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