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

Brett gazdzinski Brett.gazdzinski at mci.com
Tue Jan 10 09:32:18 EST 2006

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 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.

I also put the circuit in the push pull 812a rig, and the pair of 4d32
rf deck, with the same results.

Unless you are running a mod transformer within an inch of 
its life, I don't see the need to protect it that way.
I insulate the mod iron from ground, and never had problems.

I once had a qso with someone (cant remember who) who told 
me the circuit was no good, but would work with the 
addition of another diode...I think that is what he said...

I suppose the solid state 14kv 1 amp bricks just can not react
fast enough and let a real short pulse through?

In the old days, they used tube type rectifiers, and they
may have worked better, just a guess...

I thought about trying to add a (very high voltage) capacitor
someplace in the circuit to damp out any spikes, 
but never looked into it.

I now tend to think limiting the audio in the low level
stages and not exceeding 90% negative modulation is the cleanest
way to go.


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