the primary of an audio transformer, as shown in the diagram. The output impedance both protects the speaker and improves the tone.—C. R. Yarger, chief engineer, Station KFNF, Shenandoah, Iowa.
Bunched Leads for Sharp Tuning
The set builder who wants selectivity should bunch all leads possible, when wiring the set, and branch each lead off as it comes to its terminal point. This is a simple thing to do, considering the fact that all leads start from the terminal strip to different parts of the set. After bunching, they can be tied or laced with string at various points and fixed with a little collodion if desired. There is one thing that is very important when wiring a set in this manner and that is, that the leads from the grids of the r.f. tubes and detector should be as far away from the other wiring as possible. This also applies to the plate leads from the same tubes to the r.f. transformers; these leads must be kept clear of the grid leads. The sketch illustrates the idea, and of course it is understood that the aerial lead must also be kept clear of the other wiring. When you wire a set and string wires all over the subpanel it is highly probable that they will be energized and create a much longer field than if they were bunched together in the manner described. Use stranded insulated hookup wire and try out this method when wiring your set. I have found it highly successful.—M. M. Paggi, chief engineer, Station KFWO, Avalon, Catalina Island, Calif.
Superheterodyne Kink
In superheterodynes that employ the tuned-plate, tuned-grid type of oscillator, which is subject to spurious oscillations of a frequency other than the frequency to which the coils are tuned, the following simple remedy can be applied. We have found that a small choke coil inserted between the grid condenser and the junction of the grid coil and the grid tuning condenser effects the remedy. The choke coil need not be of special construction; a simple one may be made by coiling the busbar, at the point just mentioned, five or six times around a form 1 in. in diameter.—Wm. M. Perkins, engineer for Aero Products, Inc., Chicago, Ill.
Power Tubes Used As Detectors
One of the most valuable kinks that has come to my attention within the past few months has been the use of UX-120 and UX-171 tubes as detectors, with negative grid bias. The greatest trouble with detector tubes today is the fact that they are unable to handle the tremendous amount of current impressed upon them by radio-frequency amplifiers. Hence, the detectors distort and chatter, due to their inability to handle the necessary current.
In a set using 199-tubes, cut out the grid condenser and leak, and apply 22 volts to the plate of a UX-120 used as the detector tube. From 3 to 6 volts of negative grid bias are required when using this tube. The 171-type is used in the same manner in sets employing the large tubes with from 41⁄2 to 7 volts negative grid bias. Outside of the small additional filament-current drain, there is really less drain on the B-batteries. Far more volume and better quality can be obtained with tubes of this type as detectors, and the idea is easily applied to any set.—R. V. O. Swartwout, chief engineer, Station WCAO, Baltimore, Md.
¶ Loose or broken connections cause trouble both in manufactured and home-built sets; after checking tubes and batteries all wiring should be carefully examined.
