It will break some people's hearts to hear this perhaps, but way back in the day, around 1976, when I was at Bell Labs, we came up with a solid-state replacement for the 300Bs and other tubes used in repeaters (the word they used for an amplifier back in those days because it replaced a person who would repeat, a whole other fun story). But this solid-state replacement was called a Fetron, it used FETs and was plug compatible with the tubes. We had armies of people with asbestos gloves pulling out the hot tubes and throwing them in huge trash bins and plugging in the replacement Fetrons. We smashed 300B's by the thousands back then. Bunch of old junk! was the cry.

BTW, the repeater used to be a person's job, mostly women, they would sit in a small both and a call from say New York would come in and it could not reach Chicago with just simple copper loop so their job was to listen to what the person in New York said and repeat it into the next loop and so on until it go to Chicago, the longest loop would be maybe 100 miles. Originally the booth was 2 feet wide, just wide enough for a woman to fit into. It was a repeater bay, with little side walls to isolate the sound somewhat between repeaters. This was before electronic amplifiers.

One day, along comes an efficiency expert from HQ who figures out they can add a couple more repeater bays into the line up if you just shrunk it down by an inch to 23 inches. Then later, when electronic amplifiers came along, tube type of course, it replaced the jobs of all these women who were repeaters. Put them out of work. Later came the Fetron. But that is why bays are 23 inches wide to this day. Sort of like the old lore about railroad widths being the same as roman carriages,this was the lore passed down to me from what were the old timers then. Guess that's me now :)