Carl, you are right, they are a holdover from the tube radio days. There were even fans provided in those days. The radios at top of the stack usually failed first because they got cooked the most.
Solid state avionics are about the greatest contribution to aviation since wings! I well recall in the days with tube radios and generators rather than alternators sitting at Idlewilde, now JFK, waiting for a clearance, watching the cylinder temps go up or the battery go down. You had to decide if you were going to run the engine fast enough to get some charge and consequently run up the temps or hope that the hold wouldn't be much longer. This was in the days of Mk 12s which had dozens of tubes. When the Alpha 200 came out in the 70s, which was an early solid state navcom, we could turn everything else off, except the nav lights and beacon, and reduce the power drain somewhat.
I used to think about suggesting to the tower that I would shut every thing down, including the engine, and they could let me know with the light gun. Needless to say, I never worked up the courage to try it!
George
