Sub Tubes

The schematic calls for a 12BD6 someone has substitied another tube a 12BA6 if memory serves me right.
My question is will a substitute work 100% of the time? Secondly why did industy make another tube with the same chareristics?

There is no "Same characteristics substitute for the 12BD6".

However there are different type tubes that have similar characteristics, for audio as an example, 6L6-6F6. They are similar but have a difference in voltage at the plate. And filament current. Even though they have the same pin-outs.

If it is a direct substitute then the reason is they made some minor improvement, or change in design, such as increase in filament current, or plate power, spacing of internal grids.

Usually they will make an improvement and add a suffix such as 12BD6 to a 12BD6A, not the case always though.