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Thread: 6L6GC or 5881?

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    Supporting Member Randall's Avatar
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    6L6GC or 5881?

    I have inherited a big stash of tubes from a local amp shop, so I will be lurking in this forum for a bit as I sort thru them. Today was learn about 5881 day. The guy was a fan of the early Russian stuff, so I have some coin base, WXT, Groove Tube branded JAN Sylvania 6L6WGB, etc. I was looking in my 6L6 pile and came across a short bottle brown base tube labled 6L6GC that looks exactly like one of my Sylvania and RCA branded brown base 6L6WGB's. Other than 6L6GC the only thing I can make out is two stylized capital T's side by side like TT. Also, USA is embossed on the pin side by pin 8.

    This can't really be a 6L6GC can it? Is it possible it is a misprint? If so, I don't believe I have ever seen that before.

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    It's weird, because it WAS working fine.....

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Randall View Post
    I have inherited a big stash of tubes from a local amp shop, so I will be lurking in this forum for a bit as I sort thru them. Today was learn about 5881 day. The guy was a fan of the early Russian stuff, so I have some coin base, WXT, Groove Tube branded JAN Sylvania 6L6WGB, etc. I was looking in my 6L6 pile and came across a short bottle brown base tube labled 6L6GC that looks exactly like one of my Sylvania and RCA branded brown base 6L6WGB's. Other than 6L6GC the only thing I can make out is two stylized capital T's side by side like TT. Also, USA is embossed on the pin side by pin 8.

    This can't really be a 6L6GC can it? Is it possible it is a misprint? If so, I don't believe I have ever seen that before.
    Those 2 t's would it be a pie symbol?

    Maybe this site would help with your stash
    https://www.vivatubes.com/identifyin...-and-eia-code/
    nosaj

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    Supporting Member Randall's Avatar
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    "Those 2 t's would it be a pie symbol?"

    No, they both have serifs pointing down from the tops on either side as well as a Roman Numeral type footer at the bottom.

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    It's weird, because it WAS working fine.....

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Randall View Post
    "Those 2 t's would it be a pie symbol?"

    No, they both have serifs pointing down from the tops on either side as well as a Roman Numeral type footer at the bottom.
    i guess a picture would be best if possible.

    nosaj

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    The two gothic T's are gone, I lost them while wiping the tube to clean it. But here is a shot of the tubes. On left is a Sylvania green paint 6L6WGB, middle is tube in question labeled 6L6GC , right is RCA logo but no designation. They all three look identical in every way to me. I just now noticed they all three have the same USA raised lettering in the same place on the pin side of the base, so it must be the same base for all three.

    Click image for larger version. 

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    It's weird, because it WAS working fine.....

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    Senior Member Pedro Vecino's Avatar
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    All three seem identical to me: Sylvania 5881. The same ones that are usually labeled JAN CHS 6L6WGB in green.

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    Not quite sure they are 5881
    these are.
    Click image for larger version. 

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    Senior Member Pedro Vecino's Avatar
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    Although its parameters are not exactly identical to 5881 in all circumstances Sylvania 6L6WGB can be considered 5881 without any problem.

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    I think what we have here is a mislabled tube. The tube in question, labeled 6L6GC looks identical to and measures as a matched pair both with current and Gm with the green label Sylvania 6L6WGB. I put 462 volts on them and they pulled within 2 mA of each other. And they both measured ~5000 Gm.

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    It's weird, because it WAS working fine.....

