Pavek Museum


		3517 Raleigh Avenue. St. Louis Park, Minnesota 55416 Phone 952/926-8198 Fax 952/929-6105
		HOURS Wed – Saturday 10am - 5pm Closed Sunday - Tuesday and holidays	ADMISSION Adults $7 Students $5

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Curtain Burner Cords

I owe the world an apology. For years I’ve been telling people to use a diode to drop half the line voltage when replacing resistance line cords.

Now I find out that the effective voltage at the output of the diode is not half the RMS line voltage, but half the peak. In other words, if the RMS value of the line is 120, the peak is 169 volts. With the diode, your filament string sees half of that peak; 84.5 volts, not the 60 volts we hoped for; we’ve posted the math below.

To further complicate things, both my Simpson 260 and our bench Tektronix VOM measure the AC drop across the filament string as 60 volts AC. My GenRad 1800 shows the correct voltage of 84 volts AC.

So how do we replace that resistance line cord? Some of these sets only need 30 volts at 300 ma. across the filament string. As much as I have resisted, the answer seems to be a series dropping capacitor. The good news is that small, reliable AC components like Cornell Dubilier Type DPM Polypropylene film capacitors are finally available. Never use electrolytic capacitors to drop the voltage.

Here’s the latest solution for the line-cord replacement. A one-amp fast-blow fuse (F) should be fine for most applications. Calculating the value for the dropping capacitor (C) is not as easy as the familiar 1/2πfc. We use a program called Electronics Genius. It can be purchased online for $16, from http://www.electronicsgenius.com/ or you can call the Museum and we’ll provide the values for your radio. The surge resistor R1 is optional. Five-watt, back-to-back zeners across the dial lamp should be rated at a couple volts above the lamp voltage. For a #47 bulb (6.3 V at 150 ma.) in a 300 ma. circuit, R2 would be 42 ohms at 2 watts. For a type 44 bulb (6.3 V at 250 ma.) R2 would be 126 ohms.