[ARC5] Question on tube substituions...

[Kenneth G. Gordon]

And here is another, more or less academic, question (which I proposed 
earlier, but which got lost in an earlier discussion).

When one substitutes the 12SG7 for the 12SK7 in the RF and IF stages of 
our favorite receivers, due to the higher transconductance, the result (at least 
for the higher frequencies) is noticably reduced internally generated noise.

However, since the characteristics of the two tubes are not identical, other 
effects appear, such as, when the 12SG7 is used at the RF, the antenna 
trimmer peaks outside of its range.

I am not suggesting making any changes to the ARC-5 receivers, but this 
"tube substitution mod" is sometimes used with other vintage receivers 
where improvements are sometimes badly needed.

In those receivers, minor changes in biasing, etc., would seem to me to 
restore proper operation.

In point of fact, the 12SK7 does a remarkably good job up through 10 meters 
in my experience, and there is zero improvement at MF or even up through 7 
MHz when trying the 12SG7.

Your thoughts?

Ken W7EKB