These are some preliminary shots of a GU 81 tube, after it is heated up – just a tiny little bit.
Actually, the heater requires 12,6V with a current of 11 A.
I used a 12,5 V AC toroid, rated at 150 W, but unfortunately, the toroid that I used, under the load, … sort of … started sagging, so it gave no more than 11,7 V …
Therefore, it would be fair to say … that the tube is grossly underheated …