[Amps] Diode Bias Strings

[2]

The Semtecc diode string in question appears to be a 10 diode unit.  

>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. 

>I've done it with a Henry 3K Classic X, using and 8877, 
>and the bias soared about 6V from zero to full signal.  

6V sounded a bit high, 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 
pubs).  
- Marv - Does more RF-NFB increase or decrease IMD?

> I couldn't 
>wait to dump Henry's Semtec diode pack and put in a 50W,12V Zener.  IMHO, 

Humble ?

>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 

The red herring.

>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?
>
>73,
>
>Marv, W6FR

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
www.vcnet.com/measures.  
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