[ARC5] [BoatAnchors] GO-9: Cathode Mod Questions

[Scott Robinson]

Hi Dave,

Hi Dave,

You report that the cathode current is 200 mA. IF the DC grid bias is, 
say, 50V, then you need to drive perhaps 100 v P-P and 400 mA P-P 
current. So:

100/0.4 = 250 ohns.

Another way to look at this is that the impedance looking into the 
cathode is 1/Gm. In this case Gm = 4000 uMhos, so

A cathode = 1/4000 x 10**6 = 1000* 1/4 =250 ohms

Similar results from different methods suggest that this is about right. 
Try the 500 ohm tap; the smaller number of turns (half as many) will 
reduce the effect of the DC on the transformer, and probably make the 
modulation distortion less, particularly at low frequencies.

Also,required power looks to be about 100/2.828 volts times 0.4 x 
1/2.828 amps = only abuot 5 watts.

I hope this is helpful.

Peace,

Scott

On 11/20/19 4:30 AM, David Stinson via BoatAnchors 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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