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Groove Tubes GT6L6GE opinions?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Mickey64, Feb 19, 2009.

  1. Mickey64

    Mickey64 Member

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    What's the general opinion here of these tubes? They came as stock in my recently purchased Fender '57 Tweed Twin, but the reviews I've read are mixed.

    I've tried putting a spare pair of JAN Phillips 6L6WGB's in the amp, but cannot get the bias lower than 37Ma before the bias pot reaches the end of its range. I'm looking to get another pair of these tubes which are rated more appropriately for the amp, without changing the bias resistor. I also have a pair of Svetlana 6L6GC's which I intend to try when I have time.
    However, the Groove Tubes are supposed to be produced on original GE equipment and are the product, I believe, of intensive R&D, so should be very good, right?
     
  2. larc

    larc Member

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    When they first came out, I compared a first production set to a pair of Svets. The Svets had a strong, thick midrange flavor to them. The GT6L6GEs had a similar voicing, but provided more of everything; sounded richer, better transparency...

    If you want to compare the reissues to original GEs, they are different. Groove Tube re-issues sound more like Svets than original GEs. Real GEs have a different voicing. The mids aren't so strong and dominant (not thick)... Originals are more transparent; they sound cleaner, clearer, brighter.
     
  3. myles111

    myles111 Member

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    "the Groove Tubes are supposed to be produced on original GE equipment"

    The are not produced on the original GE equipment .... and never were. They are made in China and always were made in China.
     
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  4. myles111

    myles111 Member

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    If you compare the original GEs to the GT version make sure that the tubes are all of the same plate current output. Otherwise the test is not really valid.
     
  5. Mickey64

    Mickey64 Member

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    Thanks so much for your interesting replies. Larc, the comparison to Svets is particularly helpful, as I've used those a lot in the past in my Vibroking, and am definitely going to try them in the Twin.

    Myles, I was reading some info about GT's on your website earlier, but now I'm confused; on Groove Tubes website they state, re their GT6L6GE's: "A faithful reproduction of the original General Electric "clear top" tube made on the original GE machines and with many original materials".

    Of course, it's what they sound like that matters, but what gives?
     
  6. myles111

    myles111 Member

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    There is some truth to the statement and some "Aspenism" to the statement.

    The original design was followed and the micas were made by the same company that made the originals.

    The plate material was at one time a reproduction of the 5 clad plate material formula but a lot of the time when production needed to continue and the Chinese did not have plate material they used their own.

    The grids, winding machines, and grid wire were Chinese.

    The tubes were never made on the original machines. Those machines at GT were never used for any production at all.

    The tubes orginally were screened "made in USA" but when Aspen was called on it and learned that he could be brought up on charges for making false claims that line of print was removed from the tubes.

    There were a lot of things Aspen told people at GT where initially we believed him. Over the years many of us learned that he was basically making up his own story. I worked directly with the Chinese factory and saw the 6CA7 failures as well as the 12AX7M failures over a number of years. I watched the GEs change radiators, use the wrong plate materials and saw other things as well right down to the wrong bottles. Aspen would generally say he was going to do something about it but in the end the tubes were just sold and it was hoped that chance would make things right in the future which rarely came to pass. In the case of the 6CA7 and 12AX7M things were not resolved at the time GT was sold to Fender.

    Over on another thread on this forum that asked why GT prices were so high I have a lot of responses that go into more details on these sorts of things. I will try to find the link.

    Here is the link - http://www.thegearpage.net/board/showthread.php?t=509908
     
    Last edited: Feb 19, 2009
  7. mockoman

    mockoman Member

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    I liked the tone of them,but,of the 4 I owned,2 went up in smoke (literally)...
     
  8. Mickey64

    Mickey64 Member

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    Thanks very much Myles, I've just read that link right through, and that and your post above are eye-openers! The possible lack of consistency in GT's might explain why, before I bought my '57 Twin reissue, I first tried another whose power tubes failed right there in the shop. I then got it home, and had to replace three of the GT12AX7's because of noise issues. (There were also other issues with that amp, and it was changed for the one I have now). I also wondered why some GT tubes had the "Made in USA" on them and some didn't, and now I know.

