NOTES ON THE REFLEX

Radio Record, Volume I, Issue 26, 13 January 1928, Page 13

NOTES ON THE REFLEX

The beginner is apt to lose sight of the fact that the reflex or dual amplification principle, by means ot which a valve is made to amplify at both high and low frequency, was devised purely as aul economy measure, The,most fervent advocate of this system could hardly affirm that a reflex receiver is better than a "straight" circuit, in which the same operations are carried out by separate valves. Indeed, most of them would be ready to admit that it is mot quite as good, Bearing this fact in mind, it will be realised that the introduction of better, cheaper, and, above all, more econommi-

cal yalyes (upkeep is even more im‘portant than initial cost) tends to discount the previous advantages of the reflex receiver, and to account for its present decrease in popularity. It must be admitted, however, that these circuits are interesting, and for this reason will probably have a following for some tinie to come. Turthermore, it cannot be denied that a reflex receiver can give amplification, from the point of view of quality, indistinguishable from that obtainable from a "straight" set under average working conditions.

Such results, however, will only be obtained when the ‘dual’? receiver is really well designed, well ccnstructed, and, generally speaking, operated by someone with considerable theoretical knowledge of the subject. Overloading on a reflex set is not permissible. If the set happens to be of a type with a highly efficient R.F. system, a peculiar croaking noise will be heard on loud notes. ‘This is due to the A.J’. causing grid current, which severely damps the R.I*. transformer and interferes with the incoming signal. Because of this characteristic, reflex sets are instructive, as one learns to associate a certain volume with a certain grid swing, and can be quite sure that if the set is only just not "croaking" that the grid bias is being used to its full extent. ‘The grid swing will be about twice the value of the bias bat-

tery. CHOOSING VALVES. In choosing a radio-audio valve for reflex work, the matter must be looked at from the quality or audio point of view. In nearly every case the audiofrequency load following a reflex valve will be an inter-valve transformer or a loudspeaker, so that choice is limited to a valve with an a.c. resistance ot | 30,000 olisns in the former case, and, say, 8000 to 6000 olms in the latter. The 3000 ohm class will be the best, and the 6000 ohm the next best. If ‘maximum amplification of weak signals on ’phones is desired, one of the new yaives designed for resistance coupling and having an amplification factor of 40 for an a.c. resistance of 70,000 ohms should be used. But with such a valve quality of reproduction on any but weak signals will not be good.