Converting a monitor from 625 to 405

[ppppenguin]

Sorry, misunderstood your original post. It's a very good idea. Best to do it with the tube itself as the heater voltage will be sensitive to loading - it's hardly a low impedance supply. Little risk of damaging the tube. Short term overvoltage won't do any harm and I'd check it with the DMM before doing the accurate measurement. I already suspect that my DMM reads about 10% low.