[ARC5] ARC-5/274N/A.R.C. question

[Kenneth G. Gordon]

On 14 Jan 2014 at 7:51, D. Platt wrote:

> Ken,  Thanks very much for your insights.  I've made some notations,
> below.

Thank you, Jeep.

> > Where did you find that? I have been looking for it but have not yet
> > found it. I need the details.
> Its in the March 1960 issue of QST.  I can scan/send it if you don't
> have access to that issue.

Yes. I do have that. I was given a complete collection of QST on CD. Thanks 
a heap. :-)

> amp cathode. However, IF regen control should still be 620 ohms >
> above ground when "bottomed".
> 
> McCoy does not mention removal of the AGC, at that point.

That may possibly be because he was working with one of the "ARC-5" 
models which HAD no AGC...or see below.

>  He does
> leave the existing 510 ohm resistor, in tact, and adds a 5k pot to
> ground.

Yes. That is what I use: a 5K linear pot.

> A member of the OMRN related his doing the mod to an R-26 but did it
> to the first IF.  McCoy's mod is for the '454 and, hence, the 2nd IF
> is a 12SK7.

I could not remember that McCoy used the 2nd IF: I had always assumed 
that the selectivity should be as close to the mixer stage as possible, and 
therefore have always used the 1st IF.

McCoy MAY have used the 2nd IF since in most ARC-5 models, AGC is 
NOT connected to the 2nd IF, but only to the RF and 1st IF.

>  The R-26 uses a different tube and it may make a
> difference?

Yes. The 12SF7 is used in those models since its internal diode is used to 
provide AGC voltage independent from the BFO. That way there is steady 
AGC voltage which is NOT effected by the BFO. Therefore, AGC is always 
on.

However, that AGC voltage is so "delayed" that it only really becomes useful 
when the signal level is quite high...like when you have another aircraft on 
your wing. :-)

Ken W7EKB