[AMRadio] RE: negative cycle loading

Brett Gazdzinski brett.gazdzinski at mci.com
Mon Apr 5 09:13:19 EDT 2004

Yes, I am well aware of the stuff in the link you posted.

The problem is, the NCL circuit SHOULD work well, but does not.
Maybe its the voltage drop through the high voltage bricks, or rather
the voltage it takes for them to conduct.
On the bricks, its volts, maybe as high as 5 volts before they conduct.
Or maybe they are just too slow for higher frequency audio stuff.

Since the circuit works as far as the mod monitor indicates,
there must be something very fast making it through the circuit.
If I could just find a way to eliminate the short spikes
of over 100% negative making it through, it would work great.

My voice does not show a huge difference between positive
and negative, maybe 10%?

I suppose a low level method would work best, but like the idea
of a circuit in the RF deck that prevents negative overmod no matter 
what I do.

Looking at the circuit, maybe I will try some things, like
returning the load resistor to ground instead of the power supply,
putting a cap across some of the diodes to smooth things, etc.


> -----Original Message-----
> From: amradio-admin at mailman.qth.net
> [mailto:amradio-admin at mailman.qth.net]On Behalf Of John Coleman, ARS
> Sent: Monday, April 05, 2004 8:43 AM
> To: amradio at mailman.qth.net
> Subject: RE: [AMRadio] RE: negative cycle loading
> Hi Bret:
> 	Many of us have played with neg cycle loading to find the same
> thing that you have found.  You would need a low pass filter after the
> limiting circuit but these things make the audio have 
> resonate peaks and
> limited freq response.  (poor sounding)
> 	If you have good freq response in the lows then your audio will
> probably be lopsided anyway, especially if you talk close to the mic.
> 	SEE - http://www.qsl.net/wa5bxo/asyam/aam3.html
> Good luck, 
> John, WA5BXO
> -----Original Message-----
> From: amradio-admin at mailman.qth.net
> [mailto:amradio-admin at mailman.qth.net] On Behalf Of Brett Gazdzinski
> Sent: Sunday, April 04, 2004 10:51 PM
> To: amradio at mailman.qth.net
> Subject: [AMRadio] RE: negative cycle loading
> Hello all AM,ers.
> I was running the new 4d32 rf deck on the air over the weekend, and
> it works very well.
> I 'stole' the 811a modulator from the 812a rig to modulate
> the 4d32 rig till I get the kt90 mod deck built.
> After many and various tests of different voltage and currents on the
> plates
> and screens, etc, it seems to like running at 1100 volts and 
> 300ma, for
> about
> 220 watts out, although I can get 300 watts out.
> It modulates well, up to 900 watts pep.
> But my negative cycle loading setup seems to introduce spurs out the
> sides
> when it comes into play.
> I looked at it on the spectrum analyzer.
> I use the 3 diode, resistor, and separate power supply that sets the
> negative
> percentage (using a variac).
> I once talked with someone on the air about this circuit, and 
> they told
> me
> it had a design flaw, and needed something, another diode, 
> resistor, or
> something, but I cant remember what they said, or who said it.
> I use the 1 amp 14,000 volt diode bricks in the circuit, and maybe
> something to do with switching speeds, bias, or something, but when I
> invoke
> the circuit, I get big spurs out the sides, out to 30kc each side 40
> db down.
> The mod monitor says the modulation is getting limited to 90%
> or whatever I set it to, but running it is even worse than over
> modulating.
> I use this circuit in a number of rigs with the same results.
> The audio is clean, and I even added some resistors to the 
> plates of the
> 811a
> mod tubes, grids are swamped, adjusting the various voltages and
> drives on the 4d32 rig has zero effect on the spurs.
> Sure would like to get the circuit to work well, as I can run 
> the audio
> up much more and load the negative spikes.
> I don't want to run into it heavy, just trim some peaks off 
> the negative
> side.
> Thanks for any info,
> Brett
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