Tube pre picking up feedback/vibration?

Recently got a pair of Magnepan 3.7's and a Sanders Magtech.
Have an old ARC LS15, newly retuned.
Have noticed a muddiness in the bass, kind of a one tone bass at times. When I tap on the preamp cabinet, get a distinct sound transmitting to the speakers, so
I gather I'm picking up some feedback. The pre is rather cole to the speakers, and it doesn't have the isolation of more current models.
Can you guys help with suggestions to stop this?
I'm considering a new preamp, but never noticed this with my previous speakers.
Are there isolation devices to deal with this, tub dampers, etc???
Changes the wires from the Pre to the amp, Putting in my Verbatim wires in place of the Nordost, and that helped some. but want to remove this source of pollution from the system, obviously.
Your tubes appear to be microphonic - new low noise tubes are better than trying to quieting old ones with dampers. You said you had this retuned. Did you mean to say retubed? Also, re muddiness, are you by any chance using EH6922's?
Remove the top of your preamp and with the system on tap lightly on the tubes. The microphonic tube will be located by using this method.
Thanks . Will try to get this done today. Yes, retubed. Thank you spell checker!!
Tubes were purchased for ARC, 2 weeks ago, and I assumed they would check for defects before they were sent out. Go figure!
Again, thanks for the advice.
Sorry to say, all tubes are microphonic, at least to some degree; there simply isn't such a thing as a non-microphonic tube. This is fairly easy to demonstrate with Herbies Tube Dampers. The only way to fly.
LS15 has poor design because their tubes are overbiased for the highest unnecessary voltage gain. that also causes other circuit elements such as DC caps to go bad quicker than in any other preamp.

your consideration of a new preamp is right on spot.
"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 agree with the Herbies Tube Dampers;I use them on my pre amp with excellant results.
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.
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.


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.
I believe that preamp should be minimalistic and perform as close to passive as possible. LS15 is quite opposite to any amplifiers it drives.
And yes I've upgraded to Mytek 192 DSDDAC/preamp/headphone + Musical Fidelity XPS for analogye mainly for excellent dynamic capabilities and bunch of digital inputs. If I'd use ARC on my rig, I wouldn't be able to turn volume past 8:00. Can you imagine how dynamics would suffer? Nah I'd like to have linear performance on nearly all volume levels as I have right now. Donno about youall.
All components have to be driven with comfort -- forget the luxury because price and model has nothing to do with system synergy.
Have you ever seen BMW 5 series with plush seats and manual sunroof/side windows? I guess I don't mind driving it... You?
Marakanetz, I find your BMW 5 series analogy especially useful since that has been my car of choice for more than 10 years. Come to think of it I have never seen a 3 series with the lack of options you mentioned either.

BTW Audio Research volume pots can be easily adjusted by simply changing a single resistor.
That single resistor is a feedback resistor. Altering this resistor from default setting to some magnitude may bring the amp onto oscillation or out of stability and another level of microphony(good point to check btw). The best what can happen is sacrificed dynamic range and transparency. I believe and know that tubes are not microphonic and amps/preamps are. There are plenty of good designed tube preamps with no microphony and my eyes and hands had seen them. Did anyone have an experience with AES by Cary little tube preamp?
As to BMW 5 or 3 series, in other than US countries people are still paying top dollar for these automobilles, but have still options not to have all extra items such as auto-trans, rear climate, auto-seat, power windows etc...
The resistor change is Audio Research's recommended fix for adjusting volume control setting. Ideally the volume control should be in the 11-1 o'clock position at your desired listening level.
Still not happy with the results of the new tubes. Emailed ARC and they basically said all is well. Now I notice a low hum from one channel and reversing the wires leads me to suspect the tubed. What exactly will a bad tube sound like? Hear the hum with volume all the way down and the cd player stopped. Like I said I reversed the wires to the amp an
d the problem followed the change so I have ruled out the amp. Ideas?
Your cable routing could be causing a problem. Do some 'process of elimination' experiments to see if you can pin it down. If you have a noisy tube, it will sound more like white noise/hiss, not so much hum. A hum in only one channel is often caused by an interconnect passing close to a transformer or power cord.

1st try just reversing the tubes from channel to channel. If the sound does not change with the switch it is not a tube problem. If it does, it is a tube. Replace it.

Next, look at all of your interconnect and power cords and try to keep them well away from each other. If you must have an IC near a PC, makes sure it crosses it, NOT running parallel to it. Also make sure that the IC's are well away from any transformers in any of your components.

If that doesn't do it, disconnect each source first the PC and then the IC's. If the sound goes away your problem is in your source or its connections. If is does go away, then reconnect them one channel at a time and see if it reappears.

If doing all of that doesn't get you a fix, you might consider that you have a cap going bad. Did ARC check out your unit or did you just buy and install new tubes? If the latter then perhaps you need to have a techie check this out.
Short the input RCA jacks and see if the hum is there.
If so its the preamp, if not its your CD player or cables.

Not your power amp.

Yes some tubes are microphonic, some tubes hum.
Carefully swap the L and R channel tubes and see if it moves with the tubes. If not, then send the amp back to Audio research and have them check signal grounding.

I find Marakanetz's post about Audio Research recommending NFB resistor change to be shocking. Changing NFB to tune overall gain can shift the oscillation pole frequency and can cause stability issues.
Thanks...great suggestions . I'll try to get time and let you know what I find.
Finally dug up an old set of Sovtek tubes, probably from many years ago, that were in a box in the basement. Put them in and took out the new ones ARC had sent me ...and no more noise, microphones, whatsoever!
To say I'm disappointed that they would sell crap like this is an understatement. These tubes are marked by them for the specific spot on the circuit board, but I guess no one actually tests them for micro phonics. Do they need to actually be installed in a preamp for thai to show up?? I figured one of the reasons that I should buy from the source, so to speak, was that they would send me something that wasn't a crap shoot.
Makes me wonder if I should look at this brand when I upgrade.
Anyway, problem solved. Now to get a replacement set that only make music when the signal says to!!..))
Consider NOS 6h23's for replacement tubes for your unit. They are drop in replacements. They are very rugged and are fairly identical to Sovteks tone wise. Get them from a dealer who will test the tubes for noise (low) and microphonic's. I've used them for years in an ARC pre-amp. Interestingly the last time ARC had it they put some new Sovteks in my phone stage. Guess what, one of them failed shortly after I got it back. Shit happens - I doubt that ARC did anything wrong, and I imagine that they would give you a replacement tube if you asked (unless you got it too long ago).

Good that you solved your problem. Be thankful it was just a tube! :-)
John, I'd never suggest changing value of NFB resistor. All I advised is to check if this value had ever been changed by previous owner or any prior