The 12AY7 (also called a 6072 or 6072A) is a very sweet tube and every tweed amp from the 5E3 & up used one in the V1 position. The 5E8A low power Twin used them in the first three positions to get that extra smooth tone that it’s known for. Now for my opinion on the 12AY7, and while someone else’s thought may vary I think most would agree with me.
Each & every tube has a unique tonal character regardless of brand or designation, and swapping a few 12AX7s out can prove that to anyone. For the most part the 12AY7 has a smooth overdrive and a very well balanced tone that easy to listen to and doesn’t cause the player ear fatigue even after a long session. You might be hard pressed to find a 12AY7 that got even close to an edgy or gritty tone as they all tend to be very smooth from my experience. The 12AX7 has a gain or amplification factor of 100 (1 volt in = 100 volts out) where the 12AY7 is only 44 so there’s not as much signal hitting the next gain stage. The 12AY7 also draws more current though, which basically “browns” the out amp some, so even though there’s less gain it does overdrive quite nicely which is what gives it that special tone & feel. If you’re a harp player you’ll really appreciate the 12YA7 as there’s no feedback when you cup the mic in your hands like you get with the 12AX7. So if you’re into “over the top” rock or metal then the 12AY7 may not be for you, but if you like country, blues, jazz, fusion or straight up rock you really ought to try one out.