[JMS] Variarm Update

[Tom Elmore]

In case anyone is interested I ran the numbers for replacing the stock resistance cord with a capacitor surge resistor combination. Assuming you have the original tubes (6K7, 25L6, 25Z6)  and 120 volts as the primary supply you will need a 7.94uf (8uf should work) Non Polarized cap I used 600 watt caps) and a 33 ohm 3 watt surge resistor in series to replace the line cord. I did this recently for a pair of 1940's Mon-Key electronic keyers paying close attention to keeping the neutral at ground level with a polarized plug for added safety 

Tom KA1NVZ

> On November 8, 2021 9:06 AM Wilson Lamb via James_Millen_Society <james_millen_society at mailman.qth.net> wrote:
> 
>  
> I've been pleased to get a couple of Variarm people shaken out of the woods of the reflector.
> It's always nice to experience the thrill of breaking into a new project and turning over some new rocks to find unexpected technical pearls of wisdom!
> It's incredible that a perfectionist like Millen would choose to commercialize something so basically flawed as the AC-DC Variarm, when he could easily have added a little power transformer.  I expect he was swayed by the typical ham's famously tight budget and the relatively high cost of early components, all containing large amounts of handwork.  We live in such relative affluence, surrounded by such cheap commodity electronics, that we have lost touch with early reality.  Of course we "could" live with the AC-DC setup, now that we have three wire cords and can, more or less, guarantee that the chassis will be connected to ground.  Enough philosophy!
> 
> My curtain burner resistor cord had an open resistor, so I have converted to higher filament voltage tubes: 50Y6, 50L6, 12K7.  Dealing with the very hot 175 Ohm resistor I haywired in to get started without the curtain burner cord was more trouble than I wanted.  I had to select tubes to get reasonable filament voltage on all of them.  BEWARE, not all 50-75 yr old tubes have exactly their rated filament resistance!  19V was way too much for the 12K7, although it "worked' fine.
> Changing the osc tube is a REAL PITA!  My rubber shock mounts were shot, so I've stacked up some grommets in their place.  Too stiff, but usable.
> I had to reconnect the bandspread cap operated by the arm.  Who knows why it was disconnected!
> The electrolytics are "good," if not exploding is good, but I think the filter cap is dried out.  I could hear AC modulation of the note, so tried jumping a new cap across the B+
> That cleaned up the note.
> Hooking up an iso xfmr allowed me to ground the chassis and not be afraid to connect my scope to B- without fear.
> I'm not sure why, but the xfmr is dropping the AC to 95V, but that's no problem. 
> I noticed a tiny backwave, with the key open, caused by a leaky Micamold cap across the key jack.
> The most interesting part...I have a little dummy load/power meter, 15W full scale, that has an expanded scale at the low end.  I was running about 1/2W output, but noticed that when I connected my scope probe to the output amp plate that the output power went up, way up, like DOUBLE.  With usual perception, I deduced that the output tank was not finding resonance, which proved correct.  It took a few trial and errors, but about 20pF of fixed cap brought resonance within range of the tuning cap in the output tank!  Very exciting, getting about 1.2W, even at the reduced 95VAC supply!  Who knows what it will be with a 120V supply.
> 
> My life is busy right now and I should never have started a new project, but I've always wondered about the Variarm and now I know!
> I'd "like" to convert to an external pwr xfmr and 6V tubes, but likely won't.  I'll likely just leave things "original" and try the Variarm in the next CX contest.
> Thanks to Don Buska for getting me signed up for the reflector and to the guys on antiqueradios.com who provided tubes.  That's a great group, so check it out if you need tubes or components.
> 73,
> Wilson
> W4BOH
>  
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Thank You
Tom Elmore
Anchorage, Alaska