Quote:
Originally Posted by Catkins
The reason for its obsolescence at least in radio work was because it introduced distortion, but in the days of horn and moving iron loudspeakers that was the least of their worries. The distortion issue evidently wasn't an issue for the vision circuit in the HMV 904.
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Does everyone know how anode bend detection works? It occurs to me I only know how it works because I found a 1927/28 radio with it, and found a description in an early book. I've not found it described in any book later than 1933.
At the risk of telling people how to suck eggs. All valves from the triode upwards have a bend (or two bends upper and lower) in their response curve. The usual idea is to bias the valve so that amplification is done outside of the bend within the linear part. Anode bend detection turns that upside down. The bend is logarithmic, lower parts of the bend have very low amplification, higher parts of the bend have high amplification. We can use that to do detection if we bias the valve such that the negative part of the wave hits the low amplification of the bend, and the positive part of the wave hits the high amplification part of the bend. The end result is the negative part of the wave becomes reduced, and the positive part of the wave becomes amplified. In effect you detect (negative wave removed) and amplify (positive part of the wave amplified) at the same time.
But of course the logarithmic aspect of the resultant amplification introduces distortion, which is why it was abandoned about 1930.