"THRESHOLD HOWLS."

Press, Volume LXV, Issue 19622, 18 May 1929, Page 6

"THRESHOLD HOWLS."

METHODS FOB ELIMINATION. One of the most annoying of all faults to which home-made regenerative receivers are subject is the tendency for the set to "howl" just as the detector valve is brought by the reaction control to the point of oscillation. The trouble is particularly common in short wave receivers, which are now being used widely by broadcast listeners for picking up overseas programmes, but it is also encountered frequently in broadcast sets for ordin ary wave lengths, and it invariably makes it almost impossible for one to receive weak signals. The fault is due to a combination of circumstances resulting in the application of an incorrect bias to the grid of the valve at the point of oscillation. The most obvious method of curing "threshold howl," as the fault is ealled, is by a direct alteration of the grid bias. A most useful method of doing this, which will make it possible for the grid bias of the detector valve to be controlled at will, consists of the addition of a potentiometer to the receiver. The two ends of' the potentiometer winding should bo connected across the filament battery leads in the set. As the resistance of the potentiometer is quite high, little current will be wasted through it. The sliding arm contact of the potentiometer is connected to the filament return from the grid leak. If the grid leak is connected across the grid condenser in the usual manner the return lead from the detector tuner is connected to the sliding contact of the potentiometer instead of direct to one side of the filament. Merely by turning the potentiometer knob the grid bias of the detector valve can be adjusted, and any tendency for a howl easily checked. Apart from overcoming this nuisance it will often be found that a careful adjustment of the potentiometer will add to the sensitivity of the receiver in a striking manner. Other methods of overcoming threshold howl are the changing of the grid leak, usually for one of lower value, the readjustment —usually in the form of an increase—of the filament current of the detector valve, or the reduction of the plate pressure on the detector. The use of a reaction coil of fewer turns, or the removal of this coil farther from the grid tuning coil of the detector, will also assist in stopping the noise.