[ARC5] nbfm detector (6BN6 - a good valve)

[Leslie Smith]

Hello Ian & group,
I found your comment that the 6BN6 might operate well as a regenerative detector.  The basis for your idea is that the tube has high gain at low plate current.  I would be most interested to hear from anyone who builds or has built a regen circuit using this tube, or a tube with a similar high gain.

I used to believe that high gain and low C/high L tuned circuit made good regen detectors.  The idea of high L as a good regen system is known to be wrong.  (See comments in the RegenRx group for comment on this point.)  I wondered why this was so, and found what I believe is the answer in an article by RAMON VARGAS on the Lambda oscillator.  As a side-note in that article he urges us to build the Lambda circuit and plot the negative resistance characteristic.  I did that, and got a result similar his graph.  (See : Oscillations and Regenerative Amplification using Negative Resistance Devices by Ramon Vargas.  This is a PDF file Sorry, but I can't find a link to that article, but Google found the item for me, one of the early items in the list.)

Try:  http://www.zen22142.zen.co.uk/Theory/neg_resistance/negres.htm

Looking at Ramon's graphs, I believe the broadness of the nose (as the point of zero resistance is approached from either side in the graph) is the point of interest.  The broadness of the nose determines the ease with with the set will stay "on the edge" of regeneration.  I believe that high gain valves, circuits, tank circuits - anything that makes the nose "sharp" defeats the ability to keep the regen receiver "on the edge" of regeneration.  In other words the set is "hot", but difficult to manage.  If the nose is "blunt" the point of regeneration will be stable, because it's more difficult to slide away from the point of zero resistance.

This is an interesting point for a number of reasons.  First: it explains why many builders prefer "low C" tank circuits.  Secondly it is counter-intuitive, and that is an interesting point.  

This is an idea without an experimental basis, and I will be interested to hear from anyone who has measured the factors I describe above.  

Now, to get on topic.  I own a number of badly "hacked" R-25 sets.  Missing covers, missing IF transformers & co.  I would be most interested to pull out the 12SR7 and plug in an octal "replacement" (valve) built around Mr. Vargas' amazing cct.  At 1415kHz a bit of regeneration would sharpen that receiver's selectivity a good deal, and with care I think no changes in the circuit need be made.  The set would become useful as a receiver for use on the 80m band, in today's conditions.  A dream at the moment, but worth exploring.

I end - and end on topic!

Les Smith,
Formerly VK2BCU.



> ----- Original Message -----
> From: Ian Wilson <ianmwilson73 at gmail.com>
> To: "Discussion of AN/ARC-5 military radio equipment." <arc5 at mailman.qth.net>
> Subject: Re: [ARC5] nbfm detector
> Date: Mon, 7 Jun 2010 14:58:04 -0700
> 
> 
> The 6BN6 is a rather undervalued tube. It works very well as a product
> detector, with
> a (voltage) conversion gain in the double digits (article in the SSB manual,
> came
> originally from QST; I forget the exact title, something about Receiver
> Improvements).
> 
> *I think it would work well as a regenerative detector also.*
> *
> *
> *The key to the high gain (in both cases) is the tiny plate current - about
> 0.45mA. This*
> *lets you use a high plate load resistor and realize the available voltage
> gain. Much*
> *easier than using an audio choke or matching transformer.*
> *
> *
> *The 6BN6 is also dirt cheap, turning up frequently in box-o-tubes
> (boxes-o-tube??).*
> *
> *
> *73, ian K3IMW*
> *
> *
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