[ARC5] GO-9: Cathode Mod Questions

[J Mcvey]

 I'm certainly NOT the expert on this one, but I have a few thoughts on the subject.
The more impedance and resistance that you introduce in the cathode/filament circuit, the more yo will impact the Final's ac/dc load lines.Also, the DC current flowing through the transformer will tend to magnetize and cause the the transformer to saturate at less power than it might otherwise.So, a beefier oversized transformer might do better.
Also,  did you try using  the line voltage side to connect to the audio amp and insert the low Z side of the line transformer in the filament circuit  just like the line transformer was intended to work? 
Then, it will be able to deliver more current and have less impact on the Final load line.


    On Wednesday, November 20, 2019, 7:31:00 AM EST, David Stinson <arc5 at ix.netcom.com> wrote:  
 
   
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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