The Experimenter

Radio Record, Volume IV, Issue 23, 19 December 1930, Page 15

The Experimenter

In response to several requests, we are opening a section devoted to the interests of the experimenting amateur. Tested ideas received from enthusiasts will be published and acknowledged. Experiences of listeners relating to points raised in our Questions and Answers column will be welcomed. We, however, cannot undertake responsibility for any of the ideas published.

Protecting Valves. I HAVE two suggestions for constructors which might be useful :- (1) In sereen-grid sets using the tuned anode method of H.F. coupling there is considerable danger of blowing the valves by shorting the H.F. H.T, leads to the metal screen. Consider the circuit in the 1930 "Guide." The danger lies in the fact that the constructor thinks himself safe to fiddlé round when the switch S is off, as all the batteries are connected thereto. But this is not the case as, when the switch is off and, say, the tuning condenser marked 2 shorted across, the H.T. current can still find a path to

B- through the filaments of the valves and through the A battery. I have found it an improvement to eonnect B- permanently to the-screen -the switch S to operate on the A battery only. Under these circumstances, if the H.T. leads should come in contact with the screen the current wiF find a direct path back to B-, and by virtue of the higher resistance offered by the valve filaments will leave the valves practically unhurt. © I have found that this is the case in my set and have actually shorted the leads concerned without any apparent damage to the Valves. (2) The following method of making coils I have not seen mentioned in your paper yet, arid I have found it to be easy and satisfactory. A permanent former is dispensed with, yet the coil, when finished, is perfectly rigid, and losses are reduced to a minimum. A cylindrical object such as a cocoa tin whose diameter is considerably smaller than that required for the finished coil must be procured, The required diameter is made up of sever-

al thicknesses of thick soft paper (I found blotting-paper to be the most suitable). The paper may be secured by means of twine. The wire (cotton or silk covered) may be anchored for a start by means of a small pin dug into the paper, or it may be held under the twine. The winding is then proceeded with until the required number of turns is put on, the end being anchored. The wire is given a good coating of celluloid cement, care being taken to see that it is evenly spread and worked well into the cotton or silk covering. When this is dry the twine is removed and the cocoa tin extracted. The

paper is then taken out very carefully by rolling it between the two first fingers. Some of the paper is bound to stick to the inside of the eoil, and this should be gently scraped off with a pocket-knife, A coating of celluloid cement should then be given to the coil and the whole set aside to ary. The result is a sturdy job which can be dropped on the floor without fear of damage. The coil may be mounted

by means of a strip of thin three-ply or thick celluloid and the free ends. of the wire passed through holes im. same.-"Union" (Kilbirnie). ites ms Electrolytic Condensers [N the United States the electrolytic" condenser appears to have made. a-good deal of headway and several different types of this’ condenser are on the market and regularly advertised. Apart from the outstanding advantage of high capacity and small. volume, it is claimed that the condensers are self-healing, sothatin the event of a breakdown the condenser resumes its normal condition when the overload voltage is. removed. They are incorporated in many sets now used in New Zealand. ‘ For Small Soldering Jobs IN wireless constructional and repair work one often wants a soldering iron with a small bit for attending to small, awkward jobs. Such irons are difficult to work with, however, for owing to the small volume of copper they cool so rapidly. A very handy tool may be made from an old worn-out soldering iron. The point is cut off with a hacksaw and the end trimmed up more or less square. In the end of the old bit is drilled a hole, and a piece of }-inch copper rod, about 134 inches in length, is forced in. If necessary, it may be made perfectly secure by making deep indentations with a centre punch round its junction with the bit.-H.C.B. ‘{Lyall Bay). A Tone Control OME types of loudspeaker have = tendency to over-accentuate notes of the upper register. A tone control utilising a .01 mfd. fixed condenser and a 500,000 ohm resistance may be used to obtain better reproduction. The condenser and resistance are connected in series across the speaker terminals.