Radio broadcast .. (1922-30)

A Two-Stage Amplifier for Your Receiver By CARL GOUDY Assistant Instructor of Mechanical Engineering, Pratt Institute A home-made two-stage amplifier of sound electrical design and strong mechanical construction, which may be hooked up with any tube or crystal set, is well worth looking into. It helps those who have been receiving principally local stations, sufficiently loud really to enjoy them, bring in the distant ones louder; it enables them to use a loud speaker on signals that would operate only the phones without it, so that a roomful of people, can enjoy the programs. We put our enthusiastic "OK" on the amplifier described below. We have been trying one of these amplifiers made in exact accordance with these instructions. It has plenty of "kick" both with dry-cell and storage-battery tubes, and brings in signals unusually clear. Mr. Goudy has followed out our suggestions in designing and describing this apparatus in a manner that should make the building of it a comparatively simple matter. It is designed for use with any type of amplifying tube, and room enough has been allowed for the use of any transformer or combination of transformers and other parts now on the market. — THE EDITOR IN OFFERING this amplifier to the radio amateur and broadcast listeners, the writer has endeavored to design an instrument of marked simplicity in detail, and one that may readily be constructed with a minimum number of tools. This amplifier has the following outstanding features: 1 . Dead jacks when not in use. I n most amplifiers using jacks for the different stages of amplification, the nipples extending through the panel are alive whether the set is in operation or not. An inspection of the accompanying wiring diagrams will show that the frames of the first and second jacks are entirely out of the circuit, this being accomplished by the selection of the correct jack for the particular part of the circuit. If a high B battery voltage is carried by the usual jack, the radio fan using live jacks is exposed to a severe jolt when inserting the phone plug. 2. The entire frame and transformer cores are at ground potential, with transformers well spaced, reducing the chance of howling to a minimum. 3. Grid leads are extremely short and well separated from the plate circuit. 4. Transformer leads are mechanically secured by means of the binding post provided, but also soldered direct to the coil terminal, thus assuring absolute contact. 5. A variable C battery is used to provide a suitable grid bias for the plate voltage used and is incorporated within the set and placed so that advantage may be taken of the differ ent steps of voltage. When using high B battery voltages, say above 60, a C battery is necessary. It will be found to improve the quality of amplification quite naturally. 6. Input and output jacks and binding posts are used, making a truly universal amplifier that may be quickly connected or "plugged" to most any type of receiver. This is important, since the receivers change but the amplifier still remains the same. GOOD PARTS ARE ESSENTIAL TO HAVE a well-designed receiver or amplifier function properly after building it from a set of construction drawings, the instruction and drawings should be followed closely, good material and dependable equipment should be employed, and each item of apparatus should be tested before it is placed in the set and hooked up. It is very much like assembling an automobile engine or a watch — I have drawn a wide comparison, but fundamentally the idea is the same. Take in the first case: if the carburetor is carefully looked into, you are sure that the gas passage is clear and that it is ready to function as a unit; the magneto is given the same inspection, timing is checked — and, in so many words, every detail is examined. The result is that the motor will run; of course, it will have to be tuned or adjusted, but you have variables in the radio set that may be adjusted as well. You may try a leak here and a condenser there and find that it is worth while.