[Radio Wrangler]

The filtering effect of the shunt capacitor after the detector diode is also a function of the source and load resistances. The germanium diode will have a lot less series resistance than the original small 'metal' diode, and this will increase the turnover frequency of the filter, so you get more signal frequency ripple on the output. Adding some series resistance should restore normal operation if this is the cause

Secondly, the OA81 spec shows that it is quite leaky in the reverse direction, passing many microamps even at low voltages. Depending on circuit values, this could also cause plenty of ripple on the output. For small signal use in a high impedance circuit (valve) I'd go for a more modern small silicon diode. If low drop is important, then a Schottky one, and add a series resistor with either sort.

David