Radio Broadcast (Nov 1926-Apr 1927)

A High-Quality Amplifier for the R. B, uhsihn Receiver of an iner in IN THE November, 1926, Radio Broadcast, the author described the construction of a complete four-tube receiver embodying the fine points of the R. B. "Lab" circuit which had been developed by several engineers, and on which a very considerable amount of work had been done in the Laboratory of Radio Broadcast. Soon after, a receiver was built which was subsequently described in the January, 1927, Radio Broadcast, consisting merely of the two-tube tuner part of the original four-tube receiver. This construction was presented to our readers so that they could tie up this efficient receiver circuit with any unit audio amplifier, such as the Alden Truphonic, the National combined impedance-resistance amplifier and plate supply, the Millen audio channel, and such resistance amplifiers as the Heath, Amsco, Allen Bradley, and others. Strangely enough, practically every type of audio amplification is represented in this list excepting our old stand-by, transformer-coupled audio amplification, and it is to fill this gap that the paper herewith describes in detail the construction of a transformer-coupled audio amplifier of high merit. The "Lab" circuit in any form deserves nothing but the best of audio amplifiers. It is a circuit that will satisfy the most critical of builders on points of selectivity and sensitivity; to successfully amplify the output of the detector tube, an amplifier of proved value should be employed. In the four-tube receiver described in November, 1926, an audio channel was used which gave complete satisfaction where tone quality and volume were concerned, and it is for this reason that the use of the elements employed therein are continued in the construction of the amplifier described here. When using high-grade transformers, such as the Amertran De Luxe, it is possible that any slight defect elsewhere in the circuit will be noticeably amplified and cause distortion. This is especially true where an a. c. operated B powersupply unit is employed with an amplifier employing these transformers. Since such power supply units have a shunt tapped resistance across the output to obtain the various plate B voltages, there is a coupling effect produced, due to the resistance which is common to all plate circuits. That is to say, the common resistance couples together the various plate circuits and causes regeneration at audio frequencies which in some cases is detri Technical Editor mental to tone quality. To overcome this disadvantage, it is necessary to employ bypass condensers and r. f. and a. f. choke coils so placed that no coupling takes place. This .has been done in the amplifier described here. Radio Broadcast Laboratory would be greatly interested in receiving reports from readers who have constructed the R. B. "Lab" circuit in either the two or four-tube style and who have experienced trouble in the use of B power-supply devices with their receivers. Other notes of interest, where they concern construction details, operating notes, or results, are of especial concern to the Laboratory staff, and we welcome such reports addressed to the magazine. Photographs of interesting models built by readers are also quite welcome, for by reproducing these, it is possible to give wide circulation to valuable kinks of construction. The purpose of this article is primarily to describe the construction of a two-stage audiofrequency amplifier which may be combined with the two-tube shielded R. B. "Lab" Receiver described by the author in the January Radio Broadcast. In addition, some notes on tubes Radio Broadcast Photograph THE ARRANGEMENT THE APPARATUS In the base layout is shown the placement of the various parts. The wiring, it may be noted, is not entirely visible, for at the terminal of each piece of apparatus, a small hole is drilled in the base-board to allow the wiring to be passed through to the under side. Such an arrangement makes for neatness of appearance satisfactory for use with either the two or fourtube R. B. "Lab" receiver are also included. Aside from its especial use with this tuner circuit, the two-stage amplifier may well be worked with any other tuner unit the builder may have on hand. Such an amplifier, for instance, could very nicely be used in conjunction with a shortwave tuner. For this latter form of work it is not absolutely essential that two audio stages be employed as most of the reception is done with the aid of headphones, and two stages would produce too loud a signal for comfort. For this reason, together with others, an interstage jack has been included behind the first audio stage so as to enable the operator to plug in after the first audio amplifier. Since the output jack is of the filament-control type, the last audio tube does not remain lighted when the phone plug is inserted in the interstage jack, and, therefore, the half ampere of filament current which the semi-power tube consumes is saved. On the point of stability, the amplifier shown here incorporates all the worth while features of bypassing and audio frequency filtering that seem to be of definite value. This is especially true when it is considered that, since the amplifier is built as a unit and will quite likely be stowed away in some remote place, necessitating the use of long battery and other connecting leads, there are bound to be some deleterious coupling effects produced, unless such bypassing is resorted to. In brief observation of the functioning of the bypass condensers and audio chokes, etc., let us analyze the circuit diagram of an amplifier shown in Fig. 1. Here no bypass condensers, audio chokes, or grid filters are shown. It will be seen that audio currents in either the grid or plate circuits of the tubes, to return to the common connecting point (the minus-A lead), must first course through the battery leads which might be several yards long. In the plate circuits, the return is made to a series of B batteries which are common to all the plate circuits. Any resistance in the B batteries will tend to couple the various plate circuits together thereby setting up an audible oscillation, or tendency to produce regeneration. In cases where a. c. operated B power-supply devices are employed, the situation becomes more complicated because usually the various B voltages are obtained from the a. c. device by taking a drop through a resistance common to all the B plate circuits, as stated previously. Here we have a definite, known resistance to