The "CATHODE" Super

Radio Record, Volume V, Issue 30, 5 February 1932, Page 12

The "CATHODE" Super

(Part IID

By

CATHODE

HERE appears to be some doubt prevalent regarding the correct way to connect up a pentode of the five-pin type. If, however, the reader will remember that grid and plate are normally connected, it is not difficult to appreRiate that the "priming" or "acceleratine" grid is necessarily connected to $he pin, which in a 224 or 227 type of Yalve is connected to the cathode. A Very common error in wiring up the 247 valve (or any other pentode) is simply to connect plate and primings grid in parallel. The constructor argues that both are connected to B positive. Reference to the diagram. however {published last week), discloses that plate and priming grid are actually éeonnected to opposite ends of the output transformer, and this requirement must be observed when wiring up. The 247 pentode is biased by means of a 700 ohm resistance connected be‘ween an external "centre tap’ and earth. In view of the large audio currents involved, this resistance is bypassed with the comparatively large eapacity of 4mfd. This condenser need not be of a very high voltage type, however. The resistance must be wirewound and capable of carrying a curwent of 30 milliamps. , The 247 pentode with the voltages mtaployed is capable of feeding upward of 2 watts into a properly matched Speaker. The power detector is, as a matter of fact, fully capable of Joading tip a pair of push-pull output valves, but it is felt that any increase of the output mentioned is quite uncalled for, except, perhaps, for such exceptional purposes as entertaining in a theatre or hotel lobby. It may be noted that When the pentode is operating so as to Provide a fair volume in the speaker, a faint bluish glow may be observed just ‘within the glass envelope, this brightentng and dimming in accord with music fF Speech in the most fascinating manher. This glow need not cause any alarm. It does not indicate softness in the valve, and, in fact, seems to be most pronounced in the better types of pentode. lll

A fairly heavy component should be chosen for the smoothing choke. This has actually to carry a current not far short of 100 milliamps, so that the smaller type of choke is quite useless. It is, of course, entirely possible to construct a suitable component, and any one who wishes to do this in preference to obtaining a commercial product is recommended to refer to the last issue of the "Guide." The smoothing condensers in this unit must have a test voltage of at least 1200. It is, as a matter of fact, strongly advised that condensers of the 1500 volt test type be employed, in order to provide a satisfactory margin of safety. It is all very well for production engineers to pare their products of everything ‘but a coat of paint, but the amateur constructor will find that a little generosity in the way of safety margins is a vast comfort as.

the period of service lengthens. ‘This is, indeed, where the difference arises between cheap receivers and more highly-priced products. Safety-mar-gins are expensive, but most decidedly worth while. The reader is reminded that some of the terminals on the power unit have quite high voltages applied to them. The terminals to be particularly considered in this respect are those which make connection to the speaker field and to the output transformer of the speaker; a fair voltage is also applied to the terminal which supplies B current to the receiver proper. Therefore, unless the power unit is definitely safe from prying fingers by reason of a totally enclosed console, it is strongly advised that these terminais be mounted within the screening box and leads brought through the box to them. The same remarks apply, of course, to any terminals to which a.c. is applied. With the exception of the filament wiring, there is no object in using very heavy connecting wire. The greatest attention must, however, be paid to insulation. For instance, the covering on Glazite wire cannot be

regarded as adequate insulation for yoltages of the order encountered in this unit, and should itself be- coyered with good quality insulating sleeving, preferably. of varnished cambrie, It is felt that the constructor will now have little difficulty in satisfactorily completing the power unit. Winding the Coils. HE next item can well. be the construction of the coils. In all there are seven of these required, three be- ; ing broadcast coils; of the other four, one pair is used for the 80-metre band, the other pair for the 20-40 metre band. it will be gathered from the fact that three coils are employed on the broadcast band as against two on the shorter wave bands, that the high‘frequency stage is not employed for shortwave reception. Instead, an alternative aerial connection is made to that end of the detector coil which connects with the grid, the remaining circuit changes being effected auto-

matically Dy making use of the contacts of the plug-in coil employed for the detector tuning. Thus it is important that the instructions for the making of this coil should be aecurately followed. The first, and simplest, coil to he made is the aerial coil. This is a straightforward winding of 52 turns on a 3-inch former tapped at the 12th turn up from the low-potential end of the coil for the aerial connection. The coils in the original model are wound with 22 S.W.G. enamelled wire on a former threaded 24 turns to the inch, but a winding of 22 S.W.G. double cotton covered wire close wound will have almost exactly the same inductance and any loss of efficiency will be unimportant. This coil is mounted horizontally at the extreme left-hand end of the chassis, being kept ahout 1 inch clear of the metal by a couple of small brackets. ‘The remaining coils are all wound on 6 pin plug-in coil formers, the bases for which are located centrally in the coil sereens which should already be in position on the chassi The formers for the broadcast coith must be of the type which is thread: ed 40 turns to the inch. The connections for the detector. coil for the broadcast band are de

picted in Fig. 7% At the extréme top | o£ the coil former small flats should A be filed on the ribs of the former to accommodate the primary, which consists of 40 turns of No. 44 S.W.G.

\enamelled. This must be wound very closely and evenly in order to accommodate it in the very limited space available. The secondary winding of 68 turns of No. 30 D.S.O. is spaced about a sixteenth of an inch from the primary and is wound in the threads on the former. It will be found to take up about 1% inches. It will be noted that the pin to which the highpotential end of the secondary is con-

nected is also connected by a small scrap of wire to an adjacent pin. This connection must not be omitted. The detector coils for the shortwave bands differ from that just described in having only one winding and in the connections to the pins. The 80metre coil comprises 16 turns of No. 22 D.0.0. and the 20-40 metre coil 6 turns of the same wire spaced slightly. The threads on the former are, of course, neglected. ‘The arrangement of the connections is also shown in Vig. %, and here, too, it is important that the short connections between pins should not be neglected. The oscillator coils follow the same arrangement for all wave bands, the connections being shown in Fig. 7. The winding for the broadcast coil consists of 88 turns of No. 30 D.S.C. wound in the threads provided on the former. The former will only just accommodate about this number of turns, so that care must be taken to start the winding right at the end of the former, As shown in the dingram, this coil is tapped, and for the broadcast coil the tapping should be 24 turns up-that is, 24 turns from the plate end of the coil

The oscillator winding for the 80metre band comprises 20 turns of No. 22 D.C.C., the tapping in this instance being 5 turns from the plate end of the coil. ¥or the 20-40 metre band a ‘winding of 10 turns of the same wire spaced slightly is required, the tapping being 2 turns from the end of the coil.