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NOS 6L6 vs Currently made 6L6

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  • NOS 6L6 vs Currently made 6L6

    I have a couple of questions on interchangeablity.

    I have about 20 NOS 6L6 Vacume tubes dating back to the mid sixties. None are sorted to be "matched pairs" etc. I have installed one set in my Fender Hot Rod Deville and they sound great, however with my bias adjustment pot at the lowest setting, they check at 70 milivolts at the test point. The Fender 6L6GT "whites" were running around 66 milivolts prior to changing of the tubes.

    My questions are...

    Some of these tubes have black outer casings (metallic I assume). Does this affect tone or more importatly, inter-operability?

    Secondly, some of the tubes have one less pin than the others.

    These have all been tested and were gotten from an old ham buddy of mine and are in boxes in excellent condition. Most of them are Plain white boxes and a few are in the Red RCA boxes, The RCA's are the Black Cased Tubes.

    Thirdly, Should I generally be overly concerned with the tube numbers after the 6L6.

    I like to tinker with vintage stuff but dont care to short out my PC board in the Deville.

    Thanks in advance.


  • #2
    You do need to be concearned about the codes following the 6L6 designation for amps that have a Vp over 450. I don't think the omitted pin should pose any problems. The "cased" tubes are generally not used for audio. I have never used them but I've read that they sound inferior and/or can be microphonic. Rough matching your tubes should be more than adequate for guitar amps. You'll want an oven mit for this. With the amp on, place it in standby and remove the power tubes. Plug in just one power tube. With all knobs on 0 flip the amp off standby and let the tube warm up for a few minutes. Check it with your bias meter. Place your amp back on standby and remove the tube. Write your read number on the tube with a Sharpie. Now do the next tube the same way. When your done just match up the tubes that have similar numbers. Keep the pairs brand specific since tube construction can effect how the tubes behave during actual use. So dissimilar brands that read the same at idle could be miles apart when conducting more current. If you really want to get jiggy you could get a signal generator and a dummy load and test each tube while it conducts more current as well. But IMHE this rarely makes for a better match than just testing the idle current draw. The higher mA draw of your new tubes is normal, not very different really and could just as easily happen with a new set of currently MFG'd tubes. That's why many amps are made with an adjustable bias. So you can set the amp up properly for new tubes.


    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A


    • #3
      The metal 6L6 is a big potential problem. The metal case is connected to pin 1. If that pin is used as a tie point for other components, that point will be connected to the case. For instance, in Blackface Fender amps the grid stopper ties to pin 1. If the bear trap thingy touches the metal case, you've shorted the grid to ground. Check the socket before you plug the tube in.

      Also note that 6L6 {nothing}, 6L6G, 6L6GA, and 6L6GB (possibly others) are rated 19W plate dissipation whereas the 6L6GC is rated at 30W. The plate voltage is also rated lower as noted above. This can be a problem in some amps.

      On some data sheets you'll see a note that says tubes may be supplied with pin 6 omitted.
      WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
      REMEMBER: Everybody knows that smokin' ain't allowed in school !


      • #4
        The Hot Rod's grid-stoppers are mounted on the PCB rather than the tube socket. I still wouldn't use the metal-can 6L6's here, because of their low dissipation specs. (And you can't *see* if the plates or screens are glowing!)

        Send 'em to me instead -- I build my projects so they can use metal 6L6's without issue.

        - Scott


        • #5
          Build any *old* 5E or 5D something Pro or Super or "original Bassman", any of the cathode biased, 6L6 ones.
          You can't go trigger happy with bias and they are almost impossible to redplate, unless your grid pin is open.
          Sound will be cooked in Heaven.
          Juan Manuel Fahey