[Boatanchors] Re Why Twin Diodes In Xtal Sets?

Fri Feb 8 18:37:48 EST 2008

Thank you very much John! Great information sir.

Duane Fischer, W8DBF/WPE8CXO
dfischer at usol.com
HHI: Halligan's Hallicrafters International
http://www.w9wze.net
HHRP: Historic Halligan Radio Project
hhrp.w9wze.net

----- Original Message ----- 
From: "John Franke" <jmfranke at cox.net>
To: "Duane Fischer, W8DBF" <dfischer at usol.com>; 
<boatanchors at mailman.qth.net>
Sent: Friday, February 08, 2008 6:16 PM
Subject: Re: [Boatanchors] Re Why Twin Diodes In Xtal Sets?

> On page 5 of the Volume 3, No. 2, September 1993 issue of The Xtal Set 
> Society, John Collins submitted a very interesting note.  He indicated 
> that he had tried to parallel 4-1N82A's in an attempt to lower conduction 
> threshold.  There was a similar note on pages 3 and 4 of MRL (Modern Radio 
> Laboratories) Data Sheet Volume 2.  Ed. Peil of West Point, CA, noted 
> that, when driving a direct coupled transistor amplifier, wiring 2 diodes 
> in series increases selectivity and wiring 2 diodes in parallel increases 
> gain and reduces selectivity.
>
>
>
> Dealing first with Ed.'s note, it is easy to see how the series diode 
> connection increased the resistance of the detector and therefore reduced 
> loading on the tuned circuit  resulting in increased selectivity. 
> Similarly, it is easy to see how the paralleled diodes reduced the 
> detector resistance which increased loading and reduced selectivity.  The 
> reduced resistance also increased the circuit gain.  This increase in gain 
> was what was so interesting.  Maybe another way of looking at it is to say 
> the paralleled diodes have less loss.  I believe that this is what  John 
> Collins experienced as well.  When someone does something (like 
> paralleling diodes) and the result is good, it is worth further 
> investigation.
>
>
>
> Does paralleling diodes reduce the conduction threshold as John Collins 
> thought?  It depends on how you define conduction threshold.  If you 
> define it only by such solid-state physics terms as work function or 
> band-gap energy, the answer is no.  If however you define it as the 
> voltage necessary to generate a specific current, the answer is yes.  Let 
> me try to explain that last answer.  Lets say a diode is defined as being 
> above its conduction threshold when the diode current is equal to or 
> greater than 10 microamps. Lets further assume that you have a collection 
> of identical diodes that pass 6 microamps with a bias of 1 volt.  A single 
> diode would have a conduction threshold greater than 1 volt.  But, 2 
> diodes in parallel would have a conduction threshold less than 1 volt.  By 
> paralleling the diodes you have lowered the effective conduction 
> threshold.  But, have you really gained anything?
>
>
>
> It was interesting that the note from John Collins came to mind when I was 
> experimenting with some spiderweb coils.  I have a slightly modified MRL 
> model 2A tuner that I use as a base unit to compare other crystal sets or 
> components.  The set has a microamp meter so I can make relative 
> measurements to prove improvements (or losses) in efficiency for different 
> detectors, capacitors, coils, etc..  It was a simple matter to just 
> parallel the existing 1N277 detector with another and see if there was any 
> change.  I was quite surprised to see the detector current increase by 
> about 16%. Adding another diode increased the detector current by another 
> 10%.  It was hard to put solid numbers on the increases because of the 
> current variations caused by modulation.  So, I wired up a test circuit to 
> better evaluate the effect.  I loaded the detector with a 2.2K resistor to 
> simulate the load of a pair of high impedance headphones.  The radio 
> frequency source was a TTL crystal oscillator operating at 1.00 MHz.  A 50 
> microamp meter was used to measure the detector current.  The input level 
> was varied to produce a single diode detector current from 2 to 40 
> microamps in steps of 2 microamps.  At each step, the single diode was 
> paralleled first with one and then with two additional 1N277 diodes and 
> the detector currents noted.
>
>
>
> The percentage increase is greatest at the lower currents.  Also, adding 
> more diodes would probably produce even more of a gain but only marginally 
> so.
>
>
>
> This simple experiment does not tell the whole story.  What happens to the 
> selectivity?  Right now I cannot measure the selectivity, maybe by next 
> year?  Maybe someone else can try this experiment and go further and 
> include selectivity measurements.  I think it is a subject worth 
> investigating.
>
>
>
> I can supply the graphed data off-line.
>
>
>
> John  WA4WDL
>
>
>
> ----- Original Message ----- 
> From: "Duane Fischer, W8DBF" <dfischer at usol.com>
> To: <boatanchors at mailman.qth.net>
> Sent: Friday, February 08, 2008 6:08 PM
> Subject: [Boatanchors] Re Why Twin Diodes In Xtal Sets?
>
>
>>
>>
>> Hello All,
>>
>> I have noticed that some MW and SW crystal sets from the forties and 
>> fifties employ a pair of detectors instead of just one. Can you please 
>> explain to me the reasons why and how such a circuit would work?
>>
>> Is there 'really' a benefit to using two?
>>
>> Although I have never personally come upon plans for one, I was told by 
>> an older Ham that two diodes were used in crystal sets that received FM. 
>> Is this correct? If so, what role do two diodes play in FM signal 
>> detection specifically?
>>
>> Thank you.
>>
>> Duane W8DBF
>>
>> Duane Fischer, W8DBF/WPE8CXO
>> dfischer at usol.com
>>
>> HHI: Halligan's Hallicrafters International
>> http://www.w9wze.net
>>
>> HHRP: Historic Halligan Radio Project
>> hhrp.w9wze.net
>>
>> _______________________________________________
>>
>
>
> _______________________________________________
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