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The Anode Bend Detector.

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The so called anode bend signal detector, or grid bias detector is a high impedance answer to the problem of demodulating (detecting) AM signals. In this arrangement the valve is used with full anode voltage (say 250V) but with a large negative grid bias (around -28V) applied so that only a tiny (0.2 mA) anode current flows. The (amplified) RF input signal applied to the grid causes the anode current to increase during positive half-cycles only and an amplified audio signal is obtained at the anode which is decoupled by a small capacitor in order to suppress RF amplification.

The principal advantage of this arrangement is that the (relatively large) RF input signal is not loaded by rectification current (as in a diode detector) so a very high 'Q' value can be achieved in the tuned circuit of the preceding RF stage. This was of particular value in simple, low-cost receivers with a single TRF stage but where a high Q was required in order to provide selectivity.

Also, the anode bend detector gives a reasonable audio gain with appreciably less audio distortion than the better-known grid-leak triode detector. Use of the type 37 as a detector became obsolete when superhets with AVC and double-diode-triode became the norm.

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