For years people have been looking for a way to drop half the line voltage when replacing resistance line cords. They tried using a diode in series with the line.
Unfortunately, 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.
To further complicate things, most meters, like our Simpson 260 and our Tektronix CDM 250 VOM measure the AC drop across the filament string as 60 volts AC. Only 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 25 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 LINE 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 formerly used a program called Electronics Genius, but the creator seems to have dropped off the face of the earth. Fortunately, Paul Stenning - http://www.vintage-radio.com - has published an online program that works. Go to the bottom of page http://www.vintage-radio.com/repair-restore-information/valve_dropper-calcs.html and download the "Dropper Calculations Spreadsheet;" you'll be glad you did.
For example, four 6J5s with their filaments in series would use a 33 ohm, three watt resistor (R1) in series with a 6.9 mfd capacitor (C).
The surge resistor R1 is optional, but a good idea. Also, use a polarized plug, as shown. 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.
If you don't have a dial light there's no need for R2 and the Zeners.