[ARC5] Receiver sensitivity

[Kenneth G. Gordon]

On 10 Dec 2016 at 14:38, Richard Knoppow wrote:

>     Another thought that has nothing to do with speakers:
>     When you speak of sensitivity what exactly are you measuring?
>     Manufacturers specified different techniques, some of which are 
> useful only for determining if the gain is normal. Sometimes people plug 
> similar tubes into the RF stages and find they make the sets "hot" but 
> never measure signal to noise ratio.

Yes. I have wondered about this for a long time too. One standard is to measure the signal 
vs the signal-plus-noise. I.e., the ratio of the two.

I have never yet investigated this aspect of sensitivity.

I HAVE measured MDS or Minimum Discernible Signal for several of my receivers. But this 
value is somewhat subjective. What exactly does "discernible" mean to you? Are you 
partially deaf as I am? Or do you have more acute hearing? Is "hearing" the signal the same 
as being able to communicate with another station when signals are that weak?

I have always viewed noise generated internally by a receiver as being of far greater 
importance with regard to communications than bare sensitivity alone. This is why I have 
done everything I could with my receivers to minimize internally generated noise.

I hate multi-grid mixers for this reason. "Converters" are inherently superb noise generators. 
Also, the more grids in a tube, the more noise. For instance, the 6/12K8 is the noisiest mixer 
tube listed in Terman, having an equivalent noise resistance of something like over 840,000 
ohms, as I remember it. The 6AC7 has an ENR of something like 2000 ohms in comparison.

Also, all else being equal (which it seldom is) the tube with the higher value of 
transconductance is generally far less noisy than a tube with the lower value of 
transconductance. And this is when the tube is used as either an amplifier or a mixer.

Of course, in most cases, tubes with higher transconductance ratings are also easier to 
overload by a strong input signal, but in practice here, I have seldom had that sort of 
problem.

Ken W7EKB


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