On Wednesday, April 25, 2018 at 2:21:55 PM UTC-4, Gareth Magennis wrote:
Found 2 12AT7 valves inside a Fender (Evil) Twin today.

Both branded 12AT7, National, made in USA.

Except one is way bigger than the other one.

http://i67.tinypic.com/fc0ehi.jpg



The bigger one has a plate structure some 1.75 times taller than that of the
smaller one, yet both are apparently 12AT7.


On my valve tester, the larger valve traces showed approximately twice the
current of the smaller one.
The curves basically look like this is a double current 12AT7.



What is it?




Gareth.

It is a 'mistake' - pure and simple, nothing exotic at all.

"National" as distinct from National Union (NU, later, Philco) was a rebrander of tubes, and once upon a time, the house brand for a number of retail tube sellers including Richardson Electronics. Much of their stock came from surplus, pulls and culls from the actual manufacturers and in point of-fact sold with no further testing than fitting in the appropriate box after relabeling.

What you have is a tube getting mixed into a lot, scrubbed, painted and boxed, and sent off into the world with no further thought.

Peter Wieck
Melrose Park, PA