[Amps] analyser on output

[David Cutter]

Are there any threads, cautions, general advice, web sites to help with using an analyser on the output of an amplifier?   I seek general enlightenment and a tool for comparing one amplifier with another, ageing effects, fault finding, searching for conditions that might support parasitics and the like.

I have a AEA VIA antenna analyzer and MFJ259.

I am interested in valve and transistor PAs.  

David 
G3UNA