![]() ![]() There probably exist some exotic hi-fi amps which use fixed bias (for greater output power), but use some form of servo circuit to lock tube current to a safe value (which would then also be safe in the face of a gassy tube). If you're really concerned about the risk of gassy tubes, cathode-bias is inherently self-limiting and will either prevent a failure or minimize the scope of any resulting damage. But I'd guess you have an 85-90% chance the tube will operate perfectly fine. ![]() Watch the tube at power-on and be prepared to power off if you see signs of trouble. Ensure the fuse is the correct, rated value. It might sound silly, but your amp is the best tube tester. It's a safe (and profitable) recommendation if you're a tube vendor. So in the absence of other info and/or experience, you get the statement, "If a tube ever red-plates, it is forever suspect." It's an overstatement in my opinion, and you'll throw away perfectly good tubes. And a tube's vacuum is never absolute to start with. But your tube also has a getter flash which is there to combine with and trap such gas molecules, and prevent the gassy tube condition. So that's the possible problem, described in old tube books as a "gassy tube". And on, and on, into a runaway situation. If that situation is carried to an extreme, the additional current could run into the control grid, further reduce apparent bias, heat the grid, cause secondary emission from the grid, which furthers plate current. These ions could contaminate the control grid in a manner that would tend to counteract the bias voltage, making the tube behave as if it had less bias (more resulting plate current). The problem with overheating tube elements is that the materials may emit gas ions which were bound up in the material (outgassing). If you read around, you'll find folks saying, "If a tube red-plates, it is bad and/or can never be trusted again." Like many things tube-related, there is a basis for the statement (which is usually not provided) but then the notion is taken out of context and exaggerated somewhat. It sounds like you are describing some red-plating (gray plate starts glowing cherry-red, usually in one or 2 areas of the plate). Again, there are no physical differences I can see, and all tested fine for shorts.Īny help or advice would be greatly appreciated! Thanks in advance. So, now I have no idea which of the four is potentially a bad tube. I labelled the misfit tube, but, being a genius, used a post-it on the box which fell off. Anyways, I just put my old tubes back in and boxed the SEDs. Turns out the Jolida manual had a mistake in it and I was setting maximum plate voltage to start biasing, instead of minimum, for two out of the four tubes. After about one minute of powering up, one started glowing very brightly, so I turned the amp off immediately. ![]() Some background: I bought four SED Winged C tubes, slightly used. Physically, there is no damage that I can see at all, and I did a short-circuit test using a multimeter and everything seems fine. Would just sticking it into my amp (Jolida JD202) and giving it a go risk damaging the amp? I'd like to know if there is any way to determine if an E元4 tube is either dead, or not functioning properly, using basic tools and without using a tube tester (which I don't have).
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