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


W7QHO at aol.com W7QHO at aol.com
Tue Jan 10 21:46:46 EST 2006


In a message dated 1/10/06 6:37:54 AM, Brett.gazdzinski at mci.com writes:


> I have used the 3 diode, power supply (variac) and load
> resistor setup on various rigs, the first one was the
> pair of 813's modulated by a pair of 4x150 (4cx250b) tubes
> in AB1.
> The circuit worked very well on the mod monitor and scope, and
> I could get ungodly amounts of positive modulation.
> Like 2500 to 3000 watts pep out of a 600 watt carrier, without
> any audible distortion....
> 
> 
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.   

More simple schemes can also give some useful benefit, ex., using a weak tube 
on the side of a P-P modulator circuit that conducts on the negative half of 
the audio cycle.   Other cheap and dirty tricks include lowering the screen 
voltage on the same side or inserting a resistor in series with the plate.   Can 
end up sounding bloody awful, though, if not done carefully and with 
restraint.


> I suspect the high voltage brick diodes I use have some sort of problem,
> switching speed? that caused very bad splatter when I would exceed
> 100% neg modulation. I saw no signs of it on the scope or mod monitor,
> but it showed up on the spectrum analyzer.
> 
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.

Dennis D. W7QHO
Glendale, CA



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