Bicycle Radio is Easy and Cheap to Build (Apr, 1940)
Bicycle Radio is Easy and Cheap to Build
By ARTHUR C. MILLER
FANS who would like to install a radio on their bicycles so they can enjoy their favorite programs while riding around town or on short trips will find the inexpensive set described on these pages just what they have been looking for. Fitting in a basket mounted on the handlebars, the battery-operated, four-tube receiver contains its own loudspeaker. It gives excellent results on local broadcast stations, and if iron-core coils instead of the air-space type specified are used this range will be increased.
Owing to the directional properties of a loop antenna, a 4′ metal rod was chosen instead. The metal rod is connected directly to the grid cap of the radio-frequency tube. Both of the set’s coils are tuned by a midget two-gang tuning condenser, which is mounted on the sloping panel by means of two right-angle brackets.
The antenna rod is insulated from the metal cabinet by a ceramic standoff insulator. The tops of these insulators are usually threaded, and the best method of attaching the aluminum rod is to thread it to fit, and screw it into the insulator. For greater signal strength, the set will have to be grounded. The bicycle frame provides excellent counterpoise capacity for this purpose.
The steel cabinet used for housing the chassis and batteries measures 6-1/2″ by 7″ by 11″ and is small enough to fit inside a standard-size bicycle luggage basket. The panel is attached to the cabinet by means of self-tapping screws. The two “B” batteries that fit inside the cabinet along the back are the new small-size portable type employing the special flat cells with expanding seals. A 1-1/2 -volt “A” battery fits in between the two “B” batteries, with the 4-1/2-volt “C” battery directly in front of it. The “A” and “B” batteries are of the plug-in type and use clip-in plugs with Fahnestock terminals. This system makes it an easy matter to change batteries whenever necessary.
In order to obtain ample volume from the speaker, two stages of audiofrequency amplification are used after the detector. Both stages are resistance-coupled and use the latest-type tubes for maximum sensitivity. Type 1N5GT tubes are used in the radio-frequency, detector, and first audio-frequency stages. A beam power tube, 1T5GT, is used in the output stage, and provides a relatively high output with a very low filament drain.
A 15,000-ohm, 1/2-watt resistor is used in the grid circuit of the first audio tube. Any increase in value of this resistor will only cause instability and will fail to in- crease the amplification. Volume is controlled by a 100,000-ohm potentiometer placed in the screen circuit of the radio-frequency tube.
The key-lock switch is of the double-pole, single-throw type and breaks two circuits at one time, the positive “A” supply and the ground lead of the volume control. This is done to avoid any drain through the “B” supply while the set is turned off.
As a safety measure, it is a good idea to lock the set in the basket or to the bicycle frame. The key switch prevents anyone else from turning on the set.