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THE RESULT OF CAREFUL WORKMANSHIP Front view of the reflex set described below
A " Knock-Out' ' 3-Tube Set
A "How To Make It" Article
^VER since the publication in the November Radio Broadcast of the "knock-out" one-tube set developed by Mr. Kenneth Harkness, enthusiastic readers, sensing the possiblities of further amplification, have clamored for the addition of two audio stages. Many have added a single stage successfully, as suggested in this magazine. However, the addition of the ultimate second external step has been made impossible by howling, on which the usual remedies of grid biasing, the lowering of plate voltage, the mounting of transformers at right angles, and the use of separate A and B batteries have had little or no effect. These are methods of stabilization which are ordinarily effective in the correction of magnetic feed-back, that is, the interlocking of magnetic fields, resulting in undesirable induced effects.
Investigation in the Radio Broadcast laboratories showed that the feed-back in the case under discussion was almost entirely capacitacive — capacity between the exterior amplifier and the reflex part of the circuit through ex
ternal objects, such as the operator, near-by electric wiring, etc. When the phones were used on the last or second stage, the receiver squealed loudly when the tuning dials were approached but the squealing ceased the moment the operator was grounded. Shielding will probably suggest itself immediately to the prospective builder as the obvious solution, but, in many cases, it will .be only partially corrective. Perhaps, if the complete set were boxed and paneled in metal, the howling tendency would be totally eliminated, but this, as experiments have shown, tends to lessen the rectifying effect of the detector— probably through capacitative bypass.
The solution of the problem was found in the Radio Broadcast laboratory, by localizing the difficulty, and applying what is probably the effect of shielding, to the localized area. The grid of the second tube (the first external amplifier) appeared to be the crux of the situation, and a small condenser, C2, connected between this grid and the ground, completely and definitely eliminated the howling. In