Wireless Broadcast

Star (Christchurch), Issue 17756, 28 January 1926, Page 3

Wireless Broadcast

CONTRIBUTED BY

“ AERIAL."

REACTIVATING VACUUM TUBES Electron tubes in radio receiving sets eventually lose their sensitivity. This sometimes progresses to the point where the receiving set operates very poorly or not at all, even though the tube filament is not burned out. The user of the set frequently confuses this condition with that due to an exhausted B battery. If the tubes are of the thoriated tungsten (X—b) filament type they can usually be rejuvenated b}- a simple process and made to serve as well as new tubes in the receiving set. It happens that most of the tubes now used are of the thoriated tungsten type, and it therefore becomes of quite general interest for the public to know how to secure the full life from thenelectron tubes. The WDII and WDI2 type of tubes are the only ones extensively used which cannot be reactivated. In the tubes the source of electrons is a coating of certain oxides on the surface of the filaments, and when this has been used up no process can renew it. The thoriated tungsten filaments, however, used in most of the various other types of tubes contain the oxide of thorium throughout the whole mass of the tungsten filament, this oxide having been originally put in incandescent lamps to keep the filaments from becoming too fragile. The filaments are given a treatment which' produces a layer of atoms of thorium on the surface of the. tungsten, and this thorium, which is radioactive, emits electrons much more copiously than the tungsten wotild. After, long use or after burning the filaments too brightly the layer of thorium atoms is evaporated off, and so few electrons are then emitted that the tube docs not function properly. Reactivation is a process which boils additional thorium atoms out of the interior and the tungsten filament and forms a new layer of new thorium atojjis on the surface.

The thoriated filament was developed by the General Electric Company, which has also developed the methods of reactivating tubes of this type. The Bureau of Standards has found that the reactivation process is quite successful, and frequently makes a wonderful difference in the results obtained in the receiving set. The process is essentially the operation of the filament for a very brief interval at a specified high voltage (called “flashing”), followed by a lower voltage for a longer time (called “ageing”), all of this with no grid or plate voltage. The flashing reduces some of the thorium oxide in the filament to thorium, and the ageing forms the required surface layer. The following schedule of these operations is the result of extensive experience of the Radio Corporation of America, and is published here by courtesy of that company:— FLASHING.

Exactly the same procedures apply for C and CX tubes as for the UV and UX tubes of the corresponding number; thus. C and CX—299 correspond to UV and UX 199; C and CX—--301 A to UV and UX—2Ol—A, and CX 220 to UX—l2O. (It may be noted that some of the English tubes have similar filament characteristics: for instance. the Mullard D. 06 is similar to the UV —199. and the DFAI corresponds to the UV—2Ol—A and C—3ol —A. These English tubes have thoriated filaments also, and require the same voltage and current as the corresponding American tubes, but the writer has at present no authoritative data concerning the rejuvenating of English tubes, but the experiment is worth a trial, if a “sleepy” tube is handy.) In carrying out this schedule it is absolutely necessary to have a volt meter of a good degree of accuracy, and to use a X x X and to use a watch. (The average pocket voltmeter is of no value what soever.) No grid or plate voltages are used, and either direct or alternating current may be used for heating the filaments. The apparatus necessary for carrying out the process is simple. The filament is connected to the necessary

source of voltage, nothing being connected to the. grid or plate. A voltmeter is collected across the filament terminals. If alternating current is available the source of voltage can be a small transformer such as those used tor running door bells or electric toys. The voltage tap nearest the voltage specified should be selected and a rheostat in series, with the filament used to adjust to the exact voltage. The voltmeter in this case must be one for

alternating current. If A.C. is not available, then dry cells or a storage battery must be used. A single dry cell when new will furnish approximateh r 1.5 volts. A rheostat and D.C. voltmeter should be used to adjust to the exact voltage.—“ Technical News Bulletin” of the U.S. Bureau of Standards, October, 1925.

It may seem, on the surface, that putting such a high voltage on the filament of a receiving tube is a foolish proceeding, and one calculated to burn it out instantly. Don’t do it to a bright emitter like the UV—-200 or Phillips Dl. It is meant only for X—b filaments.

Since, however, this reactivation process was developed by the General Electric Company', and is used by the R.C.A., and recommended by the Bureau of Standards, it may be accepted confidently, and applied safely. No person need ever doubt the word of such an authority as the U.S. Bureau of Standards, but be very sure to follow the instructions carefully. JOTTINGS. The writer has it on reliable authority that some new equipment is landing this week for Station VLDN, and that it will be installed at the earliest possible moment. The apparatus is some of the very latest from the manufactory of one of the world's best producers. so some considerable improvement may be expected from VLDN in the near future. X X X A new speech amplifier has been installed at SAC during the last week, and the results are very pleasing. The local station now has plenty of “kick,” and any good receiver should bring in 3AC with ample volume. The amplifier is similar to the speech amplifier made by the Western Electric Company, and supplied to broadcast stations, and in fact uses the same circuit, differing in onlv a few minor details. x Station IYA will soon be closed. The new Auckland station is in the course of erection, and the work is being push-, ed along as expeditiously as possible. Bv the time VLAK takes the air it is hoped to have the Christchurch station on the way to completion. X X X

The little receiver with interchangeable coils, described in these columns a week or two ago, still gives satisfaction. Not only does it secure 2BL and 3LO every night under fair conditions, but also brings in some of the lowerpowered Aussies nicely, among them being 4QG, 2KY and SCL. With a vertical aerial only twenty-three feet in length and the top of it some twen-ty-eight feet above ground, 2BL and 3LO have on a few occasions been heard at loud-speaker strength, using two stages of audio amplification. X X *

VLDN put out a good entertainment on Monday evening, when “J 1 Trovatore" was broadcasted from the Festival Hall at the Dunedin Exhibition. The various items, particularly scenes 2 and 3. came through very clearly, but were somewhat marred by the fairly heavy static prevailing, and by several howling valves. It is high time the authorities took these “bloopers” in hand seriously and dealt with them as they deserve.

Filament Type of Tube. Voltage. Time. UX' and UV 199 10 30sec UX and UV 201—A 15 Imin UX — 120 10 lmin AGEING. UX and UV—199 4.5 10min UX and UV 201—A 7.5 10m in UX 120 4.5 lOmin