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GT-6L6-GE power tube?

Discussion in 'Amp Tech Center' started by itsGiusto, Jan 26, 2021.

  1. itsGiusto

    itsGiusto Tele-Holic

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    The vintage Twin reverb I just bought came with GT-6L6-GE tubes. I can't find this model on the tube bias calculator:
    https://robrobinette.com/Tube_Bias_Calculator.htm

    What's the power rating that I should use for bias dissipation calculation? Is it 30w like the 6L6GC?
     
  2. Jon Snell

    Jon Snell Tele-Holic

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    Anode dissipation values.
    6L6GT plate dissipation 11W (source The Valve Museum)
    6L6GC plate dissipation 30W (source Svetlana Electronic Devices)
    GT-6L6-GE plate dissipation 20 - 25W (source Stew Mac) Made in China!
    So, derate as required using the 6L6 data.
     
  3. itsGiusto

    itsGiusto Tele-Holic

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    Hmm, I don't know what to do if it's a range of values, 20 to 25w. That's a big range. Which value would I use for my calculation?
    I feel like it could go wrong, like what if I bias it assuming it's a 25W tube, at 17.5W = 70% dissipation, but it's actually 20W, so I've actually biased it at 87.5% dissipation! I'd generally want to bias it as close to 70% as I can, since I've found that I usually get the best sound by doing that.
     
    Last edited: Jan 26, 2021
  4. Jon Snell

    Jon Snell Tele-Holic

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    Why not set it up using a formula. Here's how to do it;
    Some like them hot some cold. Without getting too heavy with maths and algebra, a tech from a market leader amp maker (no names mentioned) who tends to bias on the hot side, told me this.

    The maximum output from an 6L6-GE is about 20 watts. So measure the anode voltage of the amp, for this exercise we will assume that to be 445 volts. Divide the 20 by the 445 then multiply by 0.7 the answer in ma. is the 70% plate load bias setting required for that amplifier.

    The 0.7 means 70% Hot anode / plate dissipation. Therefore you can use 0.5 (50%) as a Cool AB setting, 0.6 (60%) as a Warmer AB setting the one I would prefer. Or go for the 0.7 or even 0.75 (70 - 75%) if you like it hot.

    Example: 20 / 445 X 0.7 = 31.4 ma. Bias

    The bias figure will depend on the anode voltage which will vary according to the current drawn by the valve. More current less volts, less current more volts. So you may need to do the calculation several times if large adjustments are made.

    Example 2. 20 / 480 X 0.7 = 29.1 ma. Bias

    Example 3. 20 / 410 X 0.7 = 34.1 ma. Bias


    Here is an EL34 example set as Class A with 300 volts on the plate / anode.

    25 watts max o/p of the valve divide by plate volts 300 multiply by 1 = bias current requirement for class A operation. Example. 25 / 300 X 1 = 0.083 = 83ma. or 25 / 350 X 1 = 0.071 = 71ma.

    Not rocket science - dead easy elementary stuff.
     
  5. itsGiusto

    itsGiusto Tele-Holic

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    Well, yes, I understand how to do this formula, that's how I bias (I use the tube bias calculator just for convenience, and to look up what different tubes' ratings are when they're in push-pull configuration). But I was confused because you said that the range is from 20 to 25 watts. I've never seen a range for tubes before, usually I'll just see that an EL34 is supposed to be 25 watts in push-pull, or a 6V6GT is supposed to be 14W.
    Are you saying that I should actually just take the 6L6-GE it to be 20 watts when it comes to doing the biasing math?
     
    Last edited: Jan 26, 2021
  6. schmee

    schmee Doctor of Teleocity

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    They are just GT's reissue version of the old General Electric 6L6GC. I'd just bias them as that. Stew Mac likely doesn't know any more what they are than anyone else. Reportedly "medium output" though. But who knows what all the various 6L6GC vintage tubes actually were?

    "The Groove Tubes GT-6L6-GE is a faithful reproduction of the original "clear top" 6L6. The flagship of the GT 6L6 line, it looks and sounds exactly like the original G.E. tube used by Fender in the '50s and '60s. It took more than four years to develop, and has stunning clarity and power. Our most recommended 6L6."
     
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  7. itsGiusto

    itsGiusto Tele-Holic

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    Interesting. So if that the case, then it should actually be biased to 30w * .7 = 21w?
     
  8. schmee

    schmee Doctor of Teleocity

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    That's where I would start, see how it sounds.
     
  9. itsGiusto

    itsGiusto Tele-Holic

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    and if it red-plates!

    Though I've never been confident in my ability to discern red-plating from the glow from normal tube-operation.
     
  10. strat a various

    strat a various Friend of Leo's

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    You know it when you see it ... the color of the hot elements in a toaster.
     
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  11. schmee

    schmee Doctor of Teleocity

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    Nah, it won't be that far off.
     
  12. itsGiusto

    itsGiusto Tele-Holic

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    But, I mean, the tube is supposed to always have somewhat of the color of the hot elements of a toaster right? The filament inside is supposed to glow, or whatever that column is in the center:
    [​IMG]

    It's just that the plates aren't supposed to glow, if I understand correctly.

    The other thing is, it might not be red-plating while you're biasing, but that's because you bias at idle. But I don't sit and stare at my tubes while I play, when you're gonna be driving the amp harder. If I understand the operation correctly (and I don't understand it 100%), it'd be more likely to red plate when you drive it harder and the current draw goes up, no?
     
  13. strat a various

    strat a various Friend of Leo's

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    So, see the dark grey bits? When they are dull red, it's a bad thing. When I change tubes, I turn the lights off (amps like romantic atmosphere) and play loudly while looking into the back of the amp ... for several minutes. Usually if there's a problem, you'll see it. Usually.
     
  14. itsGiusto

    itsGiusto Tele-Holic

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    Alright, well, I guess I could try that.
     
  15. marshman

    marshman Poster Extraordinaire Silver Supporter

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    If memory serves, I recall reading sometime in the early-mid 90s about GT (Aspen Pittman, right?) coming into possession of one of GEs last tube manufacturing machines and some of their plate material (it's really a coating, innit?) and applying some reverse engineering to having those late-model GE 6L6GCs "recreated" at a facility abroad (don't recall where).

    Sad thing is, as a bit of a pack-rat and real nut about saving my magazines over the years, I probably still have that issue down in the man cave somewhere.
     
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  16. NTC

    NTC Tele-Meister

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    Was that the IEEE Spectrum article?
     
  17. JDB2

    JDB2 Tele-Holic

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    It’s pretty unmistakable when the plates start to turn red like red hot metal. Look at them working in a dark room.
     
  18. SoK66

    SoK66 Tele-Afflicted

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    Those are purported to be GE 6L6GC replicas and are very close. Nearly 20 years ago GT founder Aspen Pittman secured some original GE tooling for their old 6L6GC, which was OEM in most Fender combo amps through the blackface/silverface golden era of the 60s. (BTW, shorter bottle RCA 6L6GCs were OEM in the heads, not the combo amps.) Aspen was attempting USA production but couldn't do it profitably, so the tooling went to China for actual production. If you look at them side by side with a vintage GE 6L6GC they look nearly identical, and to my ears they sound fat & full like the originals. They are 30w tubes so bias as you would a regular 6L6GC.
     
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  19. hepular

    hepular Tele-Holic

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    red plating:
     
  20. milocj

    milocj Friend of Leo's

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    The only thing bad that I have heard about them is that at least when they were first brought out they seemed to have a bit higher failure rate than than GT would have liked for their premier power tube.

    I have two sets and have used one set at around 65-70% plate dissipation (for a 6L6GC) in a Fender Vibro-King for long enough to know that I know they're going to at least survive a decent amount of time at decent volume and they sound great. I've burned the second set in without issues, as well.

    Great tubes considering their cost compared to old stock.
     
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