
Every tube buyer learns the rule early: flip the Telefunken over and look for the diamond. That little raised rhombus between the pins has probably saved more wallets than any consumer protection law. But “no diamond, no Tele” has exceptions, and those exceptions are exactly the crack relabeled tubes love to crawl through. Here’s how to tell a genuine blank-bottomed Telefunken ECC83 (12AX7), ECC82 (12AU7) or ECC81 (12AT7) from the crowd of tubes that just borrowed the name.
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