Jottings

Radio Record, Volume V, Issue 26, 8 January 1932, Page 13

Jottings

HEN using a multi-range voltmeter or ammeter always assume that the volts or amps. to be tested will be a little over rather than a little under the expected voltage or amperage, for whereas a shunt resistance can easily be altered for a lower value if necessary, the switching on of excessive volts or amps. may ruin the instrument. ALTHOUGH a slavish adherence to makers’ instructions regarding "B" voltage and grid-bias voltage is not necessary, it is as well to remember that such figures are worked out

for the benefit of listeners, and therefore they should only be departed from in _ cases where the valve: owner is sure of the effect of the modification he is making. ALU grid return and earth connections,. ete., in an indirectly-heat-ed A.C. valve circuit are made, not to the heater electrode itself, but to the electron-emitting cathode placed in close proximity to it. HEN connecting up voltmeters, am. meters, efe., remember that they are marked for polarity and should always be connected the right way round. S a "B" battery tends to recover a little when not in use, voltage tests should be made after it has becn working a little while, or the reading may be misleading. F you are building a set embodying two audio-frequency transformers remember that these should be of different types, as all sorts of troubles are liable to arise if the same type of audio transformer is used in both cases,

REDUCTION of the filament volt-. age on the detector valve of a short-wave set will often help to cur threshold howl. . LITTLE lubricating oil rubbed in with the finger-tips and polished with a soft duster will often restore a discoloured ebonite panel. ‘THE incorrect adjustment of grid bias may cause an unnecessarily heavy drain upon the "B" battery and will bring about distortion. [ you cut off a piece of eardboard strip of suitable length and push a small screw through it, the cardboard strip will enable you to hold the screw steady in what appears to be an inaccessible position while you are operating the screwdriver. ND of the great objections to swing-ing-coil reaction is the fact that by this method the tuning is not so independent of the setting of the reaction, as: when a condenser is used for the latter. WHEN a long passage or hall is not available for an indoor aerial and it has to be placed in a very small room, it is generally advisable to try the effect of different positions for the wires. PD=HFLCTS in the wiring of a set can often be detected easily by testing with a pair of ’phones and dry cell. [F you are using a screen-grid_ set and have a variable high-resistance on hand, try connecting this in the B plus lead to the screening grid as very often a fine variation of the voltage to this point gives improved results.

OWING to its fairly high anode consumption, a pentode cannot be used successfully with small batteries, but should be run from the mains or from batteries of the large-capacity type. JF a small disc, such as a cigarettetin lid, is threaded on to the aerial just before it reaches the lead-in tube, The rain running down the lead-in wire will be diverted at this point and, therefore, the insulation across the lead-in tube will be improved. N ordinary on-off switch fitted across the terminals of one loudspeaker wired in series with another will enable it to be switched into circuit without affecting the other speaker. ror "family" use one of the permanent types of crystal detector is generally more satisfactory than the "cat’s-whisker" which easily gets out of adjustment. HT-STOR or audio stages in which fairly high anode resistances are used will always need far more B voltage at the battery plug than it is intended to apply to the valve, owing to the drop in voltage across the resistances.