are some phono stages more resistant to hum?

Thanks, Lewm. I will check out that possibility in my next step.

Rtilden, just FWIW, tubes are no more prone to picking up noise from lighting than transistors. That is a power supply problem, not a tube/transistor problem.

Lewm:

The cartrigde may be connected in "balanced mode", but when you connect the shield to the minus input, you convert it to single ended. You have to reference the input voltage to somewhere, and for phono inputs its the shield (-). If you could get a cartridge with a true balance output (like a center tapped transformer), with the split (or center tap) connected to the analog ground/shield, then you can have a true balanced input. Some microphones are wired this way. But phono cartriges are not.

For example, I have a PS Audio GCPH that uses a "differential" input pre-amp. The input RCAs are ground referenced via a 100 ohm resistor to analog chassis ground. The two phono inputs (V+) and (V-) are twisted pair from the tonearm/cart with a separate isolated shield. Shield is connected to chassis ground. Is this balanced? NO!!! Because the potential between the shield/chassis ground and the (V-) input is essentially zero, because no current flows in that 100 ohm reference resistor. In a true balanced system, V- and V+ would be the same, only opposite in polarity. In this phono setup, V- is 0.
So while this system may give you some CMR, it will not be anywhere nearly effective as a true balanced system.

Dhl,
To take your first paragraph, ALL cartridges (except for a very few, among which are some early Deccas and probably some others of vintage origin) are inherently balanced. The typical modern cartridge does not know or care which side of its coil is referenced to ground. Since historically 99% of phono stages were SE, most cartridge makers label one of the two pins per channel as "ground", just so that the two channels will be in phase, I guess. When we use an SE phono stage, we then treat the signal as SE.

But I did think some about your other point, whether connecting the shield to the negative phase would convert the signal to SE. This only comes up if one is using a cable that was built for handling the signal as SE, but now we are connecting that cable to a balanced phono stage. If the plug is RCA, then the negative half of the signal would have to be carried on the ground side, as far as the phono stage is concerned. As long as the "ground" side of that RCA is connected to the negative phase of the balanced circuit (and NOT to chassis ground, as is the case for your PS Audio), I don't think it would make any difference that the shield was also connected to the negative phase (it would not actually be grounded and so would operate as an unshielded cable). The shield is typically only connected at one end. But this is really a hypothetical situation; if one has a balanced phono section, one should use a balanced cable (two wires for signal, one pos one neg, plus one wire for ground) terminated with an XLR plug.

I know nothing about your PS Audio GCPH, but I take your word for it that you are listening in SE mode. Sounds like it is not a true balanced circuit, if the RCA jack is connected to chassis ground. Or if the circuit is balanced, they don't want you to operate it as such.

thanks to everyone for the responses. It is definitely hum, and not tube or other noise. I suspect that Dhl93449 correctly diagnosed a ground loop. How to solve it, in an apartment with exceptionally noisy wiring (even a flourescent light on the same circuit) and few outlets, is an open question.