So, you went with another OT instead of a 125ESE? I’m not familiar with that sort of wiring scheme for a OT so double check my advice. A single 6V6 in single-ended, class A operation with a 8 ohm load would like around 5k (5000) ohms on the primary. The exact value is not critical, it looks like you could get 5000 – 5500 ohms with that OT and that should work. That should get you around 4 watts of power with the 6V6.