While working in my office today I suddenly realized that the music had stopped. Looking at the Source Selector Preamp showed that the output levels were not moving. A little investigating revealed the culprit.
And here it is.
This is a dead JJ EZ81 (6CA4) I just removed from the Source Selector Preamp. This was a brand new tube when I finished the preamp back in October of 2023. This is a failure after just about 20 months. Even at eight hours a day that’s only about 5000 hours. And the total load on this rectifier in this application is only about 12mA.
But the failure in this tube had nothing to do with load. This tube failed by the filament circuit opening up. There is no short between either filament pin and any other pin, the filament just failed open. This is something that just shouldn’t happen after this few hours of operation.
I don’t know what’s going on with JJ tubes, but I’m not buying any more for a while. Maybe in a couple of years they’ll get their problems figured out. Until then, I’ll be buying my tubes from other manufacturers.
As always, questions and comments are welcome.
Maybe just a fluke? The heater of an indirectly heated cathode tube is usually the last thing to fail. It is a heating element rather than a filament – good and sturdy.
I have been looking through the base of the tube at the filament wiring connections. Either ends of the heater are spot bonded to the pin wires which are bent over at a 90° angle toward the center of the envelope. The spot weld on pin 4 looks good. But the spot weld on the other end (pin 5) looks like it isn’t aligned with the end of heater wire. It almost looks like the misalignment was such that it just barely caught the heater lead. I think this may be the genesis of the heater failure.
This is a quality control issue pure and simple.