Quote:
Originally Posted by Electronic M
So looking at the datasheets for the tubes you can think of the tube heaters as a single resistor that is supposed to have 75.2V across it and 0.3A running through it.
One bit of information you didn't include is what voltage dial lamps you have 6.3V, 2.5V or 12.6V.... I'm going to assume 6.3V for my math.
So if the tube heaters are 0.3A=300mA and the dial lamps are 250mA we need the resistors in parallel with the dial lamps to give the extra 50mA somewhere to go so it doesn't burn out the bulbs. R=V/A so the resistance of the 2 resistors in parallel with the dial lamps is 6.3V/0.05A=126 Ohm's...Those resistors need to be rated at P=VA or 6.3V x 0.05A= 0.315W so half watt resistors would work but 1W would run cooler.
That still doesn't account for the resistor between the tubes and dial lamp....To find that guy we can say that the tubes and dial lamps (with parallel resistance previously calculated) drop 87.8V and thus we need to drop 117V-87.8V= 29.2V across that resistor and flow 0.3A through it so by previous formulas it should be a 97.3 Ohm resistor at 8.76W (I'd use a 15-20W resistor).
If you want a soft start like the original ballast I would go with a CL-90 or similar thermistor in series with the power cord feeding the radio, or use a capacitor in place of the 97 Ohm resistor. That film cap would need to be about 27uF....a film cap would be preferable, but a motor run cap, a non-polar lytic or a couple polar lytics of double value connected in series with the polarity of one reversed (as in connect the cap positives together then connect one negative lead to the dial lamps and the other dial lamp to the tubes) would work.
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Sorry Tom! I crunched the numbers twice and I come up with 18.7uF or so.
Anyway, I would use the dropper cap without hesitation! Using a cap has a cushioning effect on the inrush current, so you don't need any other component.