[Milsurplus] ARR-15 selectivity

[B. Smith]

Maybe in some cases use a "gimmick"
k4che

On 11/6/2023 12:42 AM, Jim Whartenby via Milsurplus wrote:
> Hue
> 5pF in series with 3pF gives 2.2pF.  I just figured that it is easiest 
> to buy one value of silver mica capacitor and series it with what 
> Collins originally installed, that is why those values are in *bold* 
> type.  The new calculated bandpass is 7.5kc about half of the original 
> calculated bandpass of 15.5kc
>
> BW / fo = k = Cm / ( C1 x C2) ^ 0.5, this formula comes from the 
> Radiotron Designers Handbook aka RDH4th.
>
> Raising the product of (C1 X C2) to the half power is to take the 
> square root.  I don't know where the "square root" key is on this 
> keyboard so this notation was easier for me. <grin>  Everything needed 
> to select your own coupling capacitors is on the left side of the page.
>
> Enclosed is the analysis that Hue mentioned.
> Regards,
> Jim
>
>
> Logic: Method used to arrive at the wrong conclusion, with confidence. 
> Murphy
>
>
> On Sunday, November 5, 2023 at 07:32:12 PM CST, Hubert Miller 
> <kargo_cult at msn.com> wrote:
>
>
> This subject comes up again for me as i try to order my old emails.
> Sorry, i may have missed something, but in Jim's article, what does 
> this mean, for coupling cap, like "5+3 pF".
> Surely it does not mean, really, 8 pF ?
> Unfortunately my RDH book is hiding from me right now, but offhand i 
> don't understand how you can calculate
> selectivity from the coupling factor without knowing Q. Looking at the 
> graph from the ARR-15 manual, i have to
> guesstimate overall Q as 500 kHz / 7 kHz, my crude eyeballing of the 
> not highly defined graph seeing -3 db at
> about -7 kHz. The Q is affected by coupling, of course, but also the 
> Rp of the tube plate ( ~ 250 - 350 k ohm ?? )
> and the following tube grid. Those are probably not decisive factors 
> compared to the actual IF coil Q.
>
> After seeing Dave Stinson's impressive  vids of the BC-229 receiver 
> with the adjustable feedback, i am giving that
> kink a rethink.  I tried that kink decades ago in a Command Set 
> receiver but discarded it as near worthless.
> I cannot recall if i disabled the AGC and i'm thinking i did not. So 
> maybe if that is done, this idea is actually worthwhile ?
> On the other hand, i recall trying it in such receivers as the 
> Heathkit GR-64 and so on, where they had a regenerative
> IF by lifting grid 3 above ground via a pot, and i was not at all 
> impressed with its working.
> I recall that Roy Paffenburg booklet on the ARC-2 used the regen IF 
> idea to get more selectivity.
>
> -Hue Miller
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