    Having visited your own site on a number of occasions, and read many of your posts on the Dr Z site, I'm grateful to you for taking the time to give so much detail; thanks again! The GT6L6GE's which came stock in that amp sound OK, if a bit muddy and generally uninteresting. After reading, in the link, about your friends with the two Pro Reverbs, I just changed them for a pair of Winged C's which I had as spares, and, though they're a bit cleaner, which is not necessarily what I'm after, the muddiness is gone, the overall tone is better balanced, I can use the normal channel without it muddying up way too much, and putting an OCD in front reveals a wider, more complex, open, less one dimensional and generally pleasing tone. Didn't really have to rebias either, as they just happened to match up to the GT's. A winner so far, and the GT's are henceforth relegated to spares!
     
    Last edited: Feb 20, 2009
  9. phsyconoodler

    phsyconoodler Member

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    Put in the tubes that sound good.37ma is right in the ballpark for your amp,so that should not be a concern.
    There are so many new production tubes out there that have reliability issues these days that you can't just pin a 'bad' flag on Groovetubes.
    Svetlana's are nice sounding tubes too.But worrying about whether or not the amp sounds like it did when the original came out is just plain silly.There are too many other variables to dwell on just the tubes.
    Put some tubes in,listen to them,if you like them leave 'em.If you don't,change 'em.
    Try an EQ pedal in front of the amp.It just may surprise you.
     
  10. Funky Monkey

    Funky Monkey Member

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    + infinity.
    :AOK
     
  11. myles111

    myles111 Member

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    Thanks for the kind words.

    On the GT 6L6GEs that came in the amp... if they have white silk screening they are probably keepers. The whites are mid range (design spec) tubes. They were generally very reliable and had long life in most cases. There were not many reliability issues with the GEs. There were issues with the 6CA7s and in fact, every one ever made is a ticking time bomb as there was a major design oversight that was overlooked.
     
  12. Mickey64

    Mickey64 Member

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    Yes, they're white, so that's reassuring. However, I'm gigging with the Svets in the amp tonight, so looking forward to replicating the improvement I heard at home. Thanks again.
     
  13. myles111

    myles111 Member

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    You are quite welcome.
     
  14. larc

    larc Member

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    Good point. The reissue and original pairs I compared didn't have the same plate current. I know these differences would change their sound. Still though, from what I heard, the re-issues don't sound like the originals.
     
  15. Structo

    Structo Member

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    When I bought Aspens new edition of the Tube Amp Book I read his story about the GE tubes and got excited at the possibility of USA made tubes again.
    Then I started doing some research and found like others that his statements were a bit on the far fetched side. I think at first he may have had the intent to do this but when faced with the environmental laws due to the materials used to make these tubes he realized that he could not feasibly do it here.

    But he should have been more up front when he actually started selling these tubes that they are assembled in China.
    All kind of a moot point now because Fender bought out Groove Tubes.
    I have heard mixed reports about the quality of these tubes. Some say great while others say they are mediocre at best.
    Tubes like a lot of things to do with the perception of tone or sound is totally subjective.
    What is one mans grail may be an other's folly.

    To the guy that wants to bias the tubes lower than 37ma, you have to take into account the plate voltage when biasing power tubes.
    There is no set bias current for tubes, the bias current goes hand in hand with the plate voltage that is present.
    Where one amp may be at 70% dissipation at 37ma, another amp may only be at 40% dissipation at 37ma.

    There are some charts out there that show different plate voltages and then the associated bias current and percentage of dissipation.

    Figure out what the low (50%) and high (70%) currents are then tweak with your ears what sounds the best inside that range.
    Some amps sound better biased cold while others perform better hotter.
    Let your ears be the judge. (within reason).
    Rock On!
     
