Noise upon pre-amp tube replacement

I was replacing 7308 and 12AU7 tubes today (pair by pair) with newly purchased NOS tubes. They are RCA 7308 gold pins (made by Siemens) and Raytheon 5814. Each time I replaced the tubes, after 5 minutes I heard a "thump" from the left speaker as if a brick or wooden board fell down on the floor. I am wondering what it was? Has anyone experienced such phenomenon? My amp definitely does not like frequent tube rolling!
Once in awile you will get a NOS tube that doesn't survive the aging process..It could be a bad NOS tube..If you can flip the tubes from side to side ( left to right ) and the noise travels you have a bad tube...Its happened to me before so don't be too suprized if its a bad tube....
Hi Transl,
Thorman makes a good point, just be careful, I have seen some nos tubes short
out and cause some serious damage. When you buy NOS tubes always try to
get some warranty or at least have the retailer burn them in for 48 hrs.
If a tube fails , it usually happens within the first 48 hrs. If heard tubes go noisy
and microphonic before, and it is a shame with NOS tubes.
It was a single "thump" but then everything worked OK. There was no need even to swap the tubes from side to side.
Did you have the amplifier on while you were replacing the tubes in the preamp? The thump could have been the preamp warming up, if so then there are no worries- just make sure you turn the preamp on before the power amp(s).
This is an integrated amp. This problem did not appear again.
Transl, I had the identical problem a few months back. I would turn on my amp (Cary SLI80) and within a few minutes I would get the thump - identical to your scenario. After about a month of this behavior, one of my power tubes failed. I would swap out all the tubes one by one in the channel that's creating the noise until you find the bad tube.
Hi Cruz,

I have the Cary SLI-80 but I've never had a problem like yours, save for burnt fuses a couple of times.