57 four input 5F6 Fender Bassman. This is the one with the unusual mercury vapor rectifier tube. Rectifier tubes last a long time and if you need another they are readily available online contrary to conventional...

It's getting a lot harder to find original tweed Bassmans so maybe instead of concentrating on the well regarded 5F6-A's that have gotten kind of pricey if original, you might want to look at the very underrated 5E6....