As far as pinout is concerned, they are interchangable. The differences are that the 6AB5 is designed with a lower filament current requirement (150 mA), for use in battery and AC/DC sets. You cannot use it in place of .3 ampere tubes in AC/DC sets. However, it'll work just fine in an AC only set. The target and plate are rated for a maximum of 180 volts. Also, the plate of the triode, according to my Tung Sol manual, is supposed to be fed by a 250K resistor. The 1 Meg resistor that is most likely in your eye socket might cause the eye to fluctuate a lot more than it should. However, if it fluctuates an agreeable amount, then all is well. You can give it a try. Be sure to put suitable resistance in series with the B+ lead so that the target has a maximum of 180 volts. The triode plate is usually connected to the target via the in-socket resistor, so you don't need to worry about its voltage as long as you have the target voltage correct. At the target side of the resistor you use, run a 1 to 10 MFD cap from there to B-. Use what you feel works well. You might not need any cap at all. However, if the resistance is great enough, brightness and other things might fluctuate with eye movement. It's best to stabilize the voltage with a capacitor.
T.