Laboratory Jottings

Radio Record, Volume IV, Issue 28, 23 January 1931, Page 12

Laboratory Jottings

Gulbransen Radio

MESSRS. H. W. CLARKE, LID., have sent for test their two new Gulbransen models, the Champion Highboy and the Champion, Lowboy. These have been in use for quite a few weeks, and we have consequently been able to make a thorough examination of these new models. Both use screengrid radio frequency valves, but the large model, the Highboy, has one stage more r.f. than the Lowboy, and uses a 227 as power detector, Other than this the circuits are the same, In the Lowboy is a screen-grid detector: which feeds into two 245 valves in push-pull] through a stage of 227. The Lowboy is much smaller than the Highboy, but is very neat and efficient, and we were rather surprised

at its distance-getting abilities, Although at the present season the Australian stations are well below normal, ‘it was able to bring them in at quite comfortable strength. We heard the Japs. regnlarly, and when making comparisons with our standard, we found that it lost very little through having only two stages of screen-grid radio frequency. So far as New Zealand.stations are concerned, it brought in the main YA stations in addition to Wanganui and.an occasiona1 Dunedin station in the daytime. At night the B stations came in very. well. , ; We liked very much the tone of the -Lowboy. If anything it was slightly deeper than that of the MHighboy though it lost nothing in the higher frequencies, Incorporated in the Highboy and thc Lowboy is one of the new features fo the 1931 season, that is, a’tone control. This comprises a resistance, and a condenser shunted across the grids of the push-pull valves. By varying this a good percentage of the high frequen- . ey notes can be eliminated, giving the — ‘bass greater prominence than it would otherwise have. Tone controls are interesting features and no doubt many people will welcome them so that they may obtain whatever balance of bass and treble they require. Faithful reproductions, however, cannot be had when the higher frequencies are cut out, and for this reason we did not use the tone control when operating on the local station or for gramophone records. What we did find the control useful for was to cut out during bad conditions a great deal of statie and other interference on outside stations. ; Stations that could not be listened to with the set in the normal state could be brought in quite well when the tone control was turned round to the bass side. The volume control of both sets operatesin a dual capacity. It varies the coupling between the aerial and earth and also decreases the voltages on the sereen-grids of the. high-frequency valves. This system, which, by the way, is another feature of this year’s models, enables the volume to be turned down considerably without interfering with the quality; in-fact, we found. that the local station could be heard Without removing the aerial, and this is asking a good deal for a modern set. On the Highboy there is a local and a distance switch which cuts out part of the aerial inductance, so that the set may be used on the local station without overloading. The selectivity of both sets was stich that we could bring in 2FC and 4QG. These, of course, were received with 2YA operating fairly late at night. There was a background ‘to both stations. We consider it is a very selective set that will do this. Both models are adapted for a gramophone pick-up, and our measured frequency range showed that both brought in all the frequencies, high and low, which go to make up good reproduetion. The tone control operates as quickly and efficiently on gramophone ag on wireless. The sensitivity of the Highboy is, as is to be expected, slightly greater than that of the Lowboy. Both receiyers should be capable of bringing in the American: stations when operated’ in reasonably favourable localities. . The Highboy consumes 110 watts and the Lowboy 90,