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

[Kenneth G. Gordon]

On 16 Jun 2013 at 8:17, John Hutchins wrote:

> Ken -
> 
> So what I am reading is that in the end a slight Ham-i-fication to
> improve receiver performance, sensitivity and S/N ratio is to change
> out the first RF tube to a -(12,6)-SG7Y

Yes.

> or -(12,6)-SH7

No. The reasons being that 1) one can no longer peak the input, and 2) the 
gain appears to be so much higher than either of the other two tubes that the 
BFO voltage level is no longer "in balance", and one has to turn the RF gain 
down in order to get a clean signal on both CW and SSB. These effects are 
minimal with the 12SG7Y.

> to improve the
> front end sensitivity and S/N ratio.

Yes, but it appears that at 2 MHz, the advantage to doing this is minimal.

> I assume in your tests that the AVC line is still active in the AM
> mode to the 1st RF amp?

Yes, as much of an AVC as there is in the early model "ARC-5" receivers. 

The ARA 46104 design is very close to the early BC-454 design: the AVC 
circuit is very minimal, IMHO. This is why the later AN/ARC-5 models used a 
12SF7 as the seond IF tube instead of a 12SK7.

The 12SF7 has a diode included in the tube and this was used for a much 
better AVC circuit. That AVC circuit also works with the BFO on whereas the 
earlier one did not. In fact, the AVC circuit in the earlier receivers is so 
minimal that they don't even bother to turn it off when the BFO is activated..