Sadly I've no idea what the replacement diode actually was, it was liberated from another potted LOPT from a small 10" colour TV which had burned one of it's windings. I can tell you what the valve was that I experimented with though! It was a DY86!
I wouldn't worry about the barely visible hum bar, the hum on the picture when my set failed was really bad, the picture was dim and had a distinct 'S' shaped verticals.
I've just found a thread over on VRAT that I wrote about the set!
Extreme TV Bodgery First picture shows the hum on the picture.
Regards,
Lloyd