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    Quote Originally Posted by Randall View Post
    I think what we have here is a mislabled tube. The tube in question, labeled 6L6GC looks identical to and measures as a matched pair both with current and Gm with the green label Sylvania 6L6WGB. I put 462 volts on them and they pulled within 2 mA of each other. And they both measured ~5000 Gm.
    Well, not exactly mislabeled perhaps, but rather labeled as what the distributor could market them as because they are up to the task and other tubes that can take the heat weren't as available at that moment. That "s" happens all the time with tubes.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by Randall View Post
    I think what we have here is a mislabled tube. The tube in question, labeled 6L6GC looks identical to and measures as a matched pair both with current and Gm with the green label Sylvania 6L6WGB. I put 462 volts on them and they pulled within 2 mA of each other. And they both measured ~5000 Gm.
    That was a misleading synchronicity of parameters i.e a fluke The tolerances are something like 30% so the chances of a match like that are small. I had to buy a hundred EL34's once to get enough to match. In contrast, I got 25 MOSFETS (for a SS amp) were all from the same batch and all matched within 1%.

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    Don't forget the joker g1's Avatar
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    My Ampeg V4 running over 540V plate on those 6L6WGB JAN's says they are much more like 6L6GC than 5881's.
    Some folks insist they are a GC in a smaller bottle. Maybe that's what that one is.

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    Senior Member Pedro Vecino's Avatar
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    While waiting for the transport agency...

    Click image for larger version. 

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    Electrical characteristics of 5881, 6L6WGB and 6L6GC with voltages of 360 and 270V (plate and screen grid) are identical.
    But evaluating maximum plate and screen grid voltages, maximum current through the screen grid and maximum plate dissipation, 5881 and 6L6WGB are much closer.
    I know that the 6L6WGB support higher voltages, but not in a different way than 5881. I've also seen them in silverface Super reverb with voltages close to 500. The problem with this type of experimentation is knowing the time that they support it, the average of faults, etc. I particularly do not use them with voltages greater than 400/450. They are also very expensive.

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    Senior Member Old Tele man's Avatar
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    The 5881 was Tung-Sol's military version of the 6L6WGB, which itself was a militarized version of the commercial 6L6GB. Different interior surfaces accounted for voltage differences while different plate fabrication/coatings accounted for their wattage differences. The 6L6GC incorporated both interior surface and plate differences; but, all three had identical operating characteristics.

    More info here: http://www.tdpri.com/threads/opinion...3#post-7803156

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    Last edited by Old Tele man; 07-22-2018 at 01:58 AM.
    ...and the Devil said: "...yes, but it's a DRY heat!"

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    Quote Originally Posted by Old Tele man View Post
    The 5881 was Tung-Sol's military version of the 6L6WGB, which itself was a militarized version of the commercial 6L6GB. Different interior surfaces accounted for voltage differences while different plate fabrication/coatings accounted for their wattage differences. The 6L6GC incorporated both interior surface and plate differences; but, all three had identical operating characteristics...
    Thanks for the linked thread; any thoughts / insight / info on how the 35watt plate rating of the 7581A variant was achieved?
    http://www.mif.pg.gda.pl/homepages/f...27/7/7581A.pdf
    http://www.mif.pg.gda.pl/homepages/f...35/7/7581A.pdf

    I suspect that once a technical innovation such as the '5 ply plate' was rolled out, subsequent orders for previous tube type variants (eg 6L6WGB) would include that innovation, as the operational characteristics would be the same, and, with regard to the limiting values, 'what will be a do will do a little', ie a 30watt plate structure in a small bottle met the spec / acceptance criteria for a 5881 / 6L6WGB.
    ie it wouldn't seem to make manufacturing sense to retain whatever systems were necessary to continue making obsolete parts specific for the older, superseded tube types.

    Similarly for the 35watt plate of the 7581A.

    Hence Fender's ruggedised 6L6GC STR387 and the military JAN 6L6WGB made in the 70s and 80's may have used plate etc 7581A spec structures, and their reputation for robustness.

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    Senior Member Old Tele man's Avatar
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    The 7581 had slightly larger glass envelope (1-9/16-inch dia.) than did the earlier 6L6WGB (1-7/16-inch dia.), thus providing more surface area from which to dissipate heat from plate; however, no effect or change to screen dissipation.

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    Last edited by Old Tele man; 07-22-2018 at 03:23 PM.
    ...and the Devil said: "...yes, but it's a DRY heat!"

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