[AMRadio] RE: negative cycle loading

Brett Gazdzinski brett.gazdzinski at mci.com
Sun Apr 4 23:51:20 EDT 2004

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
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
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
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

Thanks for any info,

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