Recently I bided a pair of 6DJ8 – Amperex from Germany (USD $5, see photo) ebay item number 290488910892. This is a used tube and expect the performance is not as good as NOS. So it ends up to be one tube with much higher noise level as shown in first 3 photos. Even the noise level is higher for one tube but I really cannot hear the difference between them. Amperex tube sound good!!! Thus our ears are really not able to tell much about tube variations!?
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Another experience is a lot 10 pcs of Russian 6N23P (USD $10), ebay item number 260680492416. All of them has much higher microphonic noise than US brand tubes and 7 of them with extreme high frequency buzzing noise when knock on the tube body. The buzzing will extended few to 10 seconds! See last photo. The seller has refunded me partly due to this problem and I am happy with his compensation!!!
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I use all the tubes in my Mu-Stage pre-amp!
