In message 2009050508580977923-joseph@removespamkirtlandcom, Joe Bento
writes
Corrected schematic to show actual wiring of heater string. First
12AT6 preamp heater is closest to ground.

[ A MIME image / gif part was included here. ]

This arrangement might also help with reliability. The heater-cathode
breakdown rating of a 12AT6 might not be as high as a 50C5 (but I
haven't checked).
--
Ian