We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
Screen-Grid R.F. and Detector Circuits
DESIGN OF THE COLONIAL MODEL 32A.C.
BY DR. FULTON CUTTING
President, Colonial Radio Corporation
Dr. Fulton Cutting
In the laboratories of the Colonial Radio Corporation, experiments in the use of screen-grid tubes have been in progress for the past two years, having been started when the d.c. screen-grid tube first became available. The experiments with d.c. screen-grid tubes were found helpful in many ways in further work with the a.c. screen-grid tube. The essential results of this laboratory work have been incorporated into the latest Colonial receivers. In this article we will describe in detail some of the interesting features to be found in the Model 32 A.C. receiver.
The schematic circuit diagram of this receiver is given in Fig. 1. As the diagram indicates, the set consists essentially of a three-stage r.f. amplifier, a plate-circuit detector, and a two-stage transformercoupled audio amplifier. Screen-grid tubes
are used in the three r.f. stages and in the detector circuit. The first audio tube is a 227 and the second audio stage contains two type 245 tubes in push pull.
There are only two essential controls: the station selector and the volume control. The selector control drives the shafts of four variable condensers through the medium of a vernier gear and a series of phosphor bronze belts. A pointer, traveling across a one hundred division scale on the panel, indicates the setting of the selector.
If all four stages were accurately tuned to a single frequency, side-band cutting would result to a degree which would depend upon the shape of the top of the resonance curve for the entire tuner. However, if each stage were tuned to a different frequency, the four resultant peaks being spread out over a 5000-cycle band, much less side-band cutting would result. By properly adjusting these circuits, a resonance curve for the entire tuner could be obtained which would have a nearly flat top, 5000 cycles wide. In the Model 32 receiver, this principle has been followed, and successive stages are tuned
to carrier frequency, 2000, 3000, and 4000cycle side bands, respectively. The graph of Fig. 2 emphasizes the advantages of this system by contrasting the shapes of the resonance curve for the entire tuner before and after the "staggering" of the tuning. Mistuning in this manner of course decreases the gain (in this particular case the overall gain with all circuits in resonance was twice that indicated in Fig. 5).
System of Coupling Audio Stages
An ingenious coupling scheme is employed between the detector and first audio amplifier. This consists of a resistance and capacitance network designed to compensate automatically the deficiencies of the second stage coupling transformer, so that the amplification between the detector and the output of the second audio stage will remain constant on all frequencies in the lower half of the audio scale.
The mathematics of this arrangement are very interesting. In Fig. 3 are shown the
Fig I — The complete schematic diagram of the Colonial Model 32 A.C. receiver.
3 58
OCTOBER 1929 •