[ARC5] GO-9: Cathode Mod Questions

[David Stinson]

Good Morning.  The GO-9 (a.k.a. "The Snake") is making regular AM 
contacts on 75 meters.  I am using Cathode modulation because Suppressor 
or Screen modulation will require chopping, drilling and hacking on the 
HF Transmitter deck.  I am unwilling to do that.  The only 
"modification" to the HF Deck now is to lift one ground and add one 
wire.  Want to keep it that way.  Cathode modulation can be done 
entirely in the home-brew power supply.

A 20-Watt PA amp with output taps of 4-8-16 Ohms and 70-Volt Line.  This 
is feeding a Thordarson T-22S83 15W Line-to-Voice Coil transformer.  The 
transformer secondary side has 4-8-16 Ohm taps. The primary has 
2000-1500-1000-500 Ohm taps.   The PA amp 4-Ohm output is connected to 
the transformer 4-Ohm tap.  The transformer 2000-ohm winding is inserted 
in the 803 PA's cathode circuit at the filament transformer's center-tap 
connection to ground.  See the attached graphic.  The transformer is 
passing about 200 mA at no modulation.   With the transmitter B+ set to 
provide 100W carrier out, this provides about 80% modulation before 
flat-topping and distortion.  Reducing the output power to 60w or so 
will get it near 100%.  On-air audio reports have been good. The 
transformer does not significantly heat-up, even on long transmissions.

I have attempted to use a couple of "actual" modulation and "audio 
output" transformers, using "cut and try" methods.  None of these would 
do better than 50% modulation before flat-topping and distortion, 
probably at least in part due to impedance mismatches.  They sound crummy.

I don't know much about the "right way" to do Cathode Modulation.  How 
does one determine the "correct" impedance transform at this point, how 
much audio power is actually needed, etc?  Who's the expert?

TNX OM DE Dave AB5S
P.S. If mailman would allow Bcc, it could cut-down on duplicate mail.

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