[AMRadio] RE: negative cycle loading

W7QHO at aol.com W7QHO at aol.com
Mon Apr 5 18:51:17 EDT 2004


Sounds to me like you're hard clipping the negative peaks, either in your 
negative loading setup and/or by driving the PA down below zero.    Hard to be 
definitive here, though, not knowing exactly what circuit you're using.   The 
"three diode" circuit I'm familiar with doesn't use an additional power supply 
(see http://www.amwindow.org/tech/htm/3diode.htm).   If adjusted properly, this 
circuit will allow modulation percentages in excess of 100 on positive peaks 
without overmodulation (i.e., more that 100%) in the negative direction.   
Does distort the speech waveform to be sure but does not produce spurious 
products out 30 kHz or more.   It's also possible to misadjust this circuit to 
splatter like hell.

Dennis D. W7QHO
Glendale, CA

> -----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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