[Hallicrafters] RE: Black Beauties in SX-100

[Mark Bell]

> The bandwidth is widened by
> switching in predetermined values of resistance and capacitance to "spoil"
the "Q"
> and control the widening shape factor. Replacing these parts with
components
> having different characteristics will alter the original performance.

Good Afternoon Greg --

ER Magazine ran an article on this issue within the past year.   I followed
some discussion threads that followed the article's publication,  and one
group of thought maintained that while the analysis of the different
components (inductive, resistive, capacitive) was correct,  the practical
impact on selectivity would not be noticable, and the average operator
wouldn't notice.

To me, I think the main concern is the internal inductive reactance and
internal resistance between the BB's and modern caps.   Let L_BB be the
internal inductance of the BB capacitor,  and L_NEW be the internal
inductance of the replacement capacitor.

Now, inductive reactance is 2*Pi*f*L.  So the difference between the two
types of capacitors would be 2*Pi*f*L_BB - 2*Pi*f*L_NEW = 2*Pi*f*(L_BB -
L_NEW).    .

If one makes the assumption the L_BB >> L_New,  then the equation reduces
back down to 2*Pi*f*L_BB.
So, ignoring the 2*Pi term,  we have two terms, 50,000 cps (or 5e4)
multiplied by a value in, what, micorhenries  (1e-6),  which yields a term
on the order of 1e-2, or 0.01.ohms reactance..   If the internal inductance
was in pico or nano henries, the term is even smaller.    The same would
follow with the resistive component.

So, we're left with a net inductive reactance difference on the order of
0.01 ohms or smaller,  and a net internal resistance difference on the same
order.

Now, at 50 Khz,  an inductive reactance of 0.01 ohms yields an inductance of
about 3e-8, or 0.3e-9 henries, or about 0.0003 uH.

Again, that seem rather small to really impact the overall Q of a circuit.

Greg, I'm not saying you're wrong; in fact this email isn't really addressed
to you, but to the gang  I'm still out pondering this issue.  Just making
some thoughts to further discussion on this topic.   Comment, corrections,
more than welcomed.

73 Mark K3ZX