.... b/c back in the 1940s to the 1980s, the quality of the tube was literally a Life/Death situation during the COLD WAR and all the millions of pieces of military gear from complex communication systems to airplanes (as noted, for MIGs to withstand the Atomic Cloud after an A-bomb goes off--> since the transistor designs could NOT withstand the Atomic Cloud and would fail ... mid flight in the MIG).
