[ARC5] GO-9: Cathode Mod Questions

[D C _Mac_ Macdonald]

50 or more years ago, there was a QST article about using a vacuum tube in series with the cathode to ground lead to do cathode modulation.  I seem to remember that it used a 6Y6G and I built one to modulate a pair of 1625s or 807s.  It cut the "normal" amplifier tube cathode current to about half of the CW value.  I don't know anything about a GO-9, but if you choose a tube capable of about half the GO-9 power, this MAY give you satisfactory modulation.  I had no means (scope) in those poor high school student days, but that modulator simply plugged into the normal CW keying jack of the transmitter and seemed to give decent results until I was able to build a "real" plate modulator for my homebrew transmitter.

73 de Mac, K2GKK/5​
Since 30 Nov 1953​
Oklahoma City, OK​
USAF, Retired ('61-'81)​
FAA, Retired ('94-'10)​


________________________________
From: arc5-bounces at mailman.qth.net <arc5-bounces at mailman.qth.net> on behalf of David Stinson <arc5 at ix.netcom.com>
Sent: Wednesday, November 20, 2019 06:30
To: ARC-5 <arc5 at mailman.qth.net>; Boatanchors at mailman <boatanchors at mailman.qth.net>; milsurplus at mailman <milsurplus at mailman.qth.net>
Subject: [ARC5] GO-9: Cathode Mod Questions


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.

[cid:part1.56A7180D.C473ACC4 at ix.netcom.com]
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