[AMRadio] diode substitute for 866

[Rick]

Okay.. I tried a couple of 866 MVR's and obviously I had a defective socket
as it arced over at pin 1. I'm pretty sure the old 866 was not in good shape
either even though I burned it in for a couple of hours before applying B+.
So, I went back to the diode substitute made by RCC. It had worked
previously with no arcing and good B+ production. This time it followed the
866's arc path and blew a fuse. I know this is not unusual to continue
arcing once a path is established even though I thoroughly cleaned the
socket and chassis of all deposits, I thought. My question is, what is
attached to pin 4 in the diode that it would arc to or from? My
understanding is the anode is on the plate cap and pin 1 is the only one
used in the socket. Why would it arc to pin 4?
Thanks for any advice... Rick / K5IAR