  16. myles111

    myles111 Member

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    All your points are quite valid and your bias info is good advice.

    One point .... the environmental laws actually turned out to not be a factor at all. It was that the stuff that Aspen purchased would have required much more time and money to make it work properly and his friends in China were way more cost effective. None of the original equipment was ever used in the production of a single tube that was sold to anybody.
     
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  17. Mickey64

    Mickey64 Member

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    Well, even though I've been gigging regularly for 40 years or so, I will readily admit I'm no technician. The only reason for biasing at 33Ma, rightly or wrongly, is because that's what it states on the schematic for this amp. I bought a Weber Bias Rite a few years ago so that I don't have to find a tech (hard to find here) each time I change tubes, and it's worked well so far. I have picked up the idea from somewhere that 6L6's should be biased around 30-35Ma, and worked with this for some time now.

    However, if I assume 455 for the plate voltage, (printed next to each 6L6 on the schematic), and that Winged C's are 30 watt tubes, 70% of 30 is 21 watts, and 21/455 = .046, so is 46Ma the maximum allowable cathode current? I don't know how to actually measure plate voltage, and not sure I want to try, even with the comprehensive instructions on your website, Mike. Have I got this calculation correct?

    For the JAN Phillips 6L6WGB's, which I think are 25 watt tubes, 70% of 25 is 17.5, and 17.5/455 = 0.038, so 38Ma maximum? This suggests that the 37Ma minimum allowed by the bias pot on my amp, with the pair of these tubes I have, is very close to the maximum advisable current, correct?

    Also, the guy who suggested I was silly for trying to make the amp sound like an original has, with respect, read me wrong. I'm just trying to tweak it to sound how I want it to sound, which must also be the never ending quest of most of the folks who use this excellent forum, and I'm grateful for the wealth of information I'm getting; even after 40 years I still get excited by this stuff!
     
    Last edited: Feb 21, 2009
  18. stratman_el84

    stratman_el84 Member

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    Mickey, the plate voltage varies over a surprisingly-wide range depending on how much current the tubes are drawing at any one time and how stiff the power supply is. The voltages listed on a schematic are merely guides, not absolutes. Assuming here can get you to a bad place.

    The only way to safely and reliably calculate the plate dissipation is to actually measure the plate voltage & cathode current, then calculate. Adjust one way or another using that calculation as a guide, then re-measure and calculate everything again, because adjusting the bias will again change the plate voltage.

    If you want to get more accurate, you can measure the voltage drop across the screen resistors and then calculate how much the screens are drawing, then subtract that from the cathode current so the screen current isn't included in the cathode current and doesn't throw off the plate dissipation calculations.

    However, in most instances this isn't necessary and IMO is a bit of overkill as the screen dissipation is usually a very small fraction of the plate dissipation and can usually be disregarded for purposes of simply setting bias in the average amp.

    Nevertheless, even with all this calculation, the point is simply to make sure the tubes are operating within a safe range. What matters is how they sound. When I set bias, I do the math to find the safe adjustment range for the bias control for that set of tubes, then I try out different settings within that safe range and choose the one where the amp sounds best while the tubes are running as cool as possible for that "best sound".

    Good Luck!

    Strat
     
  19. rockon1

    rockon1 Member

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    I find it easier to find the max dis then find the percentage of that I desire.

    30 /455 = .066 X .7 (70 %) = 46 ma. Same difference. That said I rarely use 70% as "the" number. I try it then try 55% ,60% and settle on the lowest setting that sounds good to me. Some of my amps/tubes sound worse at 70% -to strident/harsh. Bob
     
  20. Mickey64

    Mickey64 Member

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    Thanks guys, much appreciated. In fact, following the great advice here, and reading around the web, I've gone into this a bit more, learnt how to measure plate voltage, and started a new thread to, hopefully, get some more expert advice on whether I'm doing it right!

    Thanks again!
     
    Last edited: Feb 22, 2009

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