OK, I'm trying to understand the failure of a 807 beam tetrode in a
homebrew push-pull audio amp.

Tubes are GE, about 45 years old, bought as new old stock several weeks
ago.

Being run with 400V on the plate and 255V on the screens.

Both behaved very nicely for a few weeks in my amp, and plate current
at idle followed the curves in the books just fine. They were
well-balanced.

But tonight under some listening stress the B+ fuse blew.

Turning the bias (supplied from a bias supply through a 100K resistor)
down to -40V, the good tube is drawing a small amount of plate current
while the other draws 40mA after warmup and current goes up and up and
up until it hits 100mA or so and I turn the amp off before blowing
another fuse. Check the grid current and as the bad tube warms up grid
current is increasing and increasing as well, to the point where the
-40V is being entirely dropped in the 100K resistor (I guess that means
about half a mA of grid current).

The tube with high current always had some violet-blue spots that
danced on the top but they were on the glass and mica so I figured that
was normal but now I don't know. There was also this strange orange
spot that was on a mica insulator there too.

Is this classic "gassy" tube failure? Towards the end it seemed to be
slightly more blueish on the top.

I've burnt up/blown up a lot of 6146 and other variant beam tetrodes
over the years but now I'm trying to get some understanding of what's
happening :-).

Will a certain chunk of NOS 807's simply be this way from age?

Tim.