Here are 5 audio pre-amp valves with just one letter in the nomenclature separating each one from the other. The one letter might be the difference in the name but they are very different in terms of characteristics in the circuit. They have quite different gain factors. They are all still in popular use in classic gear and new equipment where the vintage audio valves manufactured in UK, Europe or USA in the 50’s or 60’s are often greatly preferred over modern copies because of the way they sound.
Here’s the 12A*7 family together with alternative names for each audio valve. Sometimes the alternative tubes listed alongside each will have minor characteristic differences but in terms of gain this wont really be significant. Normally the alternative tube can be safely substituted for the 12A*7 but there can be exceptions – as should always be the case with any audio or radio valve substitution its best to check with a technician before proceeding and so avoid tears afterwards! Remember that your valve amp was originally designed for a specific tube so always get technical advice and exercise care when substituting valves.
12AT7 – ECC81, 6201, 6679
12AY7 – 6072
12AV7 – 5965
12AU7 – ECC82, 5963, 5814, 6189