[ARC5] 10 meter BC-454 - more

[Kenneth G. Gordon]

On 24 Feb 2013 at 11:28, Geoff wrote:

> > but it also puts out a lot more noise.
> 
> Which is correct since it is the ENR contribution that establishes a
> tubes NF. Anything else is RF resistance thru the circuitry adding to
> it to give you stage by stage or system NF.

That's what I had thought...

>  In your case it is likely the 12K8 you are hearing and the 12SK7
>  doesn't have enough low noise gain to overcome it.

I'm sure you're correct. The 12K8 is a particularly noisy mixer tube.

> A 12SG7 as first IF may help a little, I can hear a small 10M
> improvement in a HQ-129X with a pair of 6SG7's in RF/IF and 6K8
> converter but that is thru a bandswitching rats nest.

OK. I'll try a second 12SG7 at the first IF to see what happens.

> With no bandswitch and short leads your set should be capable of  ~
> .7uV for 10dB SNR on AM

I have to take some definitive measurements and calculations, but at this 
point, this receiver is about double that: around 2 uV. Still, as I said, I am 
quite surprised at how well this is working.

> which is a little worse than the NC-240D on
> 10M. It is hard to beat almost zero lead length and heavy wire coils
> plus the 6K8 is used just as a mixer.

Yes. With a separate oscillator, most of those mixer/HFO tubes are much 
quieter.

Ken W7EKB