[R-390] Variacs and solas: additional thoughts

[Bob Camp]

Hi

Here's a little better explanation than my last one.

The filament transformer runs 0.3 amps on the primary when it is  
running 3 amps on the secondary. A Variac would be running 3 amps the  
way most of them are metered.

There are also more than just the 10% and 5% settings. Five percent  
comes when you use the center tap of the transformer and the primary  
is on the incoming line. Ten percent shows up when you use the full  
secondary and the primary is on the incoming line. To get a few more  
variations you can hook the primary of the transformer on the output  
line side. It's a little early for heavy lifting math but I think  
that would give you a 6% and 11% setting (or 4% and 9% depending on  
the connection).

If you are going to set up something like this an external box is  
probably the best way to go. A "splice in an extension cord" approach  
is not a bad idea. More or less you would have a R390 with a wall  
wart. The transformer and fuses need to mount in a metal box. If you  
are setting up a metal box I would put a couple of surge suppressor  
MOV's in there as well. For the deluxe version you could add one of  
the current limiter thermistor.

The MOV's are an overlooked aspect of protecting an R390. As amazing  
as it sounds your normal line voltage can spike to levels that will  
damage an R390. The RFI filter on the back panel of the radio will  
take care of a lot of spikes. The problem is that a lot of radios  
have filters that are a bit over the hill.

     Enjoy!

         Bob Camp
         KB8TQ



On May 1, 2005, at 9:39 PM, Barry wrote:

> I was thinking the amperage rating of the bucking transformer was
> proportional to the total voltage.  In other words, a 3-amp. 12V  
> bucking
> transformer would be good for approximately 30A total load at  
> 120V.  Is that
> incorrect?
>
> Barry - N4BUQ
>
>
>
>
>> Hi
>>
>> For a whopping $10 Radio Shack will sell you a three amp 12 volt
>> center tapped filament transformer. The nice thing about the center
>> tap is that it will give you both a 5% and a 10% adjustment off of a
>> nominal 120 volt line. Three amps is enough to run any radio in the
>> R390 size range.
>>
>> Given that the radio works better off of a high line voltage it's not
>> real clear weather you should spend the $10 or not. As mentioned in
>> the previous thread using a fuse or two is probably a good idea.
>>
>>      Take Care
>>
>>          Bob Camp
>>          KB8TQ
>>
>
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