[R-390] R-390A IF gain setting and S/N measurement

[Larry H]

Thank you Charles - very good information.

I reread my post and I can see that I did not convey one of my questions clearly.

You wrote:
> Any difference in S/N you can accomplish by swapping tubes (with its 

  tradeoff in overload performance), you can also accomplish much more 
  easily simply by tweaking the IF gain control.  
<

The question had to do with measuring the S/N.  I tried to say that the 'measured' S/N changes depending on the setting of the IF gain, but that the IF gain control is way past the point where it could affect the 'real' S/N (as that is determined in the first few stages).  So what causes the 'measured' S/N to appear to change?  This happens in both of my 390As, of which one is kept vanilla.  So, adjusting the IF gain a little one way or another is not really changing the 'real' S/N of the RX, but it does change the measured S/N.  What I am trying to understand is what is causing the measured S/N to change?  I have an idea that in MGC mode with the lower IF gain, the audio gain must be set higher to set the db starting point for getting the 10 db increase.  This higher audio gain is causing the 10 db gain to be obtained with less RF signal input (therefore resulting in a lower measured S/N).  I would just like to know if I'm on the right track on this question.

Regards, Larry