[ARC5] Quick sensistivity comparison

[Kenneth G. Gordon]

On 13 Jun 2013 at 8:37, Richard Hankins wrote:

> Ken,
> 
> you don't say what you were surprised by?

I am surprised by the difference in the fixed noise level by the simple 
swapping to similar tubes of much higher transconductance.

>   I am surprised that /any
> /valve HF receiver gives apparent sensitivities this low (either set),
> and would suspect leakage from your signal generator.

No. I have checked this, and have conferred with other users of this same 
equipment. There is no leakage, and the sensitivities are real. I have also 
been suspicious of leakage, but have since been certain there is none...or at 
least none that is significant.

I have also had similar sensitivity-results with other receivers over the years, 
most notably, the Heathkit HW-101, SB-101 and 102. Furthermore, my 
results were independently confirmed by others who are far more 
knowledgeable and experienced in these matters than I. 

0.1 microvolt is fairly easily achieved with tubes. Sensitivities much below 
that are not, at least at HF.

Besides, as far as I am concerned, ultimate sensitivity is not the most 
important factor: the most important factor in any receiver's ultimate 
performance is internally generated noise. Eliminate or reduce that, and you 
have a good receiver.

> Either way, both sets of figures (if accurate) are academic. Aerial
> noise on the HF bands will swamp these figures by some 10 - 20 dB.

That is true, MOST of the time. 

However, that has NEVER been a reason for me to NOT attempt to achieve 
the highest performance possible from any of my receivers. I want my 
receivers to be as "transparent" as possible to any signals that might be 
there.

It has been my experience that there are times (and they are not all that 
unusual) when band noise is unusually low, even at VLF. Why mask 
anything that is possible to hear by using noisy receivers? Especially when it 
is not all that hard to achieve quietness?

Doesn't seem logical to me.

Ken W7EKB