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Half the usual size and one-quarter the weight of the average fixed condenser, the midget condensers now appearing on the market are easier to wire. Besides, the builder is often at a loss to find a convenient place to mount fixed condensers in the restricted space below the subpanel. The midgets are provided with presoldered lugs, making quick soldering possible with minimum of heat. They are said to be tested at 1,500 volts, and rated as safe for continuous operation at 1,000 volts, d.c. The marked capacity is guaranteed to be within 10 per cent.

 



Many of the commercial circuits, using B-eliminators, dispense with C-batteries entirely and use the voltage drop across a resistance to provide C-bias. Such an arrangement is entirely feasible when the B-eliminator gives a sufficiently high voltage output, so that part of this voltage can be utilized for the C-bias. Care must be used, however, or motorboating will develop due to coupling in the resistance. This is common to the plates of two or more of the tubes in the set. A simple instrument for regulating C-voltage is shown in the illustration. The extra bypass condenser for the detector control of the eliminator is very important. The choke coil illustrated may not be needed on some sets. but if there is any tendency toward motorboating it will be required. This choke can be of the commercial types, or the primary of an old audio transformer can be used. The four-terminal power potentiometer shown is connected as in the diagram. permitting the use of a C-bias potential of from 4$1/2$ to 40 volts.

 



There are many sets now in use without a suitable volume control; in others, rheostats or similar devices, which are no longer in good condition, have been used for this purpose. The little device shown in the accompanying photo is said to be capable of an extremely delicate shading, from soft, whisper-like music up to powerful volume with remarkable clarity of tone. In Fig. 1 we have a circuit diagram showing the usual circuit connections before the instrument is installed. In Fig. 2 we show the instrument connected, to improve the quality and afford a means of controlling signal volume. The instrument is available in two types, one combining a filament switch and one without; the switch type is shown in the diagram. When turned as far as possible to the left, the A-battery is off. When a C-battery is desired, the circuit is broken at the point marked X, C-positive is connected to A-negative, and C-negative is connected to F-negative of the transformer secondary.

¶Popular Mechanics' radio department offers its information service free to all readers of our magazine. We will be glad to help you with your radio problems, and will promptly answer all inquiries directed to this department. 