[Amps] Checking for IMD

[Martin AA6E]

Tom W8JI wrote:
> ...
>
> Also there is a basic flaw with two-tone tests. Two-tone 
> tests don't vary the power supply loading like a voice 
> signal would. Flaws in grid voltage or high voltage dynamic 
> regulation may not show at all. Almost always the error 
> makes things look better than they really are.
> ...
>   
Thinking a little more about the dynamic IMD with syllabic fluctuations, 
etc.,  I wonder if a narrow spaced two-tone test (10 to 30 Hz, ala 
PSK31) would be a useful proxy.  The low-frequency beat would test 
supply regulation, as others have noted.

I'm not sure I've ever seen numbers relating to dynamic linearity - just 
the simple wide spaced two-tone tests.  Do we have results, or are these 
just suspicions? 

Plate supply regulation is not very good in most amps, it seems.  
Something like -30% no load to full load?  This must have an impact, but 
it's not obvious how much it affects IMD, or how you would measure it. 

73, Martin AA6E