voltage transformer and tube gear

Most amps have a power consumption rating. Class A amps will draw a steady current until forced to A/B. Class A/B amps fluctuate draw on demand. Some, like a Carver Lightstar (300 wpc) can draw up to 15 amps at peak, however unlikely. As a guideline:

50 wpc stereo = 300 VA (watts) input, roughly 3 amps,
100 wpc = 500 VA, roughly 5 amps

As a general rule a transformer should be double the VA of the demand but 2 KVA would be the minimum I would suggest, even for a small amp. Honest three KVA, but not all, would allow 15 amps of most household outlets and weigh about 75 lbs. Wired for balanced AC, derate by half. No transformer will convert Hz but that shouldn't be a probelem for pre/amps. It could be a problem for a CDP or DAC but most of those can be either 50/60 Hz.

I use a 120 lb,5 KVA tranny for 20 amp balanced.

I should have mentioned that transformers can have an audible hum due to internal and external factors, but espescially when overworked. One might be quiet in one spot and be noisy somewhere else. Good grounding helps. Ideally, they should be in another unoccupied room.

The GE, Q-type, step down, general purpose, single phase, isolation (Ebay keywords) tranny of mine is dead quiet, but it's epoxy potted also. A shielded unit is also preferred but that can be tough to find out.