Thomas Mayer describes that although the tube manufacturers never particularly advised for TV dampers tube being used for main rectification, they are very applicable for such applications and Thomas has been using the in his power supplies for his amps reliably over many years.
From marketing point it logical, that tube manufacturers was determined to sell more expensive rectifier tubes for power supplies. Yet, TV dampers were produced for the millions of TVs sets and came cheap as mandatory. For this they are still available at large quantities at very reasonably prices. The 6AU4 can be found as 6AU4GT and as 6AU4GTA. GT additions simply stands for glass tube and A letter was added after the improved version of the tube with a slightly higher current handling capability.