Sylvania Halo model 21C508MP Fringe?

[Penthode]

Would like to see the schematic of the set. It likely employs keyed AGC. The clamper likely facilitates a delayed AGC to the RF stage in order to improve s/n performance. That is the AGC is applied to the RF stage last.

It seems to me you need to check the Keyed AGC system as a whole and not just the clamper. Easiest way to start is to take a 9volt battery with a potentiometer across it and attach it to the AGC line. As you increase the negative bias you should see the overload disappear.