[R-390] 3TF7 Ballast Tubes

[Randy and Sherry Guttery]

I'm sure that I'm going to get shot at for this - but - in 
the interest of completeness - I'm going to "stick it out 
there"...

On 2/19/2011 6:31 PM, Les Zwiebel WB6ORZ wrote:
>      It's comforting to know that the alternative installations probably yield the same amount of heat, so that a plate of kielbasa can be kept warm on top of any 390-A.
Actually - there is a way to drop that voltage *without* the 
heat...  I'm not necessarily recommending as a "fix" for 
R-390As - but it is an "option" for replacing "ballast" 
tubes / resistors in other radios.   Several of us also 
restore "vintage" and even "antique" radios.  While ballast 
tubes weren't commonly used - they are not exactly "rare" 
either.  And much more common (and much more serious to 
address) were radios with "curtain burner" power cords - 
which literally had a third "resistance" wire in the power 
cord to drop voltage to feed a series filament string.  Once 
AA5s (who's tube's filaments add up to line voltage) became 
common - curtain burners weren't made any more - but many 
examples survive today.  Obviously from the kick-name - no 
sane person is going to retain a curtain burner cord on a 
radio that will be used. The most obvious "fix" is to 
replace the resistance power cord with a power resistor.  
The problem often run into though - and a reason that wasn't 
done in the first place --  is because these are usually 
VERY compact radios - and putting that much "extra" heat 
inside can be not only hard on the other components - but a 
fire hazard as well.  Another solution is an "outboard" 
resistor - mounted in something like a "brick on a leash" 
power supply box - with adequate ventilation so as to not be 
a hazard.  A similar solution - one that reduces the heat 
but is still a "room" issue - is to use a "bucking" 
transformer.

There is, however - a much more elegant solution - a 
capacitor.  By correctly calculating the capacitance 
reactance - a capacitor can be used in place of the ballast 
tube - and while it will *not* provide any regulation (but 
then neither does a resistor - or using 2 12BA6s) - it drops 
the required 12.6 volts with nearly zero heat...  Granted 
that's a "savings" of a little less than 4 watts - but then 
again - go hold a 4W incandescent night light in your bare 
hand for a while...

just my .02  (and donning my flame-proof britches)....

-- 
randy guttery

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