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scutterflux
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Bad KT88 ???

I can't get a bias reading on one of my kt88's, but it still works. What's going on? I pulled it in the mean time, and can bias the new tube. Has anyone had this problem before, is it bad?

Jan Vigne
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Re: Bad KT88 ???

If you cannot set bias correctly on one tube but can do so on a replacement part, then you should consider the original tube to be out of spec. If it works well enough to use, you can probably continue to do so though you risk taking out other parts should the tube have a catastrophic failure. I can't imagine the sound doesn't suffer to some extent if the tube won't run at the proper bias setting.

Elk
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Re: Bad KT88 ???

When you indicate that you can't bias the tube, do you mean you literally do not get a bias reading at all? I don't understand how this could be the case and there would still be amplification by the tube.

I would just consider the tube dead as the other tube works fine. In some amps catastrophic tube failure is a very bad thing.

Monty
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Re: Bad KT88 ???

Some tubes, especially older tubes can have cathode sensitive filaments that can drive you crazy. They seem to work fine, but biasing jumps all over the place depending on how hard the tube has been worked. Many tube testers can't even diagnose these sorts of problematic tubes. They usually test OK unless you let them heat up for 10 minutes or so and then they start showing their problem.

This may or may not be the case with your KT88s, but tubes that do have this problem won't last very long anyway and should be replaced.

scutterflux
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Re: Bad KT88 ???

Can't get a reading at all, maybe 1-2 definatly not 35-60 like I should be seeing, not anything different after it's warmed up either. It's pulled, I just never saw this before Sucks because these penta kt88sc's (smooth plates) are really nice sounding but I have a %50 failure rate with them. I like them better than the kt88-98 but they're just not that reliable, I had another one blow up! There's got to be at least 500hrs on them, but still that's not very long. I had them biased pretty low in my bass amp before putting them in my stereo amp. Well, I guess I'll try another pair?

Monty
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Re: Bad KT88 ???

KT88s are notoriously delicate tubes. Usually, if you can get a few hundred hours out of them they do allright. At least the tube didn't arc and screw something else up. A lot of people like the KT88 and say that it has a cleaner, clearer sound in many amps.

Just to be sure it's the tube, and I'm almost sure that it is, you can swap sides with one of you other tubes and attempt to bias the different tube in the same socket. If it biases fine then you know it's the tube. If it doesn't bias then your problem is internal and in your bias supply.

scutterflux
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Re: Bad KT88 ???

They are very clear and have great sepeation, very nice sounding, but another one just went microphonic an hour ago or so??? I swapped out all my kt88sc's for the kt88-98's. I really thought the smooth plates had a nicer top end extension, not as forward and a slightly more liqud mid that I prefered. Oh well the other chinese kt88 tubes are supposed to be more reliable after reading a few reviews on the smooth plates. I guess it was just that quads time, they weren't biased to high in my bass amp, but I've driven them hard at times. 220W from a quad of kt88's maybe 500hrs wasn't so bad.

Elk
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Re: Bad KT88 ???


Quote:
220W from a quad of kt88's maybe 500hrs wasn't so bad.


Uh...no, that isn't bad at all.

These poor tubes have got to be exhausted. It's amazing to me that they work at all at this point.

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