Tube preamp noise

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.