Quote:
Originally Posted by Adam
Ok, you all convinced me to try these. I ordered 2. Now do I understand correctly that these gas filled glass ballasts regulate current as the set comes on, so there would be no need to add a filament transformer to protect the 7JP4? - as I had been planning to do.
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Based on tests that I did with a ballast that had only one bad filament I would have said no. That they behave just like a resistor. However, I was testing them under steady state conditions and as I raised the current the power dissipation got to a very high level before seeing any glow of the tube. I was afraid the Bakelite base was going to start burning.
Watching Bob's video of the tube as his set warm up changes my mind. The brief high current causes the filament to get hot enough to change resistance significantly and protect the 7JP4. The current when the tubes are cold must be much higher than what I tested the ballast to.
John