Laboratory Jottings

Radio Record, Volume II, Issue 45, 24 May 1929, Page 34

Laboratory Jottings

A.C. Adapter Harness, the Dongan Filament Transformer and Radiotron A.C. Valves

[THIS transformer and harness as submitted to us by Abel Smeeton, Ltd., has been thoroughly tested, and the following report is now presented. This accessory consists of a set of special valve adaptors suitably connected so that they may be attached to a filament transformer, and into. the sockets of the battery set. This is all ‘that is necessary to convert such a set into an A.C, filament. The transformer used (the Dongan) is a particularly neat arrangement fitted with three tappings for voltages of 1.5, 2.5, and 5 volts. These tappings were correct according to an accurate metre. The transformer is provided with a special socket, so that the battery eliminator may be plugged into this to obviate the necessity of another connection to the electric light mains. The harness supplied was suitable for a five-valve set. To use this all that was necessary was to connect the terminals to the transformer and plug the adaptors into the valve sockets and fit in the A.C. valves. Bias for the 227 valve is automatically applied by an extra connection to that battery or eliminator tapping.

By the courtesy of Amalgamated Wireless, Ltd., we were able to test out the following valve with this ap-paratus:-R.F. two 226’s; detector, 227; first audio, 226; power vaive, 171, The result was altogether satisfactory. The change to A.O, was made in a very few minutes, and the signals came in clear, loud, and pure without any hum-in fact, there was no telling when D.C. and A.C. was being used. The following are a few salient points in favour of the apparatus :-(1) Oorrect voltage is applied continuously to the filaments. (2) The wiring of the set does not have to be molested, the other _ bat-

tery connections remaining as before. (8) The use of a power valve is encouraged. (4) Running expenses as compared with the "A" battery are negligible. Radiotron A.C. Valves HE appeal of the A.C. receiver is growing, and we have made it a point of securing some of the A.C. valves and testing them with a view to further informing our readers on their possibilities. Most constructors avoid trying out A.C. sets though, no doubt, before this season has elapsed, many of these amateurs will have tried their hand at A.C. We hope before long to publish the full working details of the A.C. Browning-Drake. The valves submitted to us by Amalgamated Wireless, Ltd., were UX226, UY227, and UX-171A. These were used with the Dongan unit and E.B.H. harness adaptor, reviewed in another column, The 226 is a valve which resembles very much the ordinary receiving valve. The filament voltage is 1.5, requiring a current of 1.05 amperes. The amplification factor is 8.2, and the anode voltage 135 volts.

The 227 is slightly different from the | ordinary valve, in that the filament is heated indirectly. Alternating current | is applied to two terminals of the > valve, but these two are not to the © filament. This ir an independant electrode, the emission of which is a result — of the heat transferred from the heater. For this reason the valve is known as a "heater" type. This requires 2.5 volts on the heater and 1.75 amperes. The amplification factor is 8, and the plate voltage 45. This valve is most suitable as a detector, but is quite frequently used in the radio frequency stages. Through this arrangement of indirect heating, hum is kept to a minimum, On our test, there was no hum.

It is not very commonly known that alternating current can, without further adjustment, be applied to the filament of the 171A. Most power transformers have a winding that will deliver 5 volts. This can be applied directly to the filament of this valve without any hum disturbing reception. For some weeks, the writer has been using this arrangement with the other valves heated by direct current. The results have been excellent. Power transformers are now coming on to the market and there is no reason why any listener should not take the filament current fo his power valve from the filament winding. Power valves usually require a high amperage so that it is a distinct saving to take it direct from the mains.