Questions about Tube Testers
"We use this circuit since 25 years, and it never went wrong"
This is an approach below any professional level. It's called trial and error. It's only done when you know nothing else to do.
Here is an example: When I tried to learn my three years old daughter to watch out, when crossing a street, I gave her guidelines to follow. That was better than repeating: be careful darling! So I said before you cross, there are three "ANDs" and you must count them. It works like this: You must look to the left, AND to the right, AND then to the left again, AND then not run, but walk. Telling her, even when she doesn't agree with it, walking instead of running makes a stupid car driver see better what she is doing.
She says, this is not necessary. She can hear a car, and if needed, you just run away quickly. I said, no no no.... that's not a good way. . She answers, don't worry, I know how to do this. Her friends do the same, and it never went wrong.
Great! So it never went wrong. What an argument.
We very often see the same approach with tube rectifier circuits. Not so much with DIY, but rather with companies. Calling themselves experts. They just try something, violating the data sheet. When tubes survives, they say: You see? The data sheet is wrong, and we have 25 years experience doing so.
In quite some cases, we are somewhere in between. So yes, they understand the data sheet writes this not by mistake. Then, they reduce the capacitor below or at maximum, and they call me back. Saying: Now the amplifier is humming. It is really hard to explain them, that is because the chokes are too small. Larger chokes are so much more expensive. And larger size too. All I can say is, now you know whey they did that.
This is not a joke! You don't need to search very long for such "designs". If you see for instance an amplifier with a 5U4G rectifier with 100uF first capacitor on it, or 274B rectifier with 20uF, you just found a product made by a fake expert.