Try moving your TT away from your pre-amp.
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I tried moving the TT away from the preamp, and no change. I also tried putting the ground wire everyplace on the pre-amp I could see, including chassis, power supply, etc. No change.
Interestingly, I discovered something else. I still get ground-loop hum when the TT is disconnected completely, and this is the configuration:
Power outlet>Preamp>external Soundcard>computer
I don't know what to make of that. The computer is in a separate room from the preamp and plugged into an outlet that is on a separate circuit from the preamp, as far as I know. This is frustrating.
Here is even more information. In an effort to see if this had to do with the computer, I reconfigured to go through one of my receivers. Same effect:
TT>tube preamp>receiver (ground wire on outlet)=buzz
TT>tube preamp>receiver (ground wire not connected)=buzz
TT>battery preamp>receiver (ground wire on outlet)=buzz
TT>battery preamp>receiver (ground wire not connected)=NO BUZZ
I had a similar problem with a Meridian G08 cdp and Bryston B100sst. If I isolated the ground from the G08 (2 to 3 prong adapter) on the power cord the hum went away. Same if I isolated the ground on the Bryston. So I ran the G08 ungrounded (preferred to keep the component with the highest current draw grounded). Same problem with a Sim Audio Super Nova cdp. When I replaced the Bryston (with an Octave V70se) I never had the problem again with everything grounded. My initial thoughts the fault was with the G08 but in the end it was the Bryston. So my guess is the receiver is most likely the culprit. Again it's only a guess. Have you tried a 2 to 3 prong power cord adapter for the tube pre? Also from your description the TT should not be grounded with the ground wire.
What amp/pre are you using and do they both use grounded leads
Did this just start happening? - you could be experiencing a real problem in the amp or pre
Try grounding the chassis/case of the preamp
In my case I have a Naim amp, because of its design all sources must be grounded. When I connected a Pioneer DVD player it hummed - grounding the case of the player fixed that.