Tube preamp noise


I acquired an all tube preamp, and one thing I immediately noticed was that the preamp is super sensitive to any external impact, however small may it be. With a gentle tap on the cover of the preamp is echoed and heard on the speaker even when the volume is set to minimum. Same thing when I tap on the audio rack, other equipments (power amp, cdp, turntable), and even on the floor.
It is just like the sound you would hear from the PA speaker when you tap on a microphone.
Is it common on certain preamps?
Anybody experienced this?
Convert?fit=crop&h=128&rotate=exif&w=128ihcho
It would help to know which particular preamp you got and what types of tubes it uses?

Some types of tubes are more "microphonic" than others. Other times it's just a bad tube and a replacement is all that's needed. Beyond that, it's the implementation or construction of the component.
You're experiencing a microphonic tube (or tubes). Hopefully, it's isolated to one channel. You can often find the bad tube by gently tapping on each tube's glass bottle with a rubber pencil eraser. The bad tube will be obvious because it will display the microphonic effect louder than the other tubes.

At least this has been my experience.

What type of tubes does your preamp use?
Many thanks.
I will try to find a bad tube. Well, one of my friends is a strong bias on tubes and almost forced me to buy a tube preamp when I was about to upgrade, but I am not new to tubes. I may be forever like that. ;-(

Anyway, thanks for your help. My preamp uses 6SN7.
I will try to find a bad tube and replace with another, and see if the problem goes away.
6SN7. That explains a great deal.

I have owned more microphonic 6SN7 tubes than any other tube type. Many of them have been the highest priced, most sought after varieties.

Having said that, the one 6SN7 preamp I owned sounded fabulous when the tubes were all good.

Good luck. I'm sure you're going to love the preamp when the tube situation gets sorted out.

For excellent quality, guaranteed 6SN7 tubes, I suggest contacting Andy at Vintage Tube Services.
Thanks,
I may contact VTS later. The preamp is 4~5 years old and uses 4 6SN7s. It came with 4 sets of 6SN7 GT, and 2 GTAs and 2 GTBs. All tubes generated microphone tapping sound on tapping with the eraser end of a pencil. Small bumping sound appear to be OK but I found two GTAs are much worse than others. The GTA and GTB sets generate much worse cracking (or whatever) noise than 4 GTs. Among 4 GTs, one seems to generate more noise.

Furthermore, among 4 sockets, one socket always generates more tapping noise regardless whatever combination of tubes used.

I mixed (those seemingly good) 3 GTs and 1 GTB, and the tapping noise got reduced a little bit, but now there is more hum noise. Is it OK to mix GT and GTA/GTB?

I guess for many tube amp owners such problems are not uncommon, and hope my preamp is not defective other than tube problems.
You might buy some new production 6SN7 tubes...like Shuguang...and install those. The new tubes should not be microphonic, especially if they are purchased from a dealer that screens for this problem.

If the preamp has microphonics with the new production tubes, then you'll know you have a problem with the preamp and not the tubes.
Furthermore, among 4 sockets, one socket always generates more tapping noise regardless whatever combination of tubes used.
This might also indicate that a tube socket needs to be replaced.
Socket replacement? It is an easy job?
How often a socket can go bad? Is it a common problem for tube amp/preamp?

Thanks much for your help.
"You might buy some new production 6SN7 tubes...like Shuguang...and install those. The new tubes should not be microphonic, especially if they are purchased from a dealer that screens for this problem."

I got Shuguang 6SN7, and yes, I found they do not suffer from the microphonic problem. From new fresh out of the box, they do not sound as good as Sylavania or GE, but at least I am glad to find out the problem wss due to old/bad tubes, not to the sockets or other parts of the preamp.
Thanks!
Ihcho, now that you have the issue narrowed down to the tubes it's time to contact Vintage Tube Services and buy some excellent, reliable, microphonic-free, vintage 6SN7.
I don't think I've ever had a 6SN7 (or many other types) that are not microphonic to one degree or another. Unless the problem is severe it doesn't indicate that the tube is not performing well in other respects or that the tube will fail prematurely. Minimize it with tube dampers & decouple the component from the rack. If slightly microphonic NOS tubes sound better than Chinese, then use them!
I could not notice any microphonic problem with the new Chinese tubes. I have 10 used 6SN7 GT/GTA/GTB tubes form GE, Sylvania, and EH, and they all suffer microphonic rather severely. With a gentle touch on the amp case will generate an echo on the speaker. Gentle footstep near the amp does the same thing.
I just ordered tube dampers, and I will see how much they would reduce the problem. The Chinese tubes produced rather flat sound. I will see if they will make any difference after several days. I would rather use the old tubes than the Chinese tubes if the microphonic problem is lessened or if the Chinese tubes do not improve considerably over time.

I also ordered NOS Sylvania GTB tubes with green label and chrome tops, and will see how they would do.
A simple way to isolate from rack noise or foot-falls is to float the component on a partially inflated inner tube made for a child's bike.
I got NOS Sylvania GTB tubes and 1960's Russian tubes. They sound equally good, but got Russian tubes at the same price as Chinese ones, much cheaper than Sylvania tubes. Sounds notably better than the Chinese.

For the microphonic noise, the Russian tubes are much quieter. No more foot step echos. The Sylvania tubes are still prone to some noise, even though they are less noisy than the old tubes came with the preamp.

I would highly recommend those 60's Russian made 6SN7s.