[ARC5] 10 meter BC-454 - more

[Kenneth G. Gordon]

On 23 Feb 2013 at 23:46, Dennis Monticelli wrote:

> I guess I am missing something here.  If the amplitude of the
> background noise is higher and the S/N is the same (can still hear the
> weak signal with about the same readability , then the weak signal was
> amplified too and by about the same amount.  Everything is just a
> little louder so the gain is higher.  Same effect as just turning up
> the gain control.

I guess I am not making myself clear: to my ears, I am still hearing the weak 
signal with "about" the same amplitude, but I have to "dig" a bit to hear it 
through the noise contributed (obviously) by the tube.

The transconductance (for one thing) of the 12SG7 is twice that of the 
12SK7 under the same operating conditions. 4000 vs 2000.

> If the background noise goes up and covers up part of the weak signal
> (readability goes down), then additional noise was contributed that
> reduced the S/N.

Yes. That is what occurred.

>  If that was accompanied by an overall increase in
> amplitude of the desired signal then the gain went up too,

It "appears" that the overall amplitude of the desired signal remains the 
same.

> The 12SK7 is supposed to have less gain than the 12SG7, so something
> is not adding up.  Maybe the 12SK7 is running more current in the
> given bias arrangement.

I don't think so, but can check that. One thing that does somewhat concern 
me is that the screen voltage, from that fixed voltage divider in the rear of the 
receiver, is 75 volts. In the specifications that voltage is much higher and is 
close to the same voltage as the plate voltage.

> I forgot which role this tube was playing.  Was it the RF stage or one
> of the IF stages?

RF amplifier. There really isn't much improvement by using a 12SG7 at an 
IF.