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

[Clare Owens]

Hey Dave,

For many years I have been hiding a little cathode modulator (I think)
behind one of my 3 receiver racks.  Just dragged it out and it seems to
follow the general layout of the Sept. '53 QST article but has two 6Y6
tubes.  The article is at:
http://www.w7ekb.com/glowbugs/projects/SimplestModulator.pdf

Anyhow, if you'd like to try it or just have it I'd be glad yo pack it up
and send it to you.

Take care,

Clare  N2RJB


On Wed, Nov 20, 2019 at 7:30 AM 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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