[Amps] source of '3rd harmonic'

[Dan Sawyer]

All,

I am trying to add bias to a commercial class B NPN RF module. The input 
and outputs are 4:1 and 1:4 'binocular' transformers, feedback is a 
simple CR network. The output has a significant 3rd harmonic, about -10db.

I have done research on the source of odd harmonics, however I can't 
find specific references. What is the root cause of odd harmonics, what 
would affect the level of harmonic?

Thanks for your help,

Dan
kb0qil