Typically door bell transformers are in the garage high on the wall or on the ceiling. If no garage look near the breaker panel or in a closet. Back in the 1930's, most likely the door bell was operated by the stack of dry cell batteries. Latter on a transformer was probably added. This is what was used in fourth grade science class (1959?) to learn about basic electric and magnetics.
All standard door bells run on 24vac. "Ring" door bells and other electronic door bells is DC. For an old system a diode is placed at the door bell button so only a 1/2 wave ac is sent to the chime. The voltage is only positive (or negative). The voltage sensors that only sense from 70-600 volts or 50-1000 volts. Some models have a 12-1000 volts dual range that will sense 24 volts AC. These non contact voltage testers are for AC only, not DC voltages. I have a dual range, set for 12vac I was able to find a bad mini 2.5v incandescent light bulb on my prewired Christmas tree.
All is not lost, there are wireless door bells units that can be bought. Just google it.