[ARC5] S+N/N ratio results..more.

[Kenneth G. Gordon]

Using my VTVM on the audio output while being extra, extra careful to be as repeatable as 
possible and to minimize all unnecessary variables, I come up with the following approximate 
10 db S+N/N ratios at 2.0 MHz. for the following tubes in the RF amp:

12SK7: CW: 0.7 micovolts
             AM: 2.2 microvolts

12SG7Y: CW: .62 microvolts
                AM: 1.3 microvolts

12SH7: CW: .62 microvolts
             AM: 1.4 microvolts

12SH7 appears to have more overall gain than the 12SG7Y. However, I am not at all certain 
that the RF input is properly peaked with the 12SH7 since I run out of adjustment range, (the 
capacitor is all the way open) on the Align Input control. It appears to be peaked, but I am not 
certain. I AM certain about that with the other two tubes.

Secondly, the only tubes changed were the RF amp. From the mixer on out all is original.

There definitely is less over-all noise with the 12SG7 and the 12SH7 than the 12SK7, but I 
have not yet figured out a way to quantify that.

I have to turn the audio gain up somewhat in order to get my reference level at zero RF input 
set to my chosen point (200 mV) with the 12SK7, and have to turn the AF gain down to set 
that point for both of the other two tubes.

RF gain remains at maximum for all tubes. The signal generator cable was connected at all 
times.

I then set my reference level (200 mV) with the 12SK7, then swapped to the 12SG7Y and 
repeaked the Align Input: there was a 6 db increase in noise level. Swapping to the 12SH7 
and repeaking Align Input, there was another 6 db increase in noise level, or 12 db over the 
12SK7....however, I really don't know what this means.

I suspect that this simply means that the 12SG7Y has 6 db more gain than the 12SK7, and 
that the 12SH7 has 12 db more gain than the 12SK7.

Ken W7EKB