KT150 puzzle (longish)

Tubes Asylum

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I'm a tube noobie and am very much in learning mode. About 4 yrs ago I acquired my first tube amp after 40+ years as a SS guy. I am loving the music but freely admit that I am ignorant about the tech.
The amp is Ayon Spirit III, an integrated that uses 4 x KT150 power tubes. Bought it used, so I don't really know the age of the tubes, but seller claimed less than 200 hrs. Everything sounds wonderful. No problems in 4 yrs of moderate use.
Recently I went out on a limb and bought a quad of KT150 as backups through USAM. Seller claimed low hours, and after talking to him I took the risk. The Ayon user manual has instructions for running an initial pass/fail test on newly installed tubes and an "auto fixed bias" program, whatever that means. I plugged in the backup tubes and they passed the test. Listened for a couple hours and everything sounded fine, so I closed the sale.
I decided to send the backups out to be tested by a vendor that advertises testing services online. I was very pleased with the service. The price, including return shipping, was reasonable. Communication was friendly and collegial. I got the test results in one day, paid the invoice, and got my tubes back a few days later. No complaints.

Three of the KT150s tested pretty good, but one tested bad. The report said, "Heater lit. No output.". I corresponded with the vendor, who replied, "It could be that it will work in your amp but it did not on the tester". I got the tubes back today, plugged them in (including the bad one), ran the test and auto-bias, and everything sounds fine.
That was all preamble. Here is the puzzle for you experts:
How can the tube test "no output" but work okay in my amp? I would expect that if a power tube was bad, there would be no sound from one channel or no sound at all. Help me understand what's going on.
Let me stress that I have no beef with the testing vendor. They did what they were paid to do. The guy generously offered to throw in at no cost an older KT150 that still tests at 70% life expectancy. That is great customer service. I'm disappointed with the results, but I am not blaming the vendor. He simply reported the results from his equipment.
I'm trying to use this as an opportunity to learn something about the alchemy of tube amplification. Thanks in advance.

. . . in theory, practice and theory are the same; in practice, they are different . . .

Edits: 08/20/24 08/20/24

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Parts Connexion

Topic - KT150 puzzle (longish) - tlea 18:42:39 08/20/24 (7)

Lessons Learned - tlea 18:13:03 08/28/24 (0)

RE: KT150 puzzle (longish) - 6bq5 20:13:08 08/20/24 (5)

RE: KT150 puzzle (longish) - Nunki 01:09:10 08/21/24 (4)

RE: KT150 puzzle (longish) - 6bq5 11:05:35 08/21/24 (0)

Surprising - tlea 07:14:22 08/21/24 (2)

RE: Surprising - 6bq5 11:37:22 08/21/24 (0)

You'll need to crank it up a bit! - Ralph 11:12:44 08/21/24 (0)

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