Without physically examining what someone else has done by way of a conversion I am guessing as to a source of a bothersome hum.

It may be that your conversion has a contactor installed in the head itself to do the switching of the voltage to the lamp. This would typically be done to shed the load of the lamp off the enlarging timer. Contactors, which are electrically operated switches, can hum because the electromagnetic coil represents an inductive load whereas a typical incandescent lamp will be a resistive load.

If there is nothing in the way of auto focusing on the enlarger, there would typically not be a requirement for a stepping transformer. All that would be required would be to install a lamp of the proper voltage and wattage.

A regulated power supply would typically be of limited benefit in black and white printing. It would be more applicable to situations where a lamp emission color shift could occur with voltage fluctuations.

In regard to your interrupting current by blowing fuses, I would suggest that you confirm the proper sizing of the device. The way to determine this is to divide the wattage by the voltage and the result would be the amperage that would represent the load. A slight oversize of fuses may be appropriate if the device is not undersized.

Transformers are typically sized by VA (volt/amp) designations. This would be determined by determining the amperage as I described above and multiplying this by the output voltage of the transformer.

An example of determining these calculations would be as follows: The enlarger has a 1000 watt lamp and the supply voltage is 230 volt 50 hz.(Europe) the amperage requirement of the device would be 4.34 amps. The proper sizing for a transformer to supply this load would be a minimum of 998.20 or more typically expressed as a 1KVA transformer. However in actual practice one would be more accurate in sizing this as 1.5 KVA and the fuse to protect the device should be sized at 6 amp and installed on the secondary (output) winding circuit of the transformer. This would allow proper operation of the equipment and provide the necessary safety factor to protect the device.