Tossing in another $0.02...
I did a lot of research on the tube stuff a couple years ago when trying to improve the sound of my Peavey Classic 30, to my ears anyway. I wanted much clearer and unstaturated sound on the clean channel.
Preamp tubes are pretty simple as noted, plug and play. The main factor in these tubes are the models... 12AU7, 12AY7, 12AT7, 5751, 12AX7. They all are preamp tubes and are basically rated against the 12AX7 model which is classed at 100%. 12AX7s have the most gain. Others have less gain, but perhaps other characteristics and uses. For example, 12AT7s are often used in reverb circuits, sometimes 12AYs. The 5751, not a 12AX but has 70% of the gain of a 12AX and is bell-like and clear (to my ears). I used this preamp tube in the V1 position of the Peavey.
The previous poster, dancingrizz, was talking about the two tubes in one. The 12A series of preamp tubes are two part tubes w/ two separate gain stages. An amp schematic will show how the signal flows through these devices. It seems that with balanced preamp tubes, each gain stage has similar characteristics as measured. The closer they are to the measurements and within a given tolerance, the 12A series twin tube can be "balanced". I figure a good place for this type of tube is specifically in the drive/phase inverter section of the amp where the signal is prepared for the power amp section. In this spot of the amp, I used a 12AT7 balanced to reduce the drive into the power section but maintain a even signal from tube stages.
So within a category, say 12AX7, different tubes will have different values, ultimately determining gain versus noise. There is stuff like differing plate designs to try to produce different characteristics or more desirable characteristics.
All good fun. I would take a look at www.thetubestore.com
. They have the tubes and a little synopsis of the tube and its characteristics as viewed by musician testers. I have ordered from them with no issues in the past. Just gotta try your best to pick out what you may want, try it and see and be prepared to try again if you selection doesn't work the way you hoped it would.
Good luck and enjoy.