Tips and Jottings

Radio Record, Volume II, Issue 12, 16 November 1928, Page 28

Tips and Jottings

A Sereen Grid Mystery. . MNHERE have been cases in America where a new screen-grid valve has appeared to be ‘dead,’ and experts have been greatly puzzled. It appears that sometimes the small metal cap at the top is liberally coated with

lacquer, and not until this is scraped er can good contact be made with the clip. Eliminator Precaution. WHEN using an A.C. battery eliminator, always turn off the H.T. before the low tension or filament supply, and turn the low tension on before the high tension, so as to avoid gxcessive stress being set up across "che smoothing condensers. Concerning Push-Puil. A CORRESPONDENT, A. Webb (Auckland), writes: "I contemplate constructing the push-pull amplifier described in ‘Radio Record,’ of November 2, and I would like to know the following: Would it be in order to use an amperite in place of the six ohm rheostat? If so would the one amperite do for all three valves, or is it necessary to use a separate one for each valve? In the event of using this method of filament control, would I need 2 volume control, if so, what would you advise?" It depends to a great extent on the. valve to be used. If all the valves were of the 6 volt. 1 amp type, then a 4 amp amperite could be used to control the three valves. If the valves consumed .25 amp each, then the amperite would not stand up to. this work. An amperite is generally designed to control the current for one valve only, which is specified on the container. In this case it would per‘haps be advisable to know the type of valves to be used and apply for three amperites for these particular valves. Regarding volume control.-If you desire to use a gramophone pick-up, 2 volume control is supplied. If a crystal receiver, the best way to do this is to incorporate a variable condenser in the aerial lead and use this in pre-. ference to detuning the tode."