The... "Cathode" Super

Radio Record, Volume V, Issue 28, 22 January 1932, Page 12

The... "Cathode" Super

FL powerful six-valve all-wave a.c. superheterodyne, designed (\

and described

by

CATHODE

AN commencing the description (i of this highly developed design, it is perhaps advisable to be at some pains to impress upon the reader that, unlike the simpler short-wave superhet., recently described, this receiver is not a fit subject for casual experimenting by #he amateur set-builder. A great deal of time and experiment has heen gpent in working out the circuit conStants and the details of the design. and if it is desired to equal the splendid performance of which the set is eapable in its original form. the builder must be prepared to follow the instructions given in their enfirety. Such ‘modifications as are

considered permissible will be. specitically mentioned. Readers may be interested to know how the "Cathode Super’ came into being: It was this way. "Cathode" in his vocation of radio engineer was consulted by a client who wished to obtain the absolute maximum of usefnl result in both broadcast and shortwave reception. The question of cost did not enter into the matter. When the commercial receivers available came to be reviewed, however, it was found that there was not. a single receiver commercially pracurable which would give the required results. The question was then put as to Whether "Cathode" could design and

construct a suitable receiver, and the great work was undertaken. The usual minor troubles were encountered. of course, but in its essentials the "paper" design proved absolutely "right" from its commencement. In the outcome the results on both long and short waves were uniformly excellent, and the client was no less surprised and delighted than "Cathode" himself, It should be mentioned here that certain features of the receiver, notably the oscillator circuit and its coupling to the first detector, are the subject of patent applications which will doubtless be granted in due course, The designer has no objection whatever to these features being made use of by amateurs building receivers for the use of themselves and their families, the patent applications being made merely

to prevent commercial exploitation of the particular principles involyed, No fears need be entertained, therefore, regarding contravention of these parricular patents, The big receiver has been christened the "Cathode Super." partly because it employs six "indirectly-heated" cathede valves, but principally because its designer feels that it represents a fitting climax to a successful series of set " designs. "Cathode" takes a certain pride in having his now familiar nom-de-plume associated with so outstanding a design. The Cireuit. TPURNING to the circuit diagram of Fig. 1, it will be seen that the re ceiver incorporates all those features which have been found desirable in modern design. It functions on the

Ft _ superheterodyne principle. of course. and the reader who has read the earlier articles in this series will have no difficulty in following the operation of the set. The first valve amplities at radio frequency in order to provide a certain amount of selectivity ahead of the frequency changer. Without some provision of this kind, trouble might be encountered with powerful local stations in the vicinity, as superheterodynes are rather given to

receiving stations at more than one point on the tuning dials. The radio-frequency amplifier ts transformer-coupled to the first detector. A sereen grid valve is employ=d in this position, and for a very good reason. It has been shown that the efficiency of a valye operating as a eumujative grid first detector in a super is but a small fraction of that .which it shows.in its more normal sphere of re2tifying r.f. modulated at audio fraquencies. On the other hand, a three electrode valve operating as a plateband detector is notoriously insensitive to small inputs. The screen-grid valve, however, may be employed as a plateband rectifier, and in this service will show an efficiency which compares fayourably with the efficiency factor of 1 grid leak detector in normal service, The oscillator valve functions as u modified Colpitts oscillator, the regen: ration being supplied by the coupling between the two condensers which, in series, tune the oscillator coil, It has been found that this type of oscillator functions smoothly and reliably on short as well as broadcast waves, whil» being noticeably free from "dead spots" which might necessitate having an wnecessary amount of feed-back at some points to ensure its being adequate at others. The oscillator is a type 227 valre, and is coupled to the first detector yin the biasing resister of the latter, Although the operation may be a littie difficult to grasp at first sight, this coupling is effective in varying the gr d potential of the «detector in sympathy with the applied oscillations, yet wifhout introducing undesirable coupling coils into the grid cireuit directly. in case any conscientious constructor should chance to observe that the biasing resistor of the first deteetor is the

only one in the set without a by-pass condenser and hasten to rectify the omission, it should be mentioned here that the insertion of a condenser at this point will render the receiver completely inoperative. The output of the first detector feeds into the intermediate amplifier. which is tuned throughout with band pass couplings. That is to say, there are in all six coils and six semi-vari-able condensers to tune thei. The actual coupling is efiected by the mutual inductance existing between primary and secondary of the hand pass units. ‘There is not space here to go into the merits of the band-pass system of tuning, and we must content ourselves with saying that. in addition to the enhanced — selectivity whieh it lends, it seems to be the most practical method of realising the "flat top" resonance curve which is gener: ally considered desirable, There are two screen-grid intermediates, the second feeding into a plate bend or "power detector, for which i 227 type valve is employed. I'rom

this point, the constructor may be permitied some latitude in making «a choice of an audio amplifier. . The description refers to a straightforward transformer coupling to a UY 24T pentode, but to suit individual requirements this could be altered to provide push-mill coupling to a pair of power (Concluded on page 24.)

"Cathode" Super

(Continued -from page 13.3 yalves, or possibly to a first and second audio stage. The pentode seems to suit the average dynamic speaker to perfection, however. The Power Supply. JIOWER supply is drawn from altern-ating-current mains through the medium of a heavy duty power trans~ former haying two 2.5 volt filament windings for the pentode and the preceding valves, a 5 volt filament winding for a UX 280 rectifier, and 2 750 volt centre-tapped winding for the plates of the same tube. The arrangement of the filter circuit is clearly | shown in the theoretical diagram. 34° is a trifle unusual in that the first: 4mfd. smoothing condenser is split’ into two sections by a 1500 ohm resistance. This was done more to protect the condensers from seyere voltage rises due to the inductance of the first smoothing choke than for any other purpose, but it seems also to be of material assistance in reducing the slight bum which is usually encountered in a big mains-operated receiver and which is apt to be annoying in the reception of short-wave stations. The bleeder resistance which ensures sufficient eurrent being drawn through the dynamic speaker field to excite properly it must be of a heavy duty type; for that matter, so must the 1500-ohm resistance in the first section of the filter. Next week a full range of parts required to build the "Cathode" Super will be given, together with full details of the coils and b.p. filters.