Why not use a #42 tube and make a small modification to your radio. This mod will allow either tube to work. Lift the wire going to pin #5 of the tube. Add a 470 ohm 1 watt resistor in between removed wire and pin #5. Connect a 10 mfd @ 25 volt electrolytic across the resistor.
This mod will give negative grid bias for a 42 tube. A 6B5 will still work with reduced output.
Your 6B5 operates on zero grid bias and drives a speaker. A 12AT7 wasn't designed for this circuit but could be made to work. Use a 470K grid resistor.
Do you have any other tubes to select from? A 6N6 is the same as 6B5 but has a different base.
Norm
:tube manual shows 6B5 with internal resistor can I use a 12AT7. if so what size external resisor should be used soing from cathode to grid?