Grundig 80U

[pwdrive]

It's a strange one....take look at the schematic in the link below, it's for an Ekco A244....Very similar so far as the AGC is concerned insofar that the spare diode is across the AGC line just like in the 80U, the circuit description implies that it's for the voltage delay but in other receiver schematics I've seen the anode of the that diode is connected to HT via a high value resistor (10 meg or higher):

http://www.radiotechniek.nl/index.php?p=manualbrowser&l=first&s=downloadmanual&merk=Ekco&typen=Ekco_A244.pdf

There's an example here (Ekco U195) showing the forward bias arrangement with a 10 meg to HT from the anode for the AGC delay voltage:

http://www.radiotechniek.nl/index.php?p=manualbrowser&l=first&s=downloadmanual&merk=Ekco&typen=Ekco_U195.pdf

The Bush AC31 which uses an EBF80 uses the same AGC delay voltage trick as the Ekco U195 above:

http://www.radiotechniek.nl/index.php?p=manualbrowser&l=first&s=downloadmanual&merk=Bush&typen=Bush_AC31.pdf

Maybe in the first example the AGC voltage is clamped to the contact potential of the diode, until the -ve DC component from the detected signal exceeds it and takes over, in other words it keeps the AGC line voltage (and hence grid bias) constant on weak signals.

All interesting stuff....

Lawrence.