ANSWERS TO CORRESPONDENTS

Radio Record, Volume I, Issue 14, 21 October 1927, Page 14

ANSWERS TO CORRESPONDENTS

EXTENDING NIUTRODYNE. In reply to D.B. last week connected with the addition of four stages of radio and a third audio stage to a five-valve neutrodyne, it was intended to state that in No. 7 of the "Record" there is a diagram by Mr. Billing of a neutrodyne circuit that, might prove helpful so far as the radio side is concerned. Yivery effort should be made to get the present five valves to the highest efficiency before adding further to the set. Ii.J.FI. (Wellington) lias had for a moment too much current through une filament of a valve, presumably a dull emitter. The valve now refuses to give results, although the filament is intact. This filament has lost its power of emission of electrons at the temperature at which it is intended to work. Sometimes this tronble can be cured, but. it depends to some extent upon the make of valve. Soime of the .06 type are not always curable. There is no harm in trying, and the method is to run the valve at the correct voltage for an hour without any H.T. on the plate. This brings some of the special chemicals to the surface of the filament, and normal electron emission may then result, _ Another method is known as "‘flashbut is decidedly risky. It consists in connecting one filament leg of the valve to one side of the E1.T. battery, and just brushing the other H.T. counection on the other filament leg. A safe wav of doing this is to charge a 2 mfd. fixed condenser by connecting it across the H.T., and then disconnecting and putting the filament legs across the condenser, which will discharge through the filament, but there is no danger of applying the current for too long a period. The 1.7. used for either method should be 100: volts or iSO The thoriated-tungsten type of filament responds best to rejuvenation methods, If a dull emitter is frequently run at a somewhat higher tempera[arre than that specified by the makers, the thorium at the surface is dissipated more rapidly than it can be replaced hy diffusion from the interior,

M.V. asks for advice regarding a particular circuit which has baffled several experts to wliom he has referred the question of getting the set into working order, ‘Me ohm" has not a very high opinion of the circuit in question which, for the number of valves employed, gives very poor tesults, It is not, in examples that have come under his notice, a good distancegetter, and is easily beaten by an efficient 4-valve Browning-Drake. People in the radio business are probably too busy at present to give full attention to this problem. A Southern reader has built a onecontrol 7-valve super-het. and does not find it very satisfactory. There 1s trouble from harmonics, and reception only equals a four-valve set. | It is proposed to use the components in constructing a more serviceable circuit. It may be that the single control introduces trauble that takes a good amount of adjusting. Further information will be given as requested. As usual, a number of replies have been sent by post.