[ARC5] Question on tube substituions...

[Dennis Monticelli]

Ken,

It is my belief that tube subs at RF frequencies and in any stage that is
AGC controlled is not to be taken lightly.  Any change in absolute gain or
gain characteristics vs grid voltage is going to alter the gain
distribution which in almost all cases is not favorable....if we assume the
orginal designer carefully adjusted the overall gain distribution for
optimum performance (and I know you respect the original designers).  Of
course any change in capacitance requires some recalibrating for proper
tracking.

As for noise, usually the culprit in receivers of that vintage is the
pentagrid converter.  Any RF tube ahead of it will be quieter than a mixer
of that type so the mixer noise will be reduced by the RF stage gain as
reflected back to the input.  A lower noise RF tube is better still,
assuming its AGC characteristics are close enough to original or otherwise
adjusted by AGC voltage dividers to have about the same effect.  A higher
gain RF tube might help, but probably not by a lot because it doesn't take
that much gain ahead of the mixer to suppress the input reflected noise of
the mixer.

Excessive gain in the IF is not a good idea as it could lead to instability
and wideband hiss...again assuming that the original designer make a well
informed choice of IF gain in the first place.

Dennis AE6C
On Sat, Oct 18, 2014 at 8:39 AM, Kenneth G. Gordon <kgordon2006 at frontier.com
> wrote:

> And here is another, more or less academic, question (which I proposed
> earliear, 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
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