RADIO NOTES

Rotorua Morning Post, Volume 1, Issue 43, 13 October 1931, Page 5

RADIO NOTES

"Kathode."

By

Mr. j. W. Shaw, M.A., will speak from 1YA this evening. His subject will be "Humour Amongst the Nations." 2YA will relay the closing session of community singing this evening. The song leaders will be Mr. Owen Pritchard, Mr. C. Webb, Mr. and Mrs. Albert Russell and Mr. Will Mason. Mr. Frank Crowther will be at the piano. 3YA will broadcast community singing from 12.30 to 1.30 p.m., but will observe the usual silent night. The Dunedin • Garrisoh Artillery Band will be heard from 4YA this evening. The singers will he Miss R. i Cuncliff (mezzo-soprano), Miss Irene Horniblow (contralto) and Mr. F. E. Woods (bassK Mrs. C. Drake, will play "Rush Hour in Hong Kong" as a piano solo and Mr. J. McCaw will present saxaphone solos. Once again the Australian stations have fallen in volume through the YA stations maintain good strength. Conditions have not been good for long distance reeeption during the week and 'the American stations have not been up to their usual standard. The half-hour daylight savihg will probably effect trans-Tasman listening as it will nowbe' 10 p:m. before the Australian programmes commence.

Constructors a!re frequently unable to deeide which syStem of audio amplification to use. The three main systems are, transformer, fesistancecapaeity and choke-resistance, though the latter also uses capacity coupling. One of the drawbacks in resistancecapa'city coupling is the tendency for the upper musical frequeneies to become attenuated owing to the bypassing effect of stray capacities which are virtually in parallel with the plate resistance. These frequeneies. are in the neighbourhood of eight to ten thousand cycles. When the anode resistor is increased beyond certain valves, the by-passing effect becomes really serious but providing the valve is kept beloW 150,000 ohms the cut is not noticeable. Another drawback to resistance-coupling is the fact that three stages are necessary to give sufficient amplification for speaker operation. Of course, the pentode Valve has overcome this "snag" to a certain extent, though the actual '"lift' of a single pentode stage is not quite that of two transformer coupled stages. The vast improvements made in audio transformer resign hring this type of coupling to the fore, though the improvements made have not altogether be'en desirable in a.c. receivers, owing to the fact that low frequeneies are able to pass and be amplified, therefore a.c. hum is difficult to eliminate. Careful treatment is necessary with transformer coupling using two stages in order to overcome low-frequency instability. . The iron cores should be earthed and lcept at right angles to each other. Coming now to choke-resistance-capacity coupling, we find most of the above "snags" absent. A much higher quality is gained and overall amplification using two stages is good. This type of coupling is used in most broadcasting stations where speech amplification is necessary. Summing up, then, Kathode is much in favour of the latter system where absolute quality is required. On certain nights 2FC Sydney runs a theme song through the programme. During the interval between items, a piano takes up the theme song, and the announcer's voice is super-imposed on the music. This is particularly effective and the "fade outs" are remarkably well done. Immediately each item is finished, the piano is heard faintly at first, but quickly rises to full volume, to fade away again when the announcer is speaking When the announcement is finished, the operator steps the volume up slightly and merges the item gracefully in over the theme music.

Short wave reeeption has been very erratic during the past week. The American broadcasters on the 40metre band have been full speaker strength one night and almost inaudible the next. An English amateur was heard answering a call from one of the Rotorua transmitters, but was finally lost in jamming signals and static. The pentode valve is an excellent audio amplifier and makes one stage of audio amplification possible. Kathode has found one megohm most suitable for the grid return resistor, though two megohms give. slightly higher amplification. The tone is excellent and is particularly suited to dynamic speakers, as a balance is obtained between the high note amplification of the tube and the characteristic of the dynamic to emphasise the lower registers. Research has found harmonic distortion to be present in pentodes, though this is not apparent to the ear. For short wave headphone work, rather elaborate choking systems are required, as the tube, being of the screen grid type, amplifies the small amount of r.f. that passes the detector choke to large proportions, and with even the best shielding available, body capacity is vey pronounced.