Volume Control

Radio Record, Volume III, Issue 18, 15 November 1929, Page 26

Volume Control

Problems of A.C. (CoN SIDERABLE doubt exists regarding the methods of controlling volume in A.C, sets, and no constructor of these popular recei¥ers is free from this problem. The favourite method of.eontrolling volume in the batteryoperated sets was to vary the filament temperature on one or more of the R.F valves. Although some manufacturers of A.C, sets use this method to-day on A.C. valves of the 226 type, it is not considered ideal. The heating of the heavy filament is slow in following the rheostat adjustment, and this naturally introduces # very annoying time lag. It is particularly noticeable when trying for distant reception. Another method of controlling volume some. times re ‘orted to in both A.C, and D.C. sets is {> detune. This has the objectional effect of tuning the sidebands to the resonance. This causes distortion, and with these ultra-selective receivers of to-day this distortion is serious. If the bias of the r.f. valves can be reduced simultaneously with the filament temperature, this will hold up the plate current and keep out the ripple. Some. bias must be maintained at all times, however, as an even flow of grid current, and consequent modulation and distortion of the signal will result without it. A high variable resistaance across the secondary of the first audio transformer has been. used‘ by some, but this method is not recommended as it has u tendency to overload the _ detector. Varying the grid bias as a means of volume control is quite effective with cirenits using the 227 type valves, but it is not recommended with the 226’s, as this method is apt to shorten the life of the valve. A high variable resistance (100,000 ohms) in the plate supply of the R.F. plate returns will reduce the yolume, but this is not the best method. It would seem that the most logical procedure in controlling volume is to go to the heart of the matter and control the signal, instead of trying to suppress it after it is amplified or rectified. Naturally this must be done in the antenna circuit. The most approyed method is vary the coupling of the anfenna and the grid coil. This has the advantage of rendering the set more or less selective as the wishes of the over ator might be. Unfortunately this cannot be done ‘in all types of recéivers. e.g., the Browning-Drake. If it is posSible to rotate the primary antenna winding or secure a similar effect through taps, it is strongly recommended that this be done to obtain best results. A semi-aperiodic antenna coil of 8 or 10 turns of wire, wound round a bakelite former and variable 8 in its relation to the secondary, will work out very well. A variable resistance with a maximum ‘of about 500,000 ohms, in shunt with the antenna coil, will control volume quite satisfactorily. It is suggested that for. local or very strong signals that the aerial be disconnected from the set. This may be done with a snap Switch connected in series with the aerial. Another very satisfactory manner is to connected the ground wire to the. antenna circuit and leave off the aerial,