A little guide to the common preamp valve types:

ECC83 or 12AX7. High gain valves, used in most guitar amps.
ECC81 or 12AT7. Lower voltage gain high-current types, used for certain purposes in some amps.
ECC82 or 12AU7. Low gain high-current valves rarely used in guitar amps but good for dropping gain for harp.

These three types have the same pinout and can be swapped in for one another in most situations, though the higher current drawn by 12AU7 and 12AT7 types can cause minor issues. Each valve contains two triodes, ie two potential gain stages, making them very useful for designers.

The ’12’ designation means they have 12 volt filaments/heaters. But these are split into two 6 volt halves, so they can also work on the usual 6.3volt filament supplies most amps have. Really handy valves.

EF86. Used in the Vox AC15 and very early AC30s. Also used in modern Dr Z and VHT amps. Lovely sound for guitar, just one gain stage but with extremely high gain. Strong tendency to microphony and noise. Not in any way to be swapped for the above three types – very different valve type even though it has the same number of pins. These are made by the modern manufacturers but they are apparently a challenge to manufacture and the modern ones are much more prone to microphony etc than NOS ones. Which can easily cost you £40 or more.