On Wednesday, November 16, 2016 at 1:04:24 PM UTC-5, John Robertson wrote:

Keep your old tubes until you can test them and be sure they are bad.
These old tubes are not getting any more common! 2D21s were always a bit
fussy and were replaced by 2050 tubes not long after they came
out...even new out of the box there were failures. Seeburg jukeboxes
used 2D21s for their early tormat design and quickly changed the design
to 2050s part way through the second year of production due to read-out
and write-in issues. I have schematics for 2D21 testers that I can dig
up (probably find similar schematics easily enough online) and post of
my tech ftp pages if there is a demand/request. The 2D21 is pretty easy
to test, if you have access to a good tube tester it will usually show
them on the Seldom Used auxiliary tube list. The 2D21s are go/no-go
kinda things.

OA2s are also pretty basic tubes. They use a conductive gas to regulate
to 150VDC. If the tube is not broken or gassy then it should be just
fine. Check the Getter flash in the tube - it should not be showing any
white powder, that is a tube that has lost its seal and oxygen has
entered...

2D21s are/were used commonly in dental X-ray and Fluoroscope machines, and when, also back in the day, I was working for a GC doing some serious rebuilding at HUP in Philadelphia, one of the docs needed to resurrect such a machine for historical purposes. I found him the tubes he needed locally, including two 2D21s. This was "Before Internet", so not an instantaneous process.

I also keep a very good tester (Hickok 539B), so no tube would leave this house untested.

Yes, of course, keep any undamaged old tube.

Peter Wieck
Melrose Park, PA