A Corner for Beginners

Radio Record, Volume II, Issue 12, 16 November 1928, Page 11

A Corner for Beginners

By

Observer

SREGLADEEEATEDERESELI RUSE RSETATC RSE S ER SEC RES e SSeS See eee eek ae seeaASEseESES Seeseeeeasage eescesrrsss PSSESSSS CSS RS SSeS Se SSnSeSeen Notice to Beginners and Correspondents. (1) Readers in difficulty or who have solved a difficulty which may be of interest to others are invited to write to us. (2) Questions of general interest will be answered in these columns, those referring to construction will be referred to "Pentode." (3) All questions inviting reply must be accompanied by a stamped and. addressed envelope, for individual reply. (4) Address all correspondence "Observer," Radio Record, Box 1032, Wellington. (5) Where a query refers to trouble which cannot be located the following information should be given:(a) Name and model of set if factory made. (b) A diagram of layout if home constructed. (c) Particulars of valves, voltage, and grid bias. (d) As many other details as readily available.

Queries. . ()UESTION S asked on specific ques- ~ tions, and those for which a reply is expected, must be accompanied by a stamped and addressed envelope. Questions asking our opinion on manuproducts ‘cannot be answwer- & 4, except by following the general lines recommended by the manufacturers themselves. These can usually be obtained from the literature published by them. Only general recommendations can be made by us. What is wanted is short interesting questions and experiences that may be of value to anyone.. These are welcomed and, where possible, made use of in the "Corner." F.P. (North Auckland) asks whether certain valyes should be used in certain places, but does not send stamped envelope, This cannot be discussed through these columns. Manufacturers recommendations regarding yalyes should be followed. Le, Disconnecting B Battery. i P. (North Aucklaid) makes one * query which, however, is suitable for publication: "To disconnect B battery so as to relieve strain on valves, tvould the disconnecting of one terminal do or should the batteries be wholly disconnected? If B batteries are to be disconnected and there is no real reason why they should be: for on turning out the rheostat or filament switch the B is automatically: cut off, the removal of the positive lead would be sufficient. A switch put in this lead would avert the danger of making a wrong connection when replacing. | Grid Bias. C "AONORA" (Lower Hutt) writes "Wishing to change over to different valves, I find they require varying amount of grid bias; how can I supply this?" The specific valves have been given, but the general aspect only will be considered. The power valve requires 17 voltsthat is the maximum, the R.F. require 2 volts, that is the. minimum. The problem is to give to each valve the correct bias. Almost every receiver has this problem. ‘There is one solution only that is worth while. Connect all the grid bias. batteries in series, So as to get at least 17 volts. Take different negative tappings to grid bias of audio transformers of valves in question. Grid bias is applied to the R.F, valve by inserting the battery between the aerial coil and the filament negative. The detector is left unbiased. Battery Charging. id ALso how long would an A battery charger, charging a 6-volt accumulator at 2 amps. run on a unit of electricity, and would the above charger be as economical as, say, a2 3 amp. trickle charger?’ ‘This depends on the type of charger. Tungar type would require approximately 27 hours per unit. The relative 1% icienciet cannot: be compared unless principle of rectification of each is known. Earthing of Eliminators. "PINALLY, my set is earthed to a length of galvanised iron pipe driven 8 feet into clayey soil and connected by: means of a brass lug and J get serew sweated to.pipe. The eliminator is earthed similarly and the two

pipes are about a yard apart. Is this arrangement too close to ensure an efficient earth for the receiver or would further spacing prove beneficial?’ This is quite O.K. and those using eliminators should bear this in mind, particularly as the Underwriters’ rules require eliminators to be earthed. Power Valves. HE advent of power and superpower valves is not without trouble especially to the battery user and a word of warning may forestall a great deal of trouble. Such a value should not be used unless adequate means are provided to supply the necessary current. To obtain good results high voltage is necessary on the plate, as well as a large amount of current to the filament. A dry battery with a discharge rate of ten milliamperes for example, will not give satisfactory service with a set consuming thirty milliamperes, As a battery ages, it is liable (owing to the increase of its internal resistance) to give rise to howling in the set, and to distorted reproduction. Above all do not connect an old battery in series with a new one. Under these circumstances it is better where batteries only are available to use a moderate power valve. When Making Adjustments. WHEN adjusting the C battery tapping on an amplifier using a power valve the filament current should always be disconnected. The sudden surge of anode current when the © battery is disconnected may cause damage to the extremely fine wire in the speaker. Eliminator Hum. WHEN a multi-valye receiver is connected to a battery eliminator deriving its supply from the A.C. mains, a2 constant hum, which is annoying during a lull in the programme, is too often noticed. In the smoothing system of the eliminator one or two chokes are utilised, which are only designed to pass a certain current in milliamperes... If this current is exceeded the iron cores becomes mag-

netically saturated and it no longer acts as a choke to smooth the ripples in the D.C. supply from the rectifying valve. The only cure for this trouble is to use a larger eliminator or reduce the total plate current consumption of the receiver. When a negative grid potential is applied to the grid of an amplifying valve, a drep is noticed in the filament to plate current. Under working conditions bias can be applied until the valve is working on the centre of the curve of the straight portion of the curve, supplied by the makers, and in many commercial receivers insufficient bias is allowed for when working from ‘an eliminator. It is well known that when more current is drawn from the eliminator

the voltage drops also. The less current drawn, the higher the voltage. So that if more bias is given to the receiver the voltage applied to the plates of the various valves is also increased, therefore giving the valve a longer straight portion of its characteristie curve and adapting itself automatically to the extra bias supplied. The main point is that less milliampere current is drawn from the eliminator and it is amperage, not voltage, that overloads any transformer -0r choke, A miiliameter placed in the. negative B battery lead will show the total consumption of the set, and on referring to the graph, which the manufacturers of the eliminator supply, it can easily be seen the voltage that is being applied to the amplifier valves. It is stated that with short-wave sets reaction gives better results when fed into the grid circuit from the end to which the aerial lead is also connected. Failure for a short-wave receiver to oscillate is often caused by damping: introduced into the grid circuits by having the aerial coil too tightly coupled to the grid or secondary coil. Distortion. ITH many receivers distortion is a feature. In most cases, but not all, distortion can be attributed to the set, If, however, distortion is noticeable on one particular station, it may be assumed the station is at fault, but it is wise in this respect to compare notes with other listeners. i There is a rather grievous case fesulting from station distortion to be told of three new arrivals to the radio world. A short time back. a station was making certain adjustments resulting at times in indifferent transmission. Three people, reports one dealer, purchased small sets and, on hearing the transmission, returned the sets and are seeking to have the license money refunded. This certainly is a little hasty and unfair. Adjustments must be made periodically and, as they extend over only a short period, listeners should at least be indulgent.