Yesterday, with my 14 year-old daughter looking on, I performed a repair on my Audible Illusions Modulus 2D preamp (aka "The Tube Eater" as Carlman likes to call it). It had developed a wicked 60 cycle hum in the line stage.
The repair was as simple as it could be, a swap-out of the 6922's in the line stage. Reconnected it and the hum was gone. My daughter was duly impressed. She also thought it was cool that the "new" tubes I used to replace the "old' tubes were probably as old as I was (Sylvania 6DJ8...yellow and black box address said "New York, 17" which meant they were at least 40 years old).
Then the questions came:
How did you know the problem was caused be the tube?
-I did some simple troubleshooting, such as pulling the input/output cables and tinkering with the switches. They pointed at the tubes as the problem. Besides, tubes are usually the cause of the problem. [Easy one. She has the smartest Dad in the world.]
Then why do use tubes?
-Because they sound great. They are also easy to repair. [Still the smartest Dad.]
What goes wrong with them?
-Err, um. Well, they get old....
They get old?
-Yes.
But you replaced the "old" ones with something as old as you, didn't you? [There aren't many things older than me, you see.] They worked perfectly and fixed the problem, right?
-Umm. Yeah. I guess.... [Her Dad is just as dumb as every other old guy- just weirder.] They wear out.
What wears out? [She is holding the old tube and sees there are no moving parts.] Nothing looks worn. How does it wear out?
-I guess a little like a light bulb.
But when a light bulb wears out, you can see a broken filament. There's nothing I can see that's broken in here....
-It just does...OK? ["Because I said so"...that's the right parent answer, isn't it?]
Now, she is still a little impressed that I could fix a problem with a strange machine that uses flat black plastic things to make music, just to a lesser degree.
So here's a topic: Why do good tubes go bad? What breaks in over the break in period? Why do tubes sound the way they do?
Please answer in plain English...I can read white papers and not understand them all on my own.
Discuss amongst yourselves.
Thanks