Tube pre picking up feedback/vibration?

ARC checks their tubes, but tubes can change. I am sure they will gladly replace the defective tube.

"LS15 has poor design because their tubes are overbiased for the highest unnecessary voltage gain."

Whose opinion is this? Audio Research designs their preamps for performance. Isn't that what you want?

"LS15 has poor design because their tubes are overbiased for the highest unnecessary voltage gain."

This is from a guy who uses a Classe 30 and a Bryston 11B. Two solid state preamps that are inferior to the ARC LS15. Maybe you should consider a new preamp.

"Sorry to say, all tubes are microphonic, at least to some degree; there simply isn't such a thing as a non-microphonic tube."

True enough. But to find the most microphonic and thus problematic tube, lightly tapping each tube as Rrog suggests will help you isolate the bad boy so you can replace it with a new one. Just make sure you have the volume set at a low level when you do it.

I'll take a second shot at this one -

I'm not sure but it appears that Brrgrr's complaint is 'muddiness' and when he taps the case he gets feedback from his speakers caused, we must assume by microphonics. He connects the microphonics and the muddiness, or at least, he suggests a possible connection.

He has just replaced the tubes. Last I heard ARC was using EH6922's in its stuff and as replacement tubes. To be blunt, I have used these tubes in an ARC pre-amp as well as a couple of tubed CDP's and amps. I found them to be muddy in the midrange. I also found them noisy (very low level though). I never noticed a problem with microphonics, but then I never tap tubes with anything to find out. If I can't hear it it must not exist. :-)

I think it is entirely possible that the OP's only problem is tube selection and his problem could be solved by getting different tubes, and not just new replacement ones from ARC. If this amp really does stress its tubes, as Marakanetz suggests, he could try some old stock 6h23EB's. These tubes are not warm. If the OP wants some warmth he might try different tubes in the second stage such as the new production JJ's. Worked quite well in my ARC preamp.

FWIW.

FWIW.

Newbee, I used EH tubes supplied by Audio Research in an SP-8 MKII with completely different results. That preamp with EH tubes became as neutral as any preamp I have heard. The OP may have a microphonic tube or the preamp is not setting on a solid surface. Audio Research is making an effort to minimize the effects of the type of surface their equipment is setting on with built in isolation. I think this is a smart move on their part.