satisfactory compromise is to use one stage of transformer coupled amplification followed by two stages either resistance i or impedance coupled. This will give ! amplification about equal to two stages transformer coupled, but with much less distortion. If the high mu valves can be obtained then three stages of Impedance-capacity or resistance-capa-city can be used, giving results very much better than two stages transfor-mer-coupled. But as mentioned last week the final valve in this case should not be a high-mu, but a low factor power valve cr even a valve of the 201 A type. The last valve must be capable of handling a heavy plate current to operate the loud-speaker. This point may not be quite clear until it is remebered that valves are potential-con-trolled devices, while a loud-speaker is current-controlled. It follows therefore that where we are passing impulses from one valve to the next we require a large potential difference, but where we pass the impulses from the last valve to a loud-speaker we require a large current difference. Accordingly our last valve should have a low plate resistance so as to be capable of passing a heavy current in the plate circuit ,in which the loud-speaker is connected. Summing up as regards valves we can therefore say that with impedence or resistance coupling the first and second valves should be high-mu. while the last valve should be preferably a power valve such as the UXII2, the Daven MU6, the Phillips 2A watt, the Millard DFAO 1 or 2, or any similar power amplifying tube. The Milliard DFA4 is a high-mu tube suitable for the first two stages. Now as regards the respective merits of the impedancecapacity as compared with the resist-ance-capacity method, it may be said that the former gives slightly higher amplification while the latter is easier to construct. The greatWlrawback with the latter method is the excessive plate voltage required owing to the drop across the resistance. This necessitates the use of a plate voltage of 120 to I 150 volts or even more if full amplifi- , cation is to be obtained. While there- i fore the resistance amplifier is easier to construct the extra expense of sup-1 plying the high plate voltage must be considered. With the impedance coupling 674 volts may be used on the first stage, 90 volts on the second stage, and 100 volts or more on the final power i stage. It may be said, however, that theoretically, and to some extent in practice, the resistance method is somewhat superior to the impedance method as regards purity of reproduction, especially if the impedance used is not properly designed. Against this again

inch or so apart and 10,000 turns ( should be put on. The wire used should , be 44 or 46 gauge single silk covered copper. We can supply more exact . data to any amateur requiring same. It is now possible to purchase in the I’nited States properly made impedances for this type of coupling, and doubtless before long they will be procurable in New Zealand. The next question is the coupling condensers ' which arc used to pass the impulses from the plate circuit of one valve to the grid circuit of the next. These condensers must have a fairly high capacity, since they ar; passing audio-fre-quency impulses, but there seems to be uo agreement among the authorities on the subject regarding the correct value. We have seen values as high as 1 m.f.d. and as low. as .002 m.f.d. recommended. It seems probable that the best value is about .05 m.f.d. with either resistance or impedance coupling. A,t any j rate the value is not highly critical so i long as it is over .01 m.f.d. It is essential however that this condenser shall be very highly insulated so that there is no leakage of the plate current of one valve to the grid of the next. Next to the intervalve condenser comes the grid leak, which is used to pass off the accumulated charges from the grid of the valve, and thus prevent the valve from paralysing. It may seem peculiar that a grid leak is necessary is this position, since most amateurs j look upon the leak as being entirely an I adjunct of the detector valve. For the ! same reason as it is required with the detector, however, it is required here, the oniy difference being is that the 1 resistance is much lower. The best I valve is usualy about .5 megohm, but a higher value may sometimes be found more satisfactory. The value is not critical as in the case of a detector leak. The foregoing points practically deal with the whole of the extra apparatus required for cither impedance or resistance coupling. The circuit used is the same in both cases and is. very simple. If any amateur has no details of the circuit we will be pleased to supply same, but most wireless books will be found to give full details. The impendance coupling method is very often referred to as “choke” coupling, the latter term merely indicating that the impendance used chokes or tends to prevent changes in the plate current, thus causing a potential to build up across the impendance. This potent iaJ is passed through the coupling condenser to the grid of the next valve, thus carrying out the function for which the circuit is designed. • • • « General Notes. “Land.” Patea, wrote a week or so ago enquiring the price of the new power.tubes. UXI2O and I’XII2. Those tubes can be obtained from Amalgamated Wireless Ltd., Wellington, and are priced at 17s 6<l and 45s 6d respectively. The •ormer is a 3 volt dry battery tube, while the latter takes the same filament voltage and current as the 201 A. We learn that a well known local amateur almost had a serious mishap with his set recently. He was listening to a concert and became so entranced with the music that he lay down on his bed with the ’phones on. the better to enjoy the performance. During one of the intervals he fell asleep, and when the music started again with a somewhat loud selection i he was rudely awakened and in his ex- ' citement very nearly pulled the set on •fo the floor. Moral, don’t go to sleep ’ with the ’phones on! j “The Grid” listened in to 2YK a i few evenings ago and was much struck Iby the improvement in clearness and ■ tone. Though the power used is some- : what low. the various selections came • through with great clearness and quite • good volume. The announcer’s voice ivery clear, and the quality of his announcing is only eclipsed by that of 2BL and 2FC. Static has been exceedingly trouble some lately and very little opportunity has been given to amateurs to make proper use of their sets. 2BL and 3L<) are now in good form, subject to the vagaries of static. We are pleased to learn that the ridiculous action of* departmental of fleers in ordering Air Norton Coutts (2AQ) to cease transmitting has coneunder the notice of the secretary of the Department, who has advised Ah Coutts that he is at liberty to use th<250 meter wave whenever he pleases. It is expected that Mr Coutts will transmit frequent I v in the future, but he has no fixed schedule. In a certain town in England the re sidents are treated to a free broadcast ing entertainment every evening. ()ne of tin- an- lamps in the street is in some mysterious manner affected by wireloss waves, ami music and speech ran be quite clearly heard proceeding from it. We wonder whether the 8.8. C. will insist on a .eeeiving license being taken out. If the secret of the phenomenon can be discovered what a great idea it would be to make all th<* lamps act in the same way. The residents could then walk home to the strains of music every evening. The names adopted by some American manufacturers for their sets pass ail understanding. A recent catalogm contains advertisements for the Dores nadyne, the Hyperodyne, the Dymac, the Super-Autodyne, the Negadyne, the lx>godyne, the Synchrophase. the Radiodyne ami many other “dynes” ami “phases.” What in thunder all the names mean Ilea . en alone knows!

may be set the fact that unless wire resistances are used it is somewhat difficult to obtain freedom from noise due to slight variations in the resistances. The ordinary carbon plate and carbon dust types cannot be considered altogether satisfactory in this respect. Wire resistances are eminently satisfactory but must be made by the amateur, as we do not think they are procurable commercially. The wire used is 47 gauge Eureka, and a resistance of 60,000 ohms necessitates the use of about an ounce and a half of wire, which costs seven shillings an ounce in England. Me do not know where the wire ran be procured in New Zealand. Ibis type of resistance ran be almost ruled out for local amateurs, although we will be pleased to supply full data to any who may wish to send awav for the wire. The next best substitute is A fixed carbon resistance of 80,000 to f OU.OOO ohms, and these are procurable ° New Zealand. We are of the Opinion, however, that taking all things •to consideration the impedance coup- } -g is probably the most satisfactory. ■*: impedance may be the secondary im ordinary audio-transformer, used • stands with the primary unconed. The secondarv of a Ford coil ® a . also be tried as an impedance, oi >t ‘ sired special impedances may be won J. To be entirely satisfactory 1 thev should be wound on closed cores of t- j same type as that used in audio 1 tran ormers. The winding should be < Z-a sections spaced a sixtenth of an I