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When I first looked at it in the Whole Earth catalog circuit, I thought it was a regen. But a commenter correctly questioned this conclusion. I remembered the grid leak circuit (WN2A provided more info in the comments). This morning I found a Wikipedia page that explains it all very well. I especially like the description of how this detector works both with small signals in the "square law" range of the tube, and with larger signals in the linear range of the tube. The history of the discovery of the need for the large resistor is also very interesting. I remember building FET amplifiers and finding that they would -- after time -- shut down. This would happen as charge built up on the gate. I had neglected to include the normal 100k ohm resistor (that would "leak" this charge to ground). Once I put this resistor in, the amp worked fine.
Here is the Wikipedia article:
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don't forget to put a pan under the grid resistor to catch the drips from the grid leak.....
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