You would be surprised by the number of people who recommend using a 5U4 to replace a rectifier in a small amplifier that originally had a 5Y3, 5AR4 5V4, 5R4, etc. This may work, but probably not for too long since the 5U4 can draw 3A vs the others, which draw 2A or less. This is particularly problematic when the incorrect replacement goes into a collectible amp, which includes every model made by Fender from the beginning to the late-'70s, although the actual models at the end of this time are debatable- they can usually be improved to be as good as their predecessors and the reason they weren't as good is due to the fact that many of the people who worked for Fender during their hey-day left after CBS bought the company and made these changes to cut costs.

Leo Fender trained as an accountant during the Depression, but he was interested in other things, including radio repair and music. He didn't play an instrument, but he had many friends who did and they had a lot of problems with equipment reliability, so he would repair theirs. As he learned more about electronics, partnered with Doc Kaufman, making small amplifiers & lap steel guitars in the '30s and continuing to make repairs. He would improve what may have been needed and then, eventually started Fender Electric Instruments.

Leo didn't design everything- some tasks were handed of to others once the company grew to a size that required more of his attention. The amplifier in the link above is said to have been designed by Freddie Tavarez, who was a long-time Fender employee who was also a session musician in Los Angeles, playing steel guitar. He's also known for the steel guitar 'sweep' at the beginning of the theme song for those Warner Brothers cartoons most of us grew up with.

Here's one on-stage, played by Scotty Moore, with Elvis-