[ARC5] AVC in SCR-274N receivers

[Kenneth G. Gordon]

On 7 May 2014 at 9:58, Ian Wilson wrote:

> Changed the Subject since I think this is an interesting topic
> in its own right.

I agree.

> I would have thought that 100k is just too small to generate enough
> grid bias (from grid current flow in the last IF stage) to provide a
> noticeable amount of AVC.

So do I.
 
> Also, AVC could be picked off the audio detector stage.

Well, what is interesting is that the detector/BFO tube (12SR7) includes TWO 
diodes in it, one of which is always grounded, and therefore pretty much 
useless, IMHO. But then again, the engineers at ARC seemed to never have 
done anything, no matter how strange it might look to us, without some good 
reason, so perhaps there is a good reason why this diode is always 
grounded...or left "open".

As I see it, the main reason an effective AGC was NOT used in the original 
BC-series receivers was that when the BFO was turned on with the usual 
combination AGC and detector stage, the AGC would, effectively, be 
unusable.

The later, AN/ARC-5 receivers used a 12SF7 as the second IF tube. The 
12SF7 contains an added diode, and THIS diode was used for AGC in those 
later receivers.

Using this diode prevented the AGC system from being over-ridden by the 
BFO signal, since both detection and BFO injection occur AFTER the AGC. 
Furthermore, the AGC system is ALWAYS in action in those receivers. There 
is no provision for turning it off.

This is another reason why even that "better" AGC system was "marginal". It 
really only becomes effective at relatively high signal levels, from what I can 
see.

> I seem to recall reading somewhere that the receivers were designed
> with AVC, but that this was not connected up in their final form.

I have heard that too, but have no documentation on it.

> This would be consistent with the original connection of the lower
> end of the last IF grid resistor to the AVC network. The 100k/0.05uF
> filter would remove most of the audio from the AVC voltage.

Yes.

> The complete story would be interesting. Apologies if this is readily
> accessible elsewhere and I have overlooked it.

Well, although I recall, dimly, reading or hearing something about that from 
the past, I cannot recall where or when I heard it.

Ken W7EKB