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

[Brian Clarke]

Hello Ken,

What adjustments did you make for the different G1 operating points? Gain is 
related to the operating point - it is not an absolute independent of 
operating point.

As I read the circuit, the Align Input capacitor compensates for 
inter-electrode capacitance to achieve resonance with the RF input coil.

73 de Brian, VK2GCE.

On Sunday, June 16, 2013 1:36 PM, you said:


> 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