So that people can charge you extra for doing the testing for you. But really, without the cynicism, a tube tester like that won't pick up a microphonic tube. For that you need an audio amplifier like, say, a guitar amp. Get two new 12AX7s and a new 12AT7 and swap each one with the new ones. If you swap one out and there's no difference you can say that's a good one. The problem won't be in the power section so you can save yourself the cost of the 6L6s and a bias adjustment straight away. And when you've found the bad one and thrown it away, you'll still have a spare or two for when the next one goes bad
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Originally Posted by markb
So that people can charge you extra for doing the testing for you. But really, without the cynicism, a tube tester like that won't pick up a microphonic tube. For that you need an audio amplifier like, say, a guitar amp. Get two new 12AX7s and a new 12AT7 and swap each one with the new ones. If you swap one out and there's no difference you can say that's a good one. The problem won't be in the power section so you can save yourself the cost of the 6L6s and a bias adjustment straight away. And when you've found the bad one and thrown it away, you'll still have a spare or two for when the next one goes bad
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