abbey road d enfer said:Thermal agitation of electricity in conductors by JB Johnson Physical review July 1928.
Thanks. Mr Google found it for me.
Interesting that the 'working formula' is just plucked out of the air without any explanation of its derivation. Also interesting is that it makes tube noise directly proportional to idle current whereas gm tends to increase with idle current which means the 2.5/gm formula is inversely proportional to idle current