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

[John Hutchins]

Ken -

So what I am reading is that in the end a slight Ham-i-fication to 
improve receiver performance, sensitivity and S/N ratio is to change out 
the first RF tube to a -(12,6)-SG7Y or -(12,6)-SH7 to improve the front 
end sensitivity and S/N ratio.

I assume in your tests that the AVC line is still active in the AM mode 
to the 1st RF amp?

Hutch


On 6/15/2013 10:36 PM, Kenneth G. Gordon wrote:
> 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
>
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