Hi Guys,

I have a Blackstar Studio-20 and after say 10 minutes it will begin a low rumbling sound. If I tap the chassis it stops and if you tap it again it rumbles again. It only seems to occur on the clean channel as well.

Is this a sign that I need new tubes? the amp is 2 years old from new and had no changes.
If so, how would you determine which tubes need changing, the power amp or pre-amp?

Thanks for any help!!
I would replace all the tubes
2002 PRS CE22
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