1941 GE J620 Ye old ballast tube

[init4fun]

Originally Posted by Knifeswitch

This unit is the first series string radio I have entered and though I get the concept, it is new territory for me. The filaments are fed AC right off the line through what was a BL42D ballast tube. Someone apparently long ago tried to repair this as the tube was gutted and some ancient power resistors were in place under the chassis. The socket just became a terminal. It was botched pretty badly. I deciphered the RMA code for the pinout and such. After reading an article on ballast tubes I realized it wasn't just as simple as figuring the correct resistor values. They're not given and in the correct circuit shown in the diagrams, I lose the math upstairs in the brain department. Plus if I understand this correctly, we want the resistance value to change as the set comes up to temperature. This is a six tube set with 1800mA of filament and (2) 250mA pilots. R18 is the ballast. Any kind shove in the right direction would be much appreciated. Came here as a AK guy. Very glad this is here!

Hi Knifeswitch, welcome to VK

As you so rightly stated, it isn't just about a raw, fixed resistance value. During the initial power up and warm up of the tubes, the ballast changes it's resistance to regulate the current flowing through the tubes. Were this mine I'd do one of two things;

#1, the most preferable, I'd find a new ballast. If your search around they can still be found, and the circuit will function as intended.

#2, the hard way, use a 12SA7, 12SK7, 12SQ7, 12J5, 35Z5, and 35L6, and eliminate the ballast altogether. The 35Z5 has a tap for the dial light, so you won't loose that. I realize this is a fairly involved modification for someone new to this, but trust me it works, I've done it a few times to not only eliminate the ballast, but also on so called "curtain burner" sets as well (sets with the heater's dropping resistance built into the AC line cord).