[Dacious]

I personally wouldn't do this. Not to argue with MJ, but vacuum passes no heat, so the elements in the tube are not going to be affected one way or t'other by air flowing over the glass envelope. With a fan stuck randomly inside, cool air might not be flowing evenly across all the tubes, causing localised heating which is probably worse than none at all. Just blowing air inside an amp head is not likely to evenly cool without some sort of cooling fins/channels to distribute it.

The phenolic and/or ceramic tube base is not going to pass heat much via convection, especially given the localised mountings via rivets or screws. So the working core of the tube is not going to benefit much if at all. The potted and paper-wrapped transformer windings are not going to cop much either, inside their end-balls.

The other reason is - heat is power, one reason why amps seem to sound better after operating for a while. If tubes are operating in a specific range of voltage and current, they are working where they're designed and trying to alter that is not likely to improve anything. You might keep them out of 'the zone' although I doubt it.

Think of all the vacuum tube phone exchange equipment, radio transmitters etc (which still use enormous ceramic tubes) designed to operate continously around the clock with minimal maintenance. None of it had fans.

Computer gear and solidstate equipment is different as silicon chips and the components in a hard drive have much more critical temperature operating limits. Their outer surfaces are exposed solids. A 12vdc fan might help them - it's not going to help a set of red hot tubes much, but might coat them in dust sucked in from outside.

Just MHO.