The heaters in the Elyse DAC are wired in series and are current regulated. This means that a 12.6V supply feeds the two tube with 6.3V nominal heaters, in series. Because the heaters are CURRENT regulated, a fixed amount of current is 'provided' to the tubes. If a tube is used that draws a different current than the 6922/6DJ8/7308 tube, then the tubes may not have enough total heater current.
If a tube is used that draws more current than the basis of design (6922…etc.), then the first tube will light up, but the second tube may not see enough current to operate.
I expect that this is the case with this tube, although I have not looked at the data sheets to verify its operating heater current. It could also, for some reason, be a tube that draws much more than the nominal current.
Sorry for the technical explanation, but I wanted to explain WHY this could happen and why only tubes from the family on which the design was based, should be used.
Thanks,
Dan Wright