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.



** https://en.wikipedia.org/wiki/12AT7

Nothing to worry about, the shape and size of plate structures of common twin triodes vary a lot.



..... Phil