Help me Doctor, I've come out in spots...

[glowinganode]

Don't worry, nothing serious.
Further a recent conversation with my young son, I decided to demonstrate a Mercury-Arc rectifier in action.
Not having anything as impressive as Lucien, I fired up a pair of RCA 866a's http://en.wikipedia.org/wiki/866A.
One was glowing nicely with the familiar lilac tinge, however the second had a much greener glow. A bright white spot was visible in the centre of the anode, with a ring of spots around the rim.
The first rectifier struck at 16 V, dropping to 11 V once struck.
The second rectifier however struck at around 50 V, dropping to 40 V once struck.
I can only assume the second rectifier is starting to fail, I'd be interested in the failure mechanism though.
Photo's to follow.

Cheers,
Rob.