[ARC5] GO-9: Cathode Mod Questions

[Ian Wilson]

I have a small stash of 6Y6's here. Interesting tube - high current at low
voltage.
Free for postage for the first few who reply (I no longer think these are
going to pay my
doctors' bills, hi).

73, ian K3IMW

On Wed, Nov 20, 2019 at 4:31 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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