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Here's an interesting one for you guys. My amp has had a few KT88s within the last several months that will go to zero bias. The problem has been on the same channel, but in two different tube sockets. The tube will start out reading a normal bias and then drop to zero after warm-up. There are no loud noises, blown fuses or other dramatics. The last one to act up made ticking noises every 2-3 seconds similar to the sound of the end groove of a record then read zero bias. The tube then worked again and read normal the next day and then the day after a single tick sound and zero bias again. If you switch the tube that reads zero to another socket you will get the same result, zero bias after warm-up. I've had several tube amps, but never experienced this exact situation. Any help or similar experiences appreciated. Thanks.
Follow Ups:
I think Chris O's recommendation is a good one. I've seen this very issue before with 6550s and KT88s - pins/sleeves not making sufficient contact with the wires coming out of the tube envelope over which the pins are fit.
Sounds like you've already narrowed your problem down to being an issue with the tube itself. When you move that tube to a known properly operating socket and get the same result.....well, the tube is the naughty one.
It might just be inside the pins. It would be worth it to heat the pins up with a soldering iron to reflow the internal wire to the socket pins before you chuck the tube.
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i have a god undertsnading of electronics
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