The Sovtek 5751 is an affordable alternative to the 12AY7 used in original Fender tweed amps and can be subbed for a 12AT7 reverb driver tube.
Special note for 5751 types: The 5751 is not a true 12AX7, but is often used in place of a 12AX7. They only have 70% of the gain of a 12AX7, and are often used to quiet down guitar amps that have too much gain. You can learn more about this on our tone tweak info page.
One other note about the construction of these tubes is they have filaments that are almost completely encased in the plate structure. They don't "light up" brightly when working properly.