Our Crystal Corner

Radio Record, Volume II, Issue 35, 15 March 1929, Page 27

Our Crystal Corner

By

Galena

Crystal and Catwhisker. HAT is the best crystal and whis- ' ker combination to use in a set? asks "Crystal" (Wellington): A fine grained galena crystal with any coiled catwhisker wire. The advantage of one type of wire from angf her, within certain limits, is negligible. Chus agold wire is little to be preferred to’ a copper one, but there are certain observations as in the following :- A ecatwhisker of almost any metal pressing upon a crystal will produce results of a sort, but it is useless to expect the best unless a really good and suitable one is employed. It will be found that much better and more permanent results are obtainable if gold, silver or one of the special alloys is used for making contact with the crystal. Copper and brass will give good results so long as they are clean and perfectly free from tarnish, but they are really more trouble than they are worth, since the point must be perpetually brightened. The other metals mentioned are more or less non-tarnishing. , Catwhiskers should also be light and _ springy, hence only a very slender wire should be used, and it should not be seratched across the surface of the crystal, since in this way the sensitive agents are destroyed. The object should e to lower the point straight down upon the surface. and if this point is not suitable, lift it and lower it on to a fresh point. The Crystal Valve. " G.B." (Brooklyn), who is using the ¢"Orystal and Valve," wishes to know if the crystal is dismantled from the set, should signals be heard. ANSWER: Unless they are loud, they will not detract from the efficiency of the set. Where this set is being used close to a powerful transmitter, a certain amount of rectification will take place with the most carefully constructed amplifiers of this type. Suggestions to offset this were published previously. I am using a 4-vaned variable condenser. Is this large enough, or should I place a fixed condenser in parallel with this? ANSWER: The capacity is not sufficient, and a fixed condenser of at least .0004 would have to be placed in series with it. This, however, would very much limit the tuning range. 1 cannot cut out 2YA. Is this to be expected ? . ANSWER: Yes. 2YA is rather too powerful to limit to a short range of the tuning condenser of the 1-valve sei. Would the double grid valve be suitable for this set? ANSWHR: Yes. Hither Al4l or A441. The pentode would not be suitable, and the running cost would be too high. Should there be any hand capacity when tuning? ANSWHR: No. If there is, reverse the connections to the condenser.

The Full Wave Variometer Set. " Y.Z." (Wellington) writes: "You e state in reference to the 15 turns on the stator, ‘Connect one end to the aerial and the other to the primary winding of the other variometer.’ Is it meant that there are-an extra 15 turns on both stators?" ANSWER: Yes. By primary is meant the extra 15 turns on each stator. I have tried many times to get full wave results, but have always been unsuecessful. I have not tried the two carborundums. Are they likely to be matched, for I have read in an American magazine that full wave rectification with a crystal is impossible, because of the difficulty of getting two matched crystals. ANSWER: If the correspondent has not tried carborundum crystals, there is no wonder that he has not been successful, for it is absolutely essential that permanent crystals be used, for if the catwhisker or other contact is not at the most sensitive position full wave rectifieation cannot take place. The following letter from an English magazine seems rather to be of a different tone to the American. Full Wave Rectification. [? has been stated by one correspondent that full-wave rectification is impossible because of the impracticability of perfectly matching two erystals, but the following letter from "Popular Wireless": is a contradiction of this statement: "Dear Sir,- It is an amusing fact that, almost invariably, when a reader writes to ‘P.W.’ on the above subject, he preambles with information to the effect that though the owner of a multi-valve set, he occasionally ‘plays about,’ or ‘messes with’ a erystal, to entertain the kids, or kittens, or because his grandmother is afraid of an explosion in a high tension battery. May I, therefore, on behalf of less ashamed, or apologetic crystal users, herald the fact that I abandoned the valve for the ‘humble crystal’ six years ago, and that ‘since then’ I have never regretted it? I must add that this amazing act is greatly due to ‘P.W.’ which, at the time, published a method of using two erystals as rectifiers. Unfortunately, this method was technically damned in the following issue by a destructive critic. Six years’ test has, however, more than vindicated the ‘P.W.’ invention. The circuit in which the twin crystals are employed has never been published, and never will-because it is too good to appear true.-Grand Knight of the Whisker."