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[Amps] Diode Bias Strings

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Subject: [Amps] Diode Bias Strings
From: (Marv Gonsior)
Date: Fri, 19 Jul 2002 14:09:03 -0700
>>>At 08:01 AM 7/19/02 -0700, Richard Measures wrote:
>>>>The Semtecc diode string in question appears to be a 10 diode 
>>>>unit.  No, it was a 10V unit, according to Henry and the Semtech p/n 
>>>>was their SA5534. So, now we are talking about an ~18 diode device as 
>>>>used in the Henry 3-K Classic X.
>>>> >I would like to ask the proponents of series rectifier diode strings if
>>>> >they have ever measured the developed bias between the minimum and full
>>>> >power conditions?
>>>>Yes.  Please tell us about the 5408, 
>>>>or?  Previously, when I asked you for test data on your recommended 
>>>>diode biasing , your response was that I "didn't need no stinkin' 
>>>>data."  Why now?
>>>> >I've done it with a Henry 3-K Classic X, using and 8877,
>>>> >and the bias soared about 6V from zero to full signal.
>>>>V sounded a bit high,  You bet it was high and you can't compare a 
>>>>Semtech assembly with a sample of one 5408.  BTW, the early Henry 2-Ks 
>>>>used a 50W, 6V Zener.
>>>>so this morning I tested a 1n5408/3A diode for fwd
>>>>V-drop using 8877-sized currents, assuming a ZSAC of 0.1A and a signal
>>>>max Ik of c. 1.2A.  At 0.1A, the V-drop was 0.734v.  At 1.2A, the V-drop
>>>>was 0.813V.   Thus, with a 10-diode string, the change in bias V for an
>>>>8877 would be 0.79V from zero-sig to max-sig.   For improved IMD, and
>>>>less chance of overdrive with more than 78w drive, add a few ohms in
>>>>series with the diode-string to increase V-drop at max sig.
>>>> >Read that power limiting that must result in additional distortion 
>>>> products.
>>>>A diode has more V-drop as current increases.  A resistor has even more
>>>>V-drop as current increases.  A resistor can be added in series with  the
>>>>cathode to create RF-NFB.  (which I read in Svetlana, Eimac, and RCA 
>>>>- Marv - Does more RF-NFB increase or decrease IMD?  We all know the 
>>>>answer to that one but you have to know the net effect of the ratio of 
>>>>the biasing distortion to the negative feedback afforded by the resistor.
>>>> > I couldn't wait to dump Henry's Semtech diode pack and put in a 
>>>> 50W,12V Zener.  IMHO,
>>>>Humble ?  Yes, humble and it is not obvious that you know much about that.
>>>>    there is nothing worse than diode strings as a biasing element.
>>>> > Further, just ask the G-land 6M ops what they think of the bricks
>>>> >that find their way into cluttering up the band with relatively high 
>>>> levels
>>>>of splatter resulting from diode bias.  Ian et al, are you listening?
>>>>The red herring. Now, don't tell me that you've gotten into fishing?
>>>Marv, W6FR

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