01-03-2016, 10:01 PM
Paul, I have a similar Z set the 12DT8 tube is a pain. I have had several NOS fail after maybe 50-100 hours went to using 12AT7, ECC81 which are current production. The BIG DIFFERENCE is pin 9 on the ECC81 is center tap to the heater, on 12DT8 pin 9 is internal shield to ground. One must remove the ground connection from pin 9 of the tube socket or kiss your new tube good bye in a flash.