Once upon a time, there was an electrical engineering student we shall call Kim. This story happened a while back; at a time when being a female engineering student was very unusual. Kim had grown accustomed to being one of maybe 3 young ladies in a class of 90 students. She was about halfway through her 4 year program. She did not come from a rich family, but her parents were hard-working, good middle class folks. They were proud of Kim. Her father occasionally traveled for business and would take her to dinner if he happened to come through town where her university was located.
To help make some extra money, Kim had become pretty well known on campus for repairing home stereos, turntables, and guitar amplifiers. On this particular day, she had just begun to look at a fairly large guitar amplifier, when her father arrived. She let him into her tiny apartment where she had the amplifier sitting on her kitchen table. Now you have to understand that her father had no background in electrical engineering. He could barely get the TV to turn on and tune-in programs. So she was startled when he pointed to an area on the amplifier and said, “It is that place. You are going to find the problem is there. It is always the dumb ones.”
“Uh, sure Dad,” she responded. He was known as a bit of a joker, so she quickly dismissed the thought. They had a nice dinner and then he headed off to home. He had a pretty long drive ahead of him.
Back at the apartment, Kim tore into the amplifier. First she verified the reported problem. The output to the speaker was indeed very weak and distorted. She checked all of the controls and the problem did not appear to be related to any of the volume or echo knobs. She did not have a formal signal generator, but she had an FM receiver that she could use as a continuous signal source. She fired up her oscilloscope and set to probing. [That scope was her pride and joy. She had saved her earnings all summer to buy it.]
Kim meticulously worked her way through the amplifier. The audio from the radio looked okay at the input, but looked all wrong at the output. To be sure that she wasn’t being fooled by the scope display, she fed the probe signal back into the tape monitor input of her stereo receiver. The signal was good through most of the amplifier, but suddenly stopped.
Kim glanced at the top of chassis, and realized that the signal failed at the exact section where her father had pointed. She was dumbfounded.
Switching to her voltmeter, Kim verified that the DC bias voltages on that tube all looked reasonable. The filament voltage was good too. She dug deep into her parts box and was happy to find an extra tube with the right part number buried there. She replaced the tube and powered the guitar amp back up. Her radio station audio came booming through the speaker, no distortion and plenty of volume.
It was getting late, but Kim knew her father would have arrived back home about 20 minutes before. She could not help it; she had to call him and ask. “Okay, I have been debugging all evening. How in the world did you know where the problem was?”
“What problem?” He was momentarily confused. She replied, “The problem with that guitar amplifier!”
After a few seconds, he laughed and asked her, “Can’t you think of a way that I could tell just by looking?”
“Well, I guess you could have seen a resistor or capacitor smoking in that region, but that wasn’t the failure, and there was no overheating, so I am really stumped,” Kim replied.
“Remember how I tell you that you don’t have enough light in that kitchen?”
“Dad, I know, I know. You always are complaining about that. But it makes it easier to see the scope traces and I have plenty of light where I need it,” Kim replied, trying not to let too much exasperation show in her voice.
“Yes,” he responded. “But this time the darkness was helpful. When I stepped into that room, I could see about a dozen tubes; glowing a beautiful cherry red on top of that gizmo you were working on. Except that isn’t quite what I saw.”
“What Dad? What?” This time the impatience got the better of her.
“Well, I really saw a bunch of tubes glowing beautiful cherry red, and one tube that looked more like a dull brown.”
“Doh! Of course!” Kim exclaimed. “The filament wasn’t getting very hot. The electron emissions from the cathode were very low, which starved the amplifier of current. No wonder the output was low and distorted. That stage acts as the last amplifier before the power output stage.”
“But wait,” she asked. “Why did you say it is always the dumb ones?”
“Because,” he giggled, “That tube was not so bright.”
(If you are smiling or groaning, you don't need to read from here on down.)
A note of explanation for my English-as-a-second-language readers.
Humor at the end of the previous story is derived from a double meaning of the word "bright." The direct meaning of bright is the amount of light that shines from something: in this case, the vacuum tube. But the word "bright" is also commonly used to mean smart or intelligent.
Jokes are never funny when explained. Sorry about that.