You ve now effectively lowered your amps output power by half. This can be very useful with an amp as loud and clean as a Twin. The amp will operate with no other changes, but since you ve now altered the output impedance of the amp, it s advisable to double your speaker load. This can be done easily by disconnecting one of the speakers in the amp. Note however that your power tubes will not last as long as they normally would.
Some players forgo tube longevity in the interest of achieving the most desirable tone.
This is the preamp tube for the normal channel of the amp. It s located all the way to the right side of the amp as you face the back of the amp. Of course, even different brands of 12AX7 s will sound different in most amps. Which one is best for you can only be found by trial and error. Recommended by Fender for V1, V2 and V4 preamp tubes.