Radio age (Jan 1927-Jan 1928)

RADIO AGE for October, 1927 and P. leads of the coupler are connected to the loop binding posts or if an antenna will be used regularly the loop binding posts may be used for the antenna and ground and the coupler connections made in the receiver permanently. Oscillator Condensers Care should be taken to connect the rotor plates of the oscillator condenser to the plate terminal of the tube, and the stator plates to the grid. This will eliminate body capacity. C Battery The use of a C battery has been very liberal, a C bias being used on the R. F. tubes in preference to the usual potentiometer. This method is slightly more efficient and due to the fact that it decreases the B battery consumption there is practically no danger of the set motor boating on a B eliminator. Intermediate Stages One point that will probably strike the fan as being contrary to the general trend is the use of two iron core transformers and one filter. It was mentioned earlier that the sensitivity of a super depended primarily upon the efficiency of each R. F. stage. By using two long wave iron core transformers with a very high R. F. gain the efficiency or amplification is brought up very high. In order to obtain the desired selectivity with this system the filter transformer carefully designed by precision methods. Its amplification cuts off very sharp on either side of a ten kilocycle band. Beginning the Construction The foregoing points have been covered in order that the builder may have a more general idea of the points to watch for in the construction of the receiver. The list of parts should now be procured and then carefully examined to see that all parts are in good order. Construction can now be begun. The front panel should be started first. The condensers should be mounted then the rheostat, filament switch and regenerative condenser. If both condensers are the same it will be necessary to reverse the shaft in one of the Camfield condensers first This can be done by removing the cover nut at the back of the condenser, loosening up the set screw that holds the shaft and then pushing the shaft through to the other side. Tighten up the set screw again and replace the nut on the reverse side. After the front panel has been assembled lay it aside and proceed with the assembly of the baseboard. By following the layout diagram this operation will be very simple and should not take more than an hour. The Ellis oscillator coupler should be left until the last so that it will not be damaged. When the parts are all mounted on the base board wiring can be started. Wiring the Receiver It is advisable to wire the baseboard first, without the front panel attached. The wiring can be done in progressive stages beginning with the first detector. The leads for the front panel must be left quite long. When the subpaned is fully wired mount the front panel and connect the Cloyd Clevenger, world war ace and now pilot of the Alexander Airport, Denver, who gives aviation lessons to radio listeners leads to the controls on the front panel, and the receiver is then ready to test. Tubes U. X. 201 A tubes or their equivalent should be used throughout the entire receiver with the exception of the last stage where a 171 power tube is recommended. If the wiring has been carefully done, there is no reason why the set should not operate perfectly from the very start. It may be necessary to vary slightly the "C" battery voltage but it will be found in most cases, if the tubes used are O. K., that the three volt "C" bias for the radio frequency tubes as shown on the diagram will be correct. Should a "B" battery eliminator be used, one of sufficient output should be obtained, capable of delivering at least one hundred and eighty volts at about forty mills, since this is what the Thompson Super Seven will draw when a 171 power tube is used on the last stage. The fact that almost every tube in the receiver is biased with a "C" battery reduces the battery current consumption considerably and practically prevents any danger of the set motor boating with a "B" eliminator. Should this difficulty arise, as it might in the case of some eliminators with a high internal resistance, it can be eliminated by by-passing the output of the eliminator with from two to ten Mfd. bypass condensers. An eliminator that was found to work exceptionally well on the new Thompson Super Seven was the Silver Marshall Reservoir B. It delivers ample current and is sflent in the operation. A distinct advance forward has been made in the Silver Marshall Eliminator by the use of a glow tube to maintain the output voltage constant. The glow tube eliminates entirely the necessity for any variable adjustments, as it automatically delivers the correct voltage regardless of the current drawn by the Receiver. Accessories A six volt storage battery will be needed to light the filaments of the tubes. If a Trickle charger is to be used, a battery of eighty amperes capacity will be sufficient, otherwise, it would be advisable to get a little larger storage battery. The cabinet in which the set is pictured is the well known "Fritts." It needs no introduction to set builders; it is known to all for its distinguished appearance.