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Low gain 12AX7 ECC83 ???

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  • #16
    Originally posted by Enzo View Post
    Sorry I thought you asked if this meant the tube was getting weak.


    Did they make a 12AX7 with different amplification factor? No, that would then be a different tube type.
    I believe they did. Wasn't the 5751 designed to work as an electrical equivalent (for all other intents and purposes) to the 12AX7 with a mu of around 70?
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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    • #17
      The 12AY7 & 5751 are direct replacements, optimized to work at their best in an identical circuit as a circuit designed for a 12AX7, but neither should be considered a "low-gain 12AX7." They are all three very distinct tube types. It'd be like calling a 68k resistor a "low-resistance and 100k."

      If I had a real 12AX7 with a gain of 70 or 45, I'd call it "broken."

      Justin
      "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
      "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
      "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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      • #18
        5751 was an industrial tube, designed for control circuits. The fact it is useful as an audio tube is a bonus.
        Education is what you're left with after you have forgotten what you have learned.

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        • #19
          Good AX7 plate reads 168V

          Good AU7 plate reads 68V

          The International labelled 12AX7 plate reads 58V
          So you have a re/mis-labelled 12AU7 , period.

          Or any of the tubes listed in manuals as 12AU7 replacements.
          Juan Manuel Fahey

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          • #20
            Originally posted by J M Fahey View Post
            No need for a Tube tester: just plug suspect tube in a known preamp position, say the classic 1k5/100k cathode/plate pair.

            A 12AX7 will self bias around 150/160V if rail is 250/260V , a 12AT7 will not be far off .... a 12AU7 will be way off.

            I think plate will be much lower, probably less than 100V, wouldn´t be surprised at 60 to 80V , since U7 passes way more current than X7 in all cases.

            While a well worn X7 will pass less current than expected (lowered emission as Enzo said), so plate voltage should be way higher than normal.

            I bet that plate voltage difference should be enough to separate a weak X7 from a good but lower gain U7

            Even if both show low gain.
            Bloody brilliant Juan!
            Thanks for the great tip!

            Now to try and remember this the next time I decide to go wading into the tube stash. lol
            (Regardless, I saved it for posterity! )
            Start simple...then go deep!

            "EL84's are the bitches of guitar amp design." Chuck H

            "How could they know back in 1980-whatever that there'd come a time when it was easier to find the wreck of the Titanic than find another SAD1024?" -Mark Hammer

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