[ARC5] Enhancing selectivity in R-26

[Kenneth G. Gordon]

On 15 Jan 2014 at 15:12, Martin Harpen wrote:

> Ken,
> 
>      I  just recently tried  the Lew McCoy mod for the BC-454. His
>      March 1960 QST article shows the modification to the 2nd IF
>      stage. In his article, that tube was a 12SK7.

Yes. Those were the early models. Yours must be either a BC-454B or 
maybe even an R-26/ARC-5.

> In mine, I have a
>      12SF7, which has a different pinout than the other tube.

Yes, it does. The 12SF7 has an additional internal diode which provides the 
AGC voltage for the rest of the radio. Since it is ahead of the BFO, there is 
no need to turn the AGC off when switching the BFO on.

> I made
>      the modification and it didn't do much at all. I could not get
>      regeneration. So I put it all back to original and I did the mod
>      on the 1st  IF tube, which is a 12SK7.

Even "back in the day" I thought that mod should be done on the 1st IF. After 
all, we want the maximum selectivity as close to the mixer as possible. That 
is where I have always installed it.

After some discussion with Jeep, I think that the reason Lew chose the 2nd 
IF is because there is AGC connected to the cathode of the 1st IF, but not to 
the 2nd IF in the BC-454 model. He didn't want to try to tell Novices how to 
handle that. Lew worked on the KISS principle, most of the time.

> It was a very simple mod
>      and it does make a rather striking improvement in selectivity.

Yes. I have found the same thing to be true, especially for the 3 - 6 MHz 
receivers (and also for the 6 - 9.1 MHz receivers). However, it does not do 
much at all for the MF or navigation receivers.

> I
>      was extremely happy with the result. The tuning on the
>      potentiometer can be a bit touchy, and occasionally it will go
>      one way or the other

That may have something to do with the stability of your power supply. I did 
not have that problem, and, in fact, was very pleased with its stability. I could 
set it right on the edge of oscillation and it would stay there for hours...as 
long as I didn't mess too much with the RF gain.

Have you added an AF gain control to your receiver? It also is extremely 
simple and is very easily reversible.

>  if you have it right on the edge (so to
>      speak) but it makes the receiver a lot easier to use when the
>      bands are crowded.

Well, it sure does for me. Glad to see others can achieve the same result.

> The McCoy article is entilted "The Poor Man's
>      Q-Multiplier" and I would recommend this to all for reading, even
>      if people decide not to give it a try. 73s to all---Marty, KK4RF

Jeep gave me the reference and I found it in my QST-on-CD collection.

I was not surprised to see that Lew also grounded the suppressor directly as 
I did, too.

In my case, I also removed the 1st IF stage from the AGC line, by simply 
connecting the 5K regen pot directly to ground. The stage goes into 
oscillation with the pot at about the 600 ohm point, so I felt that it wasn't 
really necessary to mess with the circuit in order to get the normal cathode 
resistor in series with the pot. Doing it this way also minimally disturbs the 
rest of the circuitry, making it far easier to back the mod out if desired.

Thanks for the info, Martin.

Ken W7EKB