wow that sounds really weird... like some voltage/current going the wrong way... I would have your phono/preamp checked instead for something leaking back to the tonearm leads (?)
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Can you tell whether the cartridge body itself is vibrating so as to feed back into the tonearm wand? That is the only mechanism I can think of for this strange phenomenon. If you ignore the problem and play an LP, what happens? Is there added hum coming out of the speakers in addition to directly from the tonearm? Have you tried changing cartridges? If the insulation on the tonearm leads has a defect such that one of them is in bare metal contact with the tonearm wand, I still don't see how that would cause the phenomenon you report. The odds are vanishingly small that there are TWO defects, one on each lead such that there would be a voltage delivered to the tonearm wand (maybe). But strange things do happen.
Sometimes grounding everything to everything is a bad idea. Try removing some of those ground wires. But on the other hand, your report is clearly out of the ordinary; as you know, 99% of the time grounding problems manifest themselves on the amplified audio signal only, not in the form of causing vibration of an inert metal part.
Thanks for your considered response. My former career was as systems software developer, and I appreciate your debugging skills. Occasionally (of course) there is feedback while the stylus is tracking the groove. My gut reaction suspects some sort of intermittent feedback. as you suggested, from the cartridge itself. It is an AudioQuest 7000 now some 15 years old and my new Masseto preamp deserves something better anyway. I'd hate to think it's the shielding on the Cardas cable touching the tube, because that would require a re-wiring but that's what my hands-on experience feels like. Cartridge replacement first, then rewiring. The multiple groundings were done to successfully eliminate a constant hum at table installation time.
Dr_John, I think I did not read your first post all that clearly. Now the problem sounds to me like it might be a sort of RF oscillation in the preamp. I would not characterize this as 'feedback'... it sounds like either a very bad ground or RF in the system. The fact that you can touch the arm tube and have the sound go away for extended times suggests that its an RF problem. I don't think its anything with the Cardas cable, as when you touch the arm tube, you should at that time be able to keep the arm still.
RFI/EMI problems at tonearm wires can tend to jump around unpredictably as you describe and vary in proximity with the human body. Grounding the tonearm or TT is sometimes not enough, and screened tonearm wire may be necessary to resolve the problem.
Does the hum continue when the cartridge pins are disconnected from the tonearm wires?
You might experiment by relocating the TT to another part of the room. The current location may be in a bad RFI field or too close to in-wall AC wiring or to the AC PS of another component.
Based on your responses to Dave and Ralph, I am now thinking that I got it wrong. Am I now correct in thinking that the tonearm per se is not necessarily vibrating but that the hum you hear (over the speakers, I presume) is ameliorated when you grasp the arm tube? If so, this makes much more sense, and I agree with Dave. But I will throw in another idea; the arm tube structure is grounded to the arm pivot (i.e., makes electrical contact with) the bearings only. I once had a maddening hum problem that was only cured when I took my tonearm to Herb Papier (the inventor and original manufacturer). Herb lived about 10 miles from my house. Herb took my arm, and before my eyes he tightened the bearings by a hair, and the problem was gone. That has to have been nearly 20 years ago, and I have had no recurrence of the problem. Problem is you need a Herb or a Tri to do it so as not to over-tighten the bearings.
Yes, Lewm, the hum disappears when I grasp the covered portion of the tube. Sometimes the hum is amplified when I grasp the finger rest at the cartridge end; release the grasp, and no hum. This has been a problem since day one of installation, and several well-known analog experts and reviewers have applied various fixes, including triple-grounding the whole TT setup, wrapping the pre-amp in -yes-a chicken wire cage, and finally purchasing and inserting a $3K passive-step-up device in the chain prior to pre-amplification in order to permit such minimal gain that the hum will not be heard.
All this effort, no success.
And so, with the original dealer out of business and the non-communicative Wheaton lineage located thousands of miles away from NYC, it seems that I possibly purchased a defective tonearm, one which is not all that 'world-class precise' and which probably at this point cannot be righted.
But I do appreciate your insight and experience, Lewm. There is some solace at least in identifying the problem, though no solution seems possible.
Wow! Why so glum? Send it back to Tri, and he can tighten up the bearings to restore the proper ground connection, if that really is the issue, which I am beginning to believe it is. None of the strategies you mention could be expected to cure that problem. Just explain the symptom to him, and refer him to me if he does not get it. (We have met a few times; I was at Herb's house when Tri was there to learn how to build the tonearm. I last talked to him in person at RMAF last month.) The cost of shipping to and from Tri for you would probably be less than $25 round trip, if you use US Priority Mail. Chin up, old man! Hum due to bad, inadequate, or incorrect grounding is ALWAYS fix-able, and definitely does not indicate you have a defective sample. IOW, shit happens.
Dr John, each tonearm wire conductor that I use to eliminate hum is separately shielded("screened".) In a fully balanced configuration, each shield floats at the cartridge pin and is tied to preamp ground at the other end of the phono cable. In an RCA configuration the braided sheild can be employed both as a shield and as a conductor from the negative cartridge pin. Conductor and braided shield are both 4N silver and very thin. Of course this requires rewiring of tonearm.
Similar to your set-up, the TT is located against a wall that contains AC wiring and is some six feet from the AC main panel in the basement below. The hum increases after switching on a near-by ceiling light. After relocating the TT the hum problem was resolved without the screened wire. Then after adding the screened wire it was possble to return the TT to its former location. The relocation experiment is worth a try, if nothing else to eliminate a significant variable from the problem. Buy some cheap long Radio Shack ICs and just try it on the floor.