[AMRadio] Negative Loading circuits - good, bad, or ?

Brett gazdzinski Brett.gazdzinski at mci.com
Wed Jan 11 08:22:49 EST 2006


> I've had good results with the "three diode" circuit too.   Earliest 
> reference to the scheme I can find is in QST, October 1956 
> using 866 rectifiers.   
> Covered again in ER #3, July 1989, this time with solid state 
> diodes.   

I did not say I had good luck with the circuit, although it looks
like it works on the scope, and on the mod monitor, it results
in a very wide signal. Not much point in running it if you go
50Kc wide...

> Don't believe the type of diodes used would have any 
> significant effect on 
> this.   Splatter is generated in the PA tank circuit when 
> plate voltage is 
> suddenly cut off   on the audio negative half cycle.   Same 
> splatter would be 
> produced if the PA was being fed straight off the secondary 
> of the mod transformer.

The 3 diode circuit is supposed to prevent the plate voltage
from going to zero. I use variacs on the power supply so I can set
the point at which the circuit starts working, and no matter if I
set it to 95, 90, or 85% I get splatter if the audio would exceed
100% negative, so the circuit seems to do no good.

It LOOKS like it works, I get current in the negative cycle loading
circuit, mod monitor shows the limiting working, scope looks ok,
the signal just gets REAL wide, no matter what rig I run.

Everyone would assume it works, I did, till I got reports I was real wide
and dug out the spectrum analyzer...

Perhaps the very high frequency stuff gets past the circuit?
I should run some tests...


> Dennis D. W7QHO
> Glendale, CA

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