The New Screen—Grid Valve as Audio Amplifier

Radio Record, Volume I, Issue 37, 30 March 1928, Page 13

The New Screen—Grid Valve as Audio Amplifier

MHI. March number of ‘Radio News" gives an audio amplifier employing the UX222 valve in the first stage and a 171 type valve in the second, transformer coupling being employed. Quadruple amplification is claimed over results obtained with a 201A in the first stage, or 300 times against 216. The innovation is particularly suited for DX work, of which we get such an amount in New Zealand, but, on the. other band, if properly handled, will give excellent results on strong signals. This latter is effected by the vse of a potentiometer to cut down signal strength to au

amount that can be carried by the Jast valve. ‘The chief difference between this and the usual audio circuit consists in the use of the second transformer as an auto-transformer, by connecting the primary and secondary in series. ‘The only transformer suited to this purpose is the Pacent, which has a high secondary inductance, about 600 henries, with a high value of current through it. No further particulars are given of this transformer. ‘Ihe first transformer is 8 to 1 ratio, and the final output is through an output fransformer of the usual 1 to 1 ratio. Partial shielding of the valve is an absolute necessity to prevent any tendency of the valve to howl, and to present microphonic action from the loudspeaker. The shielding is effected by placing an inverted "beverage shaker," or metal tumbler, over the valve.

Considerable experiment and trial in the leboratory has produced a satisfactory unit, but the descriptive article impresses upon constructors the importance of adhering: to specifications as closely as possible. he schematic diagram is given here as being of particular interest at the present time. The potentiomcter above referred to is made from a 30-ohm rheostat, and in order to cut down the total resistance to 22 ohms, which will give the filament its requisite 3 volts, a section of the resistance (nearly one-third) is shorted by sweating solder across the

wires, or by any other convenient method, and both ends of the resistance are brought out to terminal screws. The arm gives a bias of }~ volt on the screen-grid. ‘Though the filament voltage will be slightly below the rated value, it makes for longer life without decreasing amplifying efficiency. } Little advautage would be gained by using the screen-grip valve unless the last valve was one equal to carrying very large volume, and this the 171 will oO