[JMS] Fwd: Variarm

[Wilson Lamb]

Spurious bits re my recent Variarm work, just in case anyone can use them. 
WL 
  
  

-----Original Message-----

From: Wilson <infomet at embarqmail.com>
To: tom <tom at telmore.com>
Date: Sunday, 2 January 2022 3:16 AM GMT
Subject: Re: Variarm

   
Good.  You don't need the caps. 
There was originally a "coupling unit," which you can see on the schematic. 
It's just a parallel tuned circuit lnk coupled to the Variarm link, 
Then the coil steps up the voltage to drivr a grid. 
There's a cap in series with the line to the grid, to avoid shorting out the bias, as stated in the literature. 
You have the 160m version, so you have to short the tank coil to get to 80m.  That's what the tap is for. 
I had to add 20pF to the tank in mine, to get it resonant on 80, but you'll have to experiment. 
You can tell when you have it right, because the tank cap will peak the output and not be at either max or min capacity. 
Your filament cap looks good.  I started testing by using 175 Ohms in place of the curtain burner resistor.  Worked OK, but made a lot of heat. 
Once it was working, I did away with dropping arrangements changing to higher filament voltage tubes; 50Y6, 50L6, 12K7. 
That woeked great, but changing the osc tube is a LOT of work, since you have to take the tuning box off, demount the 6K7 and install the 12K7, then reassemble. 
It's a great fix, but there's no real reason to do it if your filament setup is working. 
I get a solid 2W on fundamental and 1W doubling. 
I hooked it up to a tuner and worked IN, PA SC, VA, NY, from NC, with no problem.  Some of the reports were quite strong and none were very weak! 
The fuse isn't on the schematic, so who knows.  Mine is miswired.  The hot AC in (black) should go to the back end terminal, but it's not a real big deal. 
With a 3 wire cord, properly installed, I think it's usable.  Be sure green gets a solid ground to chassis.  I use old computer cords for this, the smallest gauge you can find is fine for the Variarm. 
If the line bypass fails, it will likely blow, but not damage anything. 
The coupling cap in mine was an old Micamold, which was a little leaky and causing the output tube to draw too much current.  That's the only cap I removed. 
My filter caps are likely pretty dried out, but don't get hot, just leave an audible ripple on the signal. 
I patched a new one across the final section and all is well.  I don't want to work on it anymore until I have to. 
I changed the rubber shock mounts on mine, using stacks of grommets, but they are too stff and a bear to get it all together. 
Check your filament voltages.  Mine were way off, because of tube resistance variation, but I did some selection, using spare tubes, and got them very close. 
You can't change just the rectifier and final tubes, because the 50V tubes draw only 150mA, instead of the 300ma drawn by the 6K7.  12K7 is a 150mA tube, but only comes in glass. 
The note and stability are quite good and very satisfactory for operation in normal use. 
I don't think the coax connector is original, but there's no reason not to use it. 
I think the original setup had the coupling unit hard wired to the Variarm tank.  My unit has original looking coax soldered in, about a yard long. 
Think about the function of the bias resistor on the back.  It sets the cathode bias of the final so it draws the same current on tx and rx, to provide B+ regulation. 
You set it by watchng the B+, keyed and unkeyed. 
I had some useful help from members, so I'm glad to pass it on. 
Let me know if anything else comes up. 
73, 
Wilson 
W4BOH 
  
  
  
   
From: Tom <tom at telmore.com>
To: 'Wilson <infomet at embarqmail.com>
Date: Sunday, 2 January 2022 1:00 AM GMT
Subject: RE: Variarm

  Wilson, Thank you very much for sending the information along. I attached a picture of what my VFO L2, L3 coils appeared to be wired like. Someone added capacitors to the RF coil connected to the SO-239 connector and also I had live B+ grounded to the floating chassis. What it turns out is L3 had shorted to L2 and I think someone added the caps as a quick fix. I have since removed the coil assembly and removed the bad winding it is only about 7 turns and now I will have a both coils isolated from each other.   Tom KA1NVZ   
  From: Wilson Lamb [mailto:infomet at embarqmail.com] 
Sent: Saturday, January 01, 2022 8:50 AM
To: tom
Subject: Variarm     
 
 Hi Tom,  
 I'm going to send my files on the Variarm.  
 When you have looked at them, we can talk.  
 I just got mine going.  It's the 80m version.  
 Wilson  
 W4BOH