are some phono stages more resistant to hum?

Noisy tube in the AR unit?

thank you, will try all these things. I did try different tubes, not the problem.

when I do not connect the ground wire from the tt, just leave it dangling unconnected to anything, the hum is greatly reduced. What does this imply? problem with the way the entire system is grounded? I live in an apartment, and have no control over the non-audiophile wiring setup.

There are probably a dozen reasons why one phono stage might be more likely to hum with a particular tonearm and cartridge where some other phono stage will not. First of all, lets get the terminology straight; when you say "hum" it implies a 60Hz or 120Hz tone (in countries where the mains frequency is 60Hz). Hum is almost always due to inadequate grounding. By that definition, tube phono stages are no more likely to hum than SS ones. However, true balanced phono stages of either type are much more immune to hum than are single-ended circuits, because even if the ground scheme is not perfect, hum in a balanced circuit is often cancelled by the mechanism of Common Mode Rejection (CMR). The better the CMR (i.e., the more the two halves of the balanced circuit are truly in balance), the more the phono stage is impervious to hum. One kind of noise that is akin to hum is the rare instance where the cartridge will interact with the tt motor, some types of which can induce hum in the cartridge. "Bad" tubes can sure be noisy, but the noise does not usually have the quality or pitch of hum.

Lloydc:

When you disconnect you TT ground and the hum is reduced, it implies you have a ground loop. You cannot have multple points of ground in different locations, as these will create hum as ground current flows from one location to another.

For example, if your TT base is metal and grounded to the third wire AC ground, and the analog input ground in the pre-amp is also grounded to the third wire ground, you have a ground loop, with the RCA cable shields making the connection.

The reason why some pre-amps have different hum performance may be due to how they reference the analog signal grounds at the inputs. Some are hard connected to a chassis or analog ground, and some are referenced with a capacitor or resistor. The resistor or capacitor provides a high enough impedance to interrupt or reduce ground loop currents.

Differential inputs are useful for CMR, but phono carts are almost alway connected single ended, which means the (-) and shield are common, and you cannot get a true balanced connection to the pre-amp input. So the CMR of the input is of limited